Jacopo Vizioli
email: jacopo.vizioli@univ-lille.fr

Tiziano Verri
email: tiziano.verri@unisalento.it

Patrizia Pagliara
email: patrizia.pagliara@unisalento.it

Allograft Inflammatory Factor-1 in Metazoans: Focus on Invertebrates

Keywords: AIF-1, Iba1, immunity, macrophages, invertebrates, inflammation, bacterial challenge

Simple Summary: During their life, all living organisms defend themselves from pathogens using complex strategies. Vertebrates and invertebrates share mechanisms and molecules that guarantee their overall bodily integrity. Allograft inflammatory factor-1 (AIF-1) is a protein extensively studied in vertebrates, and especially in mammals. This factor, generally involved in inflammation events occurring upon pathogenic infection or tissue injury, is linked to several important human diseases. This review collects data on the presence and role of AIF-1 in invertebrates, which are still poorly investigated organisms. Multiple alignment and phylogenetic analysis reveal that AIF-1 is conserved in vertebrates and invertebrates, suggesting similarity of functions. In some invertebrate species, the expression of AIF-1 increases considerably after a bacterial challenge, indicating that it plays a key role during the immune responses. This review highlights the importance of studying this protein in invertebrates as a way to improve our knowledge of innate immunity mechanisms and to better understand inflammatory regulation events in mammals.

Introduction

Living organisms protect themselves from pathogens or tissue injury through a complex regulatory network of processes, among which inflammation plays an important role [START_REF] Ferrero-Miliani | Chronic inflammation: Importance of NOD2 and NALP3 in interleukin-1β generation[END_REF]. Inflammation represents the first response of the immune system to harmful stimuli and is a crucial event to initiate the healing process. When tissues are injured by bacteria, trauma, toxins, heat, or any other cause, the damaged cells release molecules responsible for leaking fluid from the blood vessels into the tissues. This causes a swelling that helps isolate the foreign substance from further contact with body tissues. The released molecules also attract phagocytes able to "eat" foreign particles or damaged cells. The main purpose is the elimination or inactivation of the intruders.

During the inflammatory process, the coordinated activation of various signaling pathways regulates inflammatory mediator levels in resident tissue cells and inflammatory cells recruited from the blood [START_REF] Lawrence | The Nuclear Factor NF-κB Pathway in Inflammation[END_REF]. In this sense, deregulated inflammatory states can drive many chronic diseases, including cardiovascular and bowel diseases, diabetes, arthritis, and cancer [START_REF] Libby | Inflammatory mechanisms: The molecular basis of inflammation and disease[END_REF].

Allograft inflammatory factor-1 (AIF-1) is a calcium-binding scaffold/adaptor protein often associated with inflammatory diseases. AIF-1 was originally cloned from active macrophages in rat and human atherosclerotic allogenic heart grafts undergoing chronic transplant rejection [START_REF] Utans | Cloning and characterization of allograft inflammatory factor-1: A novel macrophage factor identified in rat cardiac allografts with chronic rejection[END_REF][START_REF] Utans | Allograft inflammatory factory-1. A cytokine-responsive macrophage molecule expressed in transplanted human hearts[END_REF]. AIF-1 belongs to a family of proteins including three other molecules identical to AIF-1, which were reported under the names of MRF-1, Iba1, and Daintain, respectively, and a series of proteins sharing a large, but not total, identity with AIF-1 (i.e., IRT-1, BART-1, and G1) [START_REF] Deininger | The allograft inflammatory factor-1 family of proteins[END_REF]. Subsequently, AIF-1 has been identified and characterized in many other species, so we can consider it as one of the most evolutionarily conserved inflammatory genes. AIF-1 is a 17-kDa protein with a central pair of EF-hand motifs [START_REF] Imai | A novel gene iba1 in the major histocompatibility complex class III region encoding an EF hand protein expressed in a monocytic lineage[END_REF]. This feature is common to a large family of Ca 2+ -binding proteins known as EF-hand proteins [START_REF] Kawasaki | Classification and evolution of EF-hand proteins[END_REF]. Moreover, the AIF-1 protein and gene are also known as ionized calcium-binding adapter molecule 1 (Iba1), a 147-amino-acid Ca 2+ -binding protein widely used as a marker for microglia [START_REF] Prinz | Microglia and brain macrophages in the molecular age: From origin to neuropsychiatric disease[END_REF]. AIF-1 is an important regulator of immune response and is involved in a large panel of inflammation-associated pathologies. An overview of inflammatory diseases mediated by AIF-1 was published by Zhao et al. [START_REF] Zhao | Role of AIF-1 in the regulation of inflammatory activation and diverse disease processes[END_REF]. In this work, the authors report the involvement of this factor in allograft rejections, vasculopathies, autoimmune diseases, central nervous system (CNS) injuries, and cancers, enhancing its role as a regulator of inflammatory mediators such as iNOS, cytokines, and chemokines.

In this review, we will present, after a short mention of AIF-1 in vertebrates, an overview of this factor as identified and characterized in phylogenetically distant invertebrate species, including sponges, cnidarians, mollusks, annelids, and echinoderms.

The more we learn about these distantly related ancestors, the more we realize how much we share with them. To date, it is possible to find several disparate, and often unrelated, studies on the expression and role of AIF-1 within different invertebrate species. Furthermore, it is worth considering the role and modulation of AIF-1 in the immune response of invertebrate organisms as well as its functional and structural conservation along metazoan evolution. A thorough understanding of ancient immune systems will not only help us to identify chinks in the armor of invertebrate pests but also provide a window to the better comprehension of our own innate immune mechanisms.

Vertebrata

All multicellular organisms, vertebrates or invertebrates, possess an immune system that is an essential component of the defense strategies to recognize and neutralize parasites, microorganisms, viruses, and more. Vertebrates have two lines of defense, defined as innate, or non-specific, immunity and adaptive, or acquired, immunity. The success of the immune response is guaranteed by a complex interweaving of interactions between different types of molecules, each with specific functions.

In mammals, the expression of AIF-1 has been reported in different cell types such as activated macrophages, microglial cells, and dendritic cells (DC). The main immunomodulatory role of the protein during the inflammatory response has also been highlighted in these cells. However, this gene plays different roles in the nervous and immune systems [START_REF] Chen | Identification, isolation, and characterization of daintain (allograft inflammatory factor 1), a macrophage polypeptide with effects on insulin secretion and abundantly present in the pancreas of prediabetic BB rats[END_REF][START_REF] Schluesener | Allograft-inflammatory factor-1 in rat experimental autoimmune encephalomyelitis, neuritis, and uveitis: Expression by activated macrophages and microglial cells[END_REF]. Furthermore, AIF-1 is also expressed in muscle, liver, spleen, and thymus in rats [START_REF] Autieri | cDNA cloning of human allograft inflammatory factor-1: Tissue distribution, cytokine induction, and mRNA expression in injured rat carotid arteries[END_REF] and humans [START_REF] Kuschel | Allograft inflammatory factor-1 is expressed by macrophages in injured skeletal muscle and abrogates proliferation and differentiation of satellite cells[END_REF], and it is considered a marker of activated human vascular smooth muscle cells and arterial injury [START_REF] Autieri | Expression of allograft inflammatory factor-1 is a marker of activated human vascular smooth muscle cells and arterial injury[END_REF]. A possible link between skeletal muscle cell proliferation and AIF-1-induced inhibition of satellite cell proliferation has also been revealed [START_REF] Kuschel | Allograft inflammatory factor-1 is expressed by macrophages in injured skeletal muscle and abrogates proliferation and differentiation of satellite cells[END_REF], expanding the possible fields of action of this protein. In 2017, Elizondo et al. underlined the importance of AIF-1 in antigen presentation by DC [START_REF] Elizondo | Inhibition of allograft inflammatory factor-1 in dendritic cells restrains CD4 + T cell effector responses and induces CD25 + Foxp3 + T regulatory subsets[END_REF]. In this study, they reported that AIF-1 is expressed in CD11c + dendritic cells and that expression silencing restrains induction of antigen-specific CD4 + T cell effector responses. Moreover, because AIF-1 knockdown in murine DC resulted in impaired T cell proliferation, the same authors demonstrated that AIF-1 expression in DC serves as a potent governor of cognate T cell responses [START_REF] Elizondo | IL-10 producing CD8 + CD122 + PD-1 + regulatory T cells are expanded by dendritic cells silenced for Allograft Inflammatory Factor-1[END_REF]. Furthermore, Miyata et al. [START_REF] Miyata | DNA cloning and characterization of an allograft inflammatory factor-1 homologue in red sea bream (Chrysophrys major)[END_REF] showed an upregulation of AIF-1 transcripts in red seabream (Teleostean) leukocytes upon LPS stimulation and suggested a similar function in Vertebrata. More recently, a novel role of AIF-1 as a Ca 2+ -responsive scaffold protein involved in cell differentiation emerged [START_REF] Elizondo | Allograft Inflammatory Factor-1 governs hematopoietic stem cell differentiation into cDC1 and monocyte-derived dendritic cells through IRF8 and RelB in vitro[END_REF]. In particular, the requirement of AIF-1 expression in hematopoietic progenitors for differentiation into Mo-DC and cDC1 subsets has been evidenced.

The AIF-1 protein represents a crucial element for macrophages' survival and pro-inflammatory activity [START_REF] Yang | Allograft inflammatory factor-1 (AIF-1) is crucial for the survival and pro-inflammatory activity of macrophages[END_REF][START_REF] Tian | Inhibition of AIF-1 expression by constitutive siRNA expression reduces macrophage migration, proliferation, and signal transduction initiated by atherogenic stimuli[END_REF]. It is involved in inflammation and immune responses associated with autoimmune diseases [START_REF] Zhao | Daintain/AIF-1 (Allograft Inflammatory Factor-1) accelerates type 1 diabetes in NOD mice[END_REF], and also with vasculopathy [START_REF] Sommerville | Increased atherosclerosis and vascular smooth muscle cell activation in AIF-1 transgenic mice fed a high-fat diet[END_REF] and CNS injury [START_REF] Deininger | The allograft inflammatory factor-1 in Creutzfeldt-Jakob disease brains[END_REF]. In a recent review, Sikora et al. [START_REF] Sikora | Role of allograft inflammatory factor-1 in pathogenesis of diseases[END_REF] summarized the role of AIF-1 in the pathogenesis of some diseases including endometriosis, breast cancer, atherosclerosis, rheumatoid arthritis, and fibrosis. Indeed, several authors evidenced its importance in rheumatoid arthritis progression [START_REF] Kimura | A critical role for allograft inflammatory factor-1 in the pathogenesis of rheumatoid arthritis[END_REF][START_REF] Harney | Fine mapping of the MHC Class III region demonstrates association of AIF1 and rheumatoid arthritis[END_REF][START_REF] Pawlik | Association of allograft inflammatory factor-1 gene polymorphism with rheumatoid arthritis[END_REF]. AIF-1 is also considered as a new risk factor for the development of atherosclerosis [START_REF] Berglund | NFAT regulates the expression of AIF-1 and IRT-1: Yin and yang splice variants of neointima formation and atherosclerosis[END_REF], and, when overexpressed, it influences the intensification of atherosclerotic plaque calcification [START_REF] Albiero | Myeloid calcifying cells promote atherosclerotic calcification via paracrine activity and allograft inflammatory factor-1 overexpression[END_REF]. Recent studies report a tight link between AIF-1 and cancer. It is involved in breast cancer development by interacting with several proteins such as metalloproteinases [START_REF] Yu | Effects of AIF-1 inflammatory factors on the regulation of proliferation of breast cancer cells[END_REF] or transcription factors [START_REF] Liu | Daintain/AIF-1 promotes breast cancer proliferation via activation of the NF-kB/cyclin D1 pathway and facilitates tumor growth[END_REF] and activating the downstream pathways, inducing cell proliferation. Interestingly, AIF-1 is also expressed in the CNS of vertebrates. Mostly known under the name of Iba1, this factor is largely recognized as a specific microglial marker allowing the distinction of these brain-resident immune cells from neurons and other brain glial cells [START_REF] Prinz | Microglia and brain macrophages in the molecular age: From origin to neuropsychiatric disease[END_REF]. AIF-1 is modulated upon different brain injuries and pathologies, indicating a link with CNS inflammatory states [START_REF] Zhao | Role of AIF-1 in the regulation of inflammatory activation and diverse disease processes[END_REF]. AIF-1 expression is generally linked to the presence of activated brain microglia/macrophages. Beschorner et al. [START_REF] Beschorner | Differential regulation of the monocytic calcium-binding peptides macrophage-inhibiting factor related protein-8 (MRP8/S100A8) and allograft inflammatory factor-1 (AIF-1) following human traumatic brain injury[END_REF] reported the expression of AIF-1 in a limited subpopulation of microglial cells and did not observe a significant upregulation of this gene upon traumatic brain injury. In contrast, Schwab et al. [START_REF] Schwab | AIF-1 expression defines a proliferating and alert microglialrmacrophage phenotype following spinal cord injury in rats[END_REF] observed an important accumulation of AIF-1 + cells in microglia/macrophages in association with experimental spinal cord injury. Indeed, the authors indicate this accumulation as essential for the initiation of an effective response to CNS injury and repair events. Interestingly, AIF-1 expression has been reported in human microglia following cerebral infarction [START_REF] Postler | Allograft-Inflammatory-factor-1 is upregulated in microglial cells in human cerebral infarctions[END_REF]. This suggests that AIF-1 can also be upregulated by non-inflammatory brain lesions such as hypoxia.

Invertebrata

Despite the limited amount of published data, it appears that in vertebrates, like in vertebrates, AIF-1 is mainly involved in the inflammatory response. To date, AIF-1 genes have been characterized functionally in phylogenetically distant group of species, such as sponges, cnidarians, mollusks, annelids, and echinoderms. Many invertebrate species are of commercial interest, and more and more research groups are directing their investigations to these animals and their defense mechanisms. Aquaculture of numerous species suffers from massive infections and severe mortality. Therefore, it is necessary to understand and solve disease-related problems. Furthermore, due to the relative simplicity of their immune system, invertebrates also represent good models for studying innate immunity and providing insight into the evolution of their defense mechanisms.

Invertebrates are characterized by a lack of acquired immunity; thus, in such organisms the innate immune systems can provide the host with an immediate defense against pathogens in a non-specific manner. During this event, the immune system is able to differentiate self from non-self. The recognition process is carried out by circulating cells named amebocytes, hemocytes, or coelomocytes according to the various animal groups. In many invertebrate species, these cells have a macrophage-like appearance and function. They are characterized by the presence on their surface of pathogen recognition receptors (PRRs) recognizing pathogen-associated molecular patterns (PAMPs). These are well-conserved molecular structures expressed by various pathogens whether they are viruses, bacteria, or other foreign particles. The receptor-ligand binding triggers a complex cascade of cellular reactions with the production of a wide array of effector molecules.

AIF-1 is a ubiquitously expressed and well-conserved molecule involved in innate immunity response from sponges [START_REF] Kruse | Differential expression of allograft inflammatory factor 1 and of glutathione peroxidase during auto-and allograft response in marine sponges[END_REF][START_REF] Müller | Histocompatibility reaction in tissue and cells of the marine sponge Suberites domuncula in vitro and in vivo: Central role of the allograft inflammatory factor 1[END_REF] to humans [START_REF] Deininger | The allograft inflammatory factor-1 family of proteins[END_REF]. Notably, when compared all together, AIF-1-related proteins of vertebrates and invertebrates easily branch according to the major metazoan groups. In addition, AIF-1-related proteins are also detectable in choanoflagellates, i.e., the closest living relatives of animals (Figure 1A). As summarized in Figure 1B, percent identity values remain invariably high (never lower than 40%) among metazoan groups. For details on the actual percent identity values, see also Supplemental Figure S1.

In spite of the evolutionary distance of the selected species, multiple alignment of the AIF-1 sequences from vertebrates, invertebrates, and choanoflagellates reveals highly conserved motifs and structural features (Figure 2). In particular, AIF-1 protein sequence lengths span from 142 to 158 amino acids, with major sequence differences mainly discernable at the amino-and carboxy-terminal ends. Multiple sequence alignment shows the presence of 19 fully conserved amino acid residues (asterisks) and 32 positions exhibiting high conservation, thanks to the presence of amino acids with strongly similar properties (colons). The core of the proteins invariably contains two adjacent EF-hand (EFh) calcium-binding motifs, the second being less conserved than the first. Canonically, this type of domain consists of a 12-residue loop flanked on both sides by a 12-residue α-helical domain. Ca 2+ binding induces a conformational change in the EFh motif, leading to the activation or inactivation of target proteins.

Porifera

Porifera are the phylogenetically oldest still existent metazoans in which AIF-1 has been identified [START_REF] Kruse | Differential expression of allograft inflammatory factor 1 and of glutathione peroxidase during auto-and allograft response in marine sponges[END_REF][START_REF] Müller | Histocompatibility reaction in tissue and cells of the marine sponge Suberites domuncula in vitro and in vivo: Central role of the allograft inflammatory factor 1[END_REF]. In these organisms, the protein presents a high sequence similarity with vertebrates. Kruse et al. [START_REF] Kruse | Differential expression of allograft inflammatory factor 1 and of glutathione peroxidase during auto-and allograft response in marine sponges[END_REF] observed that in the sponges Suberites domuncula and Geodia cydonium the expression of AIF-1 mRNA was induced in cytokine-mediated allogeneic responses during wound repair. Interestingly, its expression does not occur in autografts, suggesting a possible function in immunocytes involved in alloimmune rejection [START_REF] Kruse | Differential expression of allograft inflammatory factor 1 and of glutathione peroxidase during auto-and allograft response in marine sponges[END_REF]. Cloning of S. domuncula AIF-1 allows demonstration that the distribution of the six exon/intron borders is, with one exception, strictly conserved between sponge, human, and mouse genes. This also suggests a close evolutionary distance between these species [START_REF] Müller | Histocompatibility reaction in tissue and cells of the marine sponge Suberites domuncula in vitro and in vivo: Central role of the allograft inflammatory factor 1[END_REF]. Moreover, in the same sponge it has been documented, both at tissue and in vitro level, that the expression of AIF-1 and certain Tcf-like transcription factor genes is closely correlated with histoincompatibility reactions [START_REF] Müller | Histocompatibility reaction in tissue and cells of the marine sponge Suberites domuncula in vitro and in vivo: Central role of the allograft inflammatory factor 1[END_REF]. Indeed, AIF-1 expression is upregulated in transplants, especially in grafts deriving from different donors. According to Kruse et al. [START_REF] Kruse | Differential expression of allograft inflammatory factor 1 and of glutathione peroxidase during auto-and allograft response in marine sponges[END_REF], these data suggest that in sponges, in addition to an adaptive immunity, an effector system involving a cytokine-mediated activation of immunocytes occurs. Although we do not know which cells produce AIF-1 in sponges, it is possible to hypothesize that the 'gray cells', which are equivalent to the immune leukocytes within the vertebrates, are the main AIF-1 producers [START_REF] Müller | Histocompatibility reaction in tissue and cells of the marine sponge Suberites domuncula in vitro and in vivo: Central role of the allograft inflammatory factor 1[END_REF].

Figure 1. (A)

Neighbor-joining (NJ) optimal tree based on selected metazoan allograft inflammatory factor-1 (AIF-1)-related proteins. The tree was generated using MEGA X including AIF-1 from different species ranging from mammals to porifera. The amino acid sequences of two choanoflagellates are also included in the tree. All the sequences used were obtained from GenBank at NCBI. The evolutionary history was inferred using the NJ method. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site.

Cnidaria

Apart from a gene sequence from Nematostella vectensis present in GenBank (Acc. No. XP_001635454), data on AIF-1 in Cnidaria only derive from the sea anemone Anemonia viridis, where an AIF-1 homologue was first identified and characterized [START_REF] Cuttitta | Evolutionary conserved mechanisms pervade structure and transcriptional modulation of allograft inflammatory factor-1 from sea anemone Anemonia viridis[END_REF]. The predicted protein shows the common elements of AIF-1 family members, possessing the evolutionarily conserved EF-hand Ca 2+ -binding motifs, the typical post-transcriptional modification sites, and a 3D structure that can be superimposed with human members of this family [START_REF] Cuttitta | Evolutionary conserved mechanisms pervade structure and transcriptional modulation of allograft inflammatory factor-1 from sea anemone Anemonia viridis[END_REF]. Probably, in A. viridis AIF-1 serves as a general protective factor under normal physiological conditions. However, after challenges with different stresses (i.e., biotic or physical challenge) a transcriptional activation can be observed, confirming the involvement of AIF-1 in the inflammatory response [START_REF] Cuttitta | Evolutionary conserved mechanisms pervade structure and transcriptional modulation of allograft inflammatory factor-1 from sea anemone Anemonia viridis[END_REF]. In this anthozoan species, AIF-1 transcripts are detected in different tissues including the tentacles, oral disk, body wall, pharynx, and basal disk. Considering the basic diblastic organization of Cnidaria, these results evidence that in A. viridis the expression pattern of AIF-1 could be distributed in the cells of both the ectoderm and endoderm layers [START_REF] Cuttitta | Evolutionary conserved mechanisms pervade structure and transcriptional modulation of allograft inflammatory factor-1 from sea anemone Anemonia viridis[END_REF]. However, it is not clear at present whether the presence of this protein is limited to immunocytes, which possibly infiltrate the tissues.

Mollusca

Among invertebrates, due to their commercial interest, mollusks represent the most investigated species. Furthermore, frequently suffering from environmental bacterial infection, investigations in mollusks are prevalently targeted at the identification of genes modulated upon bacterial challenge. The first report of AIF-1 in mollusks was in Haliotis diversicolor hemocytes, where AIF-1 was one of the 34 genes involved in different cellular pathways upregulated after bacterial challenge [START_REF] Wang | Identification of the up-regulated expression genes in hemocytes of variously colored abalone (Haliotis diversicolor Reeve, 1846) challenged with bacteria[END_REF]. Moreover, in this gastropod a high expression level of AIF-1 mRNA was found in gills, suggesting that it could also have a significant contribution in the prevention of microbial infection [START_REF] Zhang | Cloning and characterization of allograft inflammatory factor-1 (AIF-1) from manila clam Venerupis philippinarum[END_REF].

AIF-1 cDNA has also been cloned from Bivalves including oysters (Pinctada martensii, Crassostrea gigas, and Crassostrea ariakensis) [START_REF] Li | Expression of allograft inflammatory factor-1 (AIF-1) in response to bacterial challenge and tissue injury in the pearl oyster, Pinctada martensii[END_REF][START_REF] Zhang | Allograft inflammatory factor-1 stimulates hemocyte immune activation by enhancing phagocytosis and expression of inflammatory cytokines in Crassostrea gigas[END_REF][START_REF] Xu | Allograft inflammatory factor 1 functions as a pro-inflammatory cytokine in the oyster, Crassostrea ariakensis[END_REF], triangle sail mussel (Hyriopsis cumingii) [START_REF] Li | Characterization of allograft inflammatory factor-1 in Hyriopsis cumingii and its expression in response to immune challenge and pearl sac formation[END_REF], scallops (Chlamys farreri) [START_REF] Wang | Molecular cloning and transcriptional regulation of an allograft inflammatory factor-1 (AIF-1) in Zhikong scallop Chlamys farreri[END_REF], and clams (Venerupis philippinarum) [START_REF] Zhang | Cloning and characterization of allograft inflammatory factor-1 (AIF-1) from manila clam Venerupis philippinarum[END_REF], and also from gastropods such as abalones (Haliotis discus discus) [START_REF] De Zoysa | Allograft inflammatory factor-1 in disk abalone (Haliotis discus discus): Molecular cloning, transcriptional regulation against immune challenge and tissue injury[END_REF]. Upregulation of AIF-1 has been observed in some species after tissue injury such as tissue implant in H. cumingii [START_REF] Li | Characterization of allograft inflammatory factor-1 in Hyriopsis cumingii and its expression in response to immune challenge and pearl sac formation[END_REF] or shell damage and mantle injury in P. martensii [START_REF] Li | Expression of allograft inflammatory factor-1 (AIF-1) in response to bacterial challenge and tissue injury in the pearl oyster, Pinctada martensii[END_REF]. In all these species, AIF-1 molecules were active in the host immune responses against pathogenic challenges. Indeed, in V. philippinarum, P. martensii, and H. discus discus this gene was upregulated in hemocytes after infection by both Gram-positive and Gram-negative bacteria [START_REF] Zhang | Cloning and characterization of allograft inflammatory factor-1 (AIF-1) from manila clam Venerupis philippinarum[END_REF][START_REF] Li | Expression of allograft inflammatory factor-1 (AIF-1) in response to bacterial challenge and tissue injury in the pearl oyster, Pinctada martensii[END_REF][START_REF] De Zoysa | Allograft inflammatory factor-1 in disk abalone (Haliotis discus discus): Molecular cloning, transcriptional regulation against immune challenge and tissue injury[END_REF], indicating its possible role in clearing pathogens soon after the infection. In support of this, there are also data showing the increased expression of AIF-1 after LPS stimulation in H. cumingii [START_REF] Li | Characterization of allograft inflammatory factor-1 in Hyriopsis cumingii and its expression in response to immune challenge and pearl sac formation[END_REF] and C. ariakensis [START_REF] Xu | Allograft inflammatory factor 1 functions as a pro-inflammatory cytokine in the oyster, Crassostrea ariakensis[END_REF]. In the latter mollusk, using a recombinant AIF-1 protein, Xu et al. [START_REF] Xu | Allograft inflammatory factor 1 functions as a pro-inflammatory cytokine in the oyster, Crassostrea ariakensis[END_REF] highlighted that AIF-1 acts in the regulation of some immune-related genes such as LITAF, MyD88, and TGFβ. Interestingly, LITAF is an important transcription factor and is believed to regulate the expression of inflammatory-related factors IL-1α, TNFα, and IFN-γ in mammals [START_REF] Park | Signaling pathways and genes that inhibit pathogen-induced macrophage apoptosis-CREB and NF-kB as key regulators[END_REF][START_REF] Tang | LPS induces the interaction of a transcription factor, LPS-induced TNF-alpha factor, and STAT6(B) with effects on multiple cytokines[END_REF].

Studies on the role of AIF-1 in H. cumingi evidenced a significant increase in the phagocytosis rate [START_REF] Wang | Molecular cloning and transcriptional regulation of an allograft inflammatory factor-1 (AIF-1) in Zhikong scallop Chlamys farreri[END_REF]. This is not surprising since mollusk hemocytes, the main component of cellular immune responses in invertebrates, can be considered functionally analogous to vertebrate leukocytes, acting as macrophages and playing a crucial role in the recognition and removal of foreign materials [START_REF] Kuchel | Haemocyte morphology and function in the Akoya pearl oyster, Pinctada imbricata[END_REF]. Recently, the functional role in hemocytes has been better pointed out by confocal imaging, which revealed that AIF-1 regulates phagocytosis via a functional interaction with filamentous actin [START_REF] Beltran | iTRAQ-based quantitative proteomic profiling of the immune response of the South African abalone, Haliotis midae[END_REF]. In hemocytes, AIF-1 appeared diffused in the cytoplasm and colocalized with F-actin bundles. After a bacterial challenge, a disruption of the AIF-1 and F-actin association and an increase in cell extension occurred. In all the investigated mollusks, the AIF-1 gene resulted constitutively expressed in various tissues such as mantle, gill, hepatopancreas, muscle, and foot, with the highest level always being recorded in hemocytes [START_REF] Zhang | Cloning and characterization of allograft inflammatory factor-1 (AIF-1) from manila clam Venerupis philippinarum[END_REF][START_REF] De Zoysa | Allograft inflammatory factor-1 in disk abalone (Haliotis discus discus): Molecular cloning, transcriptional regulation against immune challenge and tissue injury[END_REF]. Some authors report the constitutive expression of AIF-1 transcripts in a variety of unstimulated tissues. This is probably due to its involvement in various processes other than inflammation, pathogenic challenges, or tissue injury, which still need to be explored. However, although AIF-1 has been found in several organs, we cannot currently say whether it is expressed in cells other than hemocytes generally present in various tissues.

Gust et al. [START_REF] Gust | Effects of short-term exposure to environmentally relevant concentrations of different pharmaceutical mixtures on the immune response of the pond snail Lymnaea stagnalis[END_REF] used the AIF-1 gene as a marker to evaluate the immune effects of environmentally relevant concentrations of pharmaceutical mixtures on the pond snail Lymnaea stagnalis. Results indicate that this factor, together with other immune and inflammatory markers, is modulated in the presence of drug mixtures and municipal effluent water in this gastropod. In particular, the AIF-1 gene in L. stagnalis is downregulated in response to antibiotic and psychiatric drug mixtures. This study demonstrates the interest of AIF-1 as a potential biomarker for environmental studies on water chemical pollution.

Annelida

AIF-1 was initially characterized in annelids by Drago et al. [START_REF] Drago | Microglia of medicinal leech (Hirudo medicinalis) express a specific activation marker homologous to vertebrate ionized calcium-binding adapter molecule 1 (Iba1/alias Aif-1)[END_REF], who reported its presence, under the name of Iba1, in the CNS of the medicinal leech Hirudo medicinalis. Iba1 is a largely recognized microglial marker in vertebrates, though it has not been detected in parenchymal brain microglia of zebrafish and birds [START_REF] Geirsdottir | Cross-Species single-cell analysis reveals divergence of the primate microglia program[END_REF]. According to the authors, this work constitutes the first report of such a factor in the CNS of an invertebrate species [START_REF] Drago | Microglia of medicinal leech (Hirudo medicinalis) express a specific activation marker homologous to vertebrate ionized calcium-binding adapter molecule 1 (Iba1/alias Aif-1)[END_REF]. Like its vertebrate counterpart, the leech gene Iba1/AIF-1 is upregulated in nervous cells upon experimental injury or ATP stimulation. The predicted Iba1 protein shows an average identity of about 50% and 55% with AIF-1 proteins described in vertebrates and invertebrates, respectively. Immunohistochemistry analyses demonstrated its presence in activated microglia accumulated at the injury site of connective fibers and in those surrounding neuron cell bodies. Results from Schorn et al. [START_REF] Schorn | Homolog of allograft inflammatory factor-1 induces macrophage migration during innate immunity response in leech[END_REF] established the constitutive presence of the Iba1/AIF-1 protein in CD68+ and CD45+ macrophage-like cells spread in leech body wall tissues. The number of AIF-1 immunopositive cells strongly increases upon bacterial challenge. In addition, the injection of recombinant AIF-1 in leeches induces massive angiogenesis and, similarly to AIF-1 in vertebrates, promotes macrophage-like cell recruitment at the injured site. Recent works from the same team [START_REF] Baranzini | AIF-1 and RNASET2 Play Complementary Roles in the Innate Immune Response of Medicinal Leech[END_REF][START_REF] Baranzini | Hirudo verbana as an alternative model to dissect the relationship between innate immunity and regeneration[END_REF] demonstrated that RNASET2, a protein belonging to the T2 ribonuclease family, and AIF-1 are released from the same immunocompetent cells in the leech H. verbana8. Both factors would contribute to the recruitment of immune cells (granulocytes and macrophages) upon LPS injection or wound healing, resulting in the activation of an effective response against pathogen infection. AIF-1 has also been identified in Hirudo telocytes [START_REF] Pulze | A new cellular type in invertebrates: First evidence of telocytes in leech Hirudo medicinalis[END_REF], special resident cells involved in the immune-surveillance system of the leech body wall. Like in vertebrates, these cells also play a role in regulating immune and neuroendocrine functions in leeches. All together, these data indicate the involvement of AIF-1 in immune cell activation and migration as well as in regulating the inflammatory response in leeches.

Echinodermata

Echinoderms represent the most developed invertebrates and the bridge leading to the primitive chordates, cephalochordates, and urochordates, in which many autologous genes and functions from their ancestors can be found.

The evolutionary position of echinoderms among the Deuterostomia, the same evolutionary branch where vertebrates are found, represents one of the reasons that makes these animals attractive as upcoming model systems. Echinoderms, besides being an important source of food and medicine for humans, provide important clues to understand immune functions that are common with vertebrates. Firstly, Elie Metchnikoff, by introducing the comparative approach to immunology [START_REF] Cammarata | Elie Metchnikoff and the multidisciplinary link novelty among Zoology, Embryology and Innate Immunity[END_REF], postulated the inflammation concept applicable to mammals and other animals with closed circulatory systems. Metchnikoff's study in this field started from the observation of phagocyte recruitment around foreign material in the larvae of an echinoderm (sea star) [START_REF] Metchnikoff | Phagocytosis and Immunity[END_REF].

Only many years later were the immune and non-self recognition capabilities established in these organisms [START_REF] Hildemann | Transplantation reactions of tropical Australian echinoderms[END_REF][START_REF] Karp | Specific allograft reactivity in the sea star Dermasterias imbricata[END_REF][START_REF] Coffaro | Immune response in the sea urchin Lytechinus pictus[END_REF][START_REF] Coffaro | Memory and specificity in the sea urchin Lytechinus pictus[END_REF] when molecular approaches were developed to evidence novel immune effectors.

The first report on the presence of AIF-1 in echinoderms [START_REF] Nair | Microarray analysis of coelomocyte gene expression in response to LPS in the sea urchin. Identification of unexpected immune diversity in an invertebrate[END_REF] was based on the EST analysis of genes upregulated in coelomocytes in response to LPS challenge. Indeed, Nair et al. [START_REF] Nair | Microarray analysis of coelomocyte gene expression in response to LPS in the sea urchin. Identification of unexpected immune diversity in an invertebrate[END_REF] identified one gene, named Sp1086, matching allograft inflammatory factor-1 in the purple sea urchin, Strongylocentrotus purpuratus.

Within the Echinodermata phylum, the sea urchin represents an excellent model organism for studies on inflammation, including those on the expression and regulation of AIF-1. Indeed, the Antarctic sea urchin Sterechinus neumayeri [START_REF] Ovando | Cloning and expression analysis of allograft inflammatory factor type 1 in coelomocytes of antarctic sea urchin (Sterechinus neumayeri)[END_REF] and the common sea urchin Paracentrotus lividus [START_REF] Barca | Molecular and expression analysis of the Allograft inflammatory factor 1 (AIF-1) in the coelomocytes of the common sea urchin Paracentrotus lividus[END_REF][START_REF] Chiaramonte | Allograft Inflammatory Factor AIF-1: Early immune response in the Mediterranean sea urchin Paracentrotus lividus[END_REF] have furnished intriguing information about the association of AIF-1 with the immune response. AIF-1 has been identified in coelomocytes of S. neumayeri, where an increase in expression during the first phase of the immune response to a bacterial challenge was evidenced. Interestingly, with S. neumayeri being a sea urchin living in circumpolar waters, the protein primary structure presents some molecular adaptations to cold [START_REF] Ovando | Cloning and expression analysis of allograft inflammatory factor type 1 in coelomocytes of antarctic sea urchin (Sterechinus neumayeri)[END_REF], and this report indicates that AIF-1 can participate in the inflammatory in extremely cold environments.

Further data have been derived from investigation in the common sea urchin P. lividus, where the molecular identification and functional characterization of AIF-1 have been recently reported [START_REF] Ji | Cloning and gene expression of allograft inflammatory factor-1 (AIF-1) provide new insights into injury and bacteria response of the sea cucumber Apostichopus japonicus (Selenka, 1867)[END_REF]. In this work, the authors found a significant increase in AIF-1 expression, at both the mRNA and protein level, in coelomocytes after Gram+ bacterial challenge. In addition, immunocytochemical analysis conducted on different coelomocyte populations revealed the presence of the AIF-1 protein in the perinuclear cytoplasmic zone of amoebocytes and inside red sphaerula cell granules. With these cells being involved in the inflammatory reaction, it is possible to support that AIF-1 plays a crucial role in the defense processes within echinoderms. More recently, information on the AIF-1 gene in P. lividus has been enriched by the work of Chiaramonte et al. [START_REF] Chiaramonte | Allograft Inflammatory Factor AIF-1: Early immune response in the Mediterranean sea urchin Paracentrotus lividus[END_REF], who reported its modulation following LPS challenge and the bioinformatically characterized protein structure.

In the sea cucumber Apostichopus japonicus, the full-length cDNA of AIF-1 has been cloned [START_REF] Ji | Cloning and gene expression of allograft inflammatory factor-1 (AIF-1) provide new insights into injury and bacteria response of the sea cucumber Apostichopus japonicus (Selenka, 1867)[END_REF]. Like the sea urchin, in this organism a significant increase in the expression levels of AIF-1 transcripts has also been detected in coelomocytes after bacterial challenge and papilla injury. Based on these results, the authors supported the idea that AIF-1 is involved in acute inflammatory response. Furthermore, data reported the constitutive expression of AIF-1 in all the tested tissues, including body wall, intestine, respiratory tree, tube feet, and longitudinal muscle.

Conclusions

Genes belonging to the AIF-1 family have been observed in many metazoan phyla. The sequences of the predicted proteins display a high level of conservation, particularly concerning the structure of the two EF-hand Ca 2+ -binding regions typical of this factor. Interestingly, although AIF-1 genes have been described in phylogenetically distant species, they have not been identified in yeast or plants and are also absent in some Protostomia, such as in Drosophila melanogaster or Caenorabditis elegans, the most studied animal models within the arthropod and nematode groups, respectively.

From a phylogenetic/evolutionary perspective, we evidence the steadily increasing numbers of allograft inflammatory factor-like (nucleotide and amino acid) sequences from invertebrates now available in the various databases/databanks online. A systematic functional genomics approach is now probably required to analyze, rationalize, and possibly re-classify the gene/gene product(s) assortment detectable in the genomes of the various metazoan species. This will also help in re-considering, re-assessing, and better defining the roles and functions of the various AIF-1 genes and proteins. In addition, better knowledge of the role of AIF-1/Iba1 in inflammatory pathways might lead to the identification of new therapeutic targets for some neuroinflammatory diseases [START_REF] Chinnasamy | Loss of Allograft Inflammatory Factor-1 ameliorates experimental autoimmune encephalomyelitis by limiting encephalitogenic CD4 T-Cell expansion[END_REF].

AIF-1 activity is clearly linked to inflammation and immune response events in all the animal phyla in which this protein has been characterized. The spectrum of action and the involvement of this factor in the immune response of metazoans need to be further investigated. In invertebrates, in particular, AIF-1 is generally upregulated in hemocytes after bacterial challenge, confirming its involvement in inflammatory/immune processes. Indeed, its presence in a large variety of invertebrates' organs and tissue could be explained by the infiltration of circulating hemocytes. Another common feature of most cells expressing AIF-1 is their ability to migrate upon inflammatory signals [START_REF] Chinnasamy | Loss of Allograft Inflammatory Factor-1 ameliorates experimental autoimmune encephalomyelitis by limiting encephalitogenic CD4 T-Cell expansion[END_REF]. This suggests that the AIF-1 protein might play a crucial role in cell mobility events, possibly regulating the calcium metabolism of activated cells. Although the AIF-1 protein is generally present in cells belonging to the monocyte/macrophage lineage, it can also be associated with other very peculiar cell types, such as the elongated spermatids present in the luminal aspect of mouse testes [START_REF] Köhler | Allograft inflammatory factor-1/Ionized calcium-binding adapter molecule 1 is specifically expressed by most subpopulations of macrophages and spermatids in testis[END_REF]. AIF-1 presence is correlated with spermatid development and differentiation stages, suggesting that this protein could be involved in the reorganization of the actin cytoskeleton during spermatogenesis and cytoplasmic residue elimination, occurring in the final stage of spermiogenesis.

Invertebrates constitute a largely unexplored source of experimental models. Future studies on AIF-1 in these organisms will bring new knowledge on its biological functions, which are probably not relegated to inflammatory regulation. It could also open up unexpected insights for the comprehension of AIF-1 functions in mammal immune response and the control of human inflammatory diseases.

  Figure 1. (A)Neighbor-joining (NJ) optimal tree based on selected metazoan allograft inflammatory factor-1 (AIF-1)-related proteins. The tree was generated using MEGA X including AIF-1 from different species ranging from mammals to porifera. The amino acid sequences of two choanoflagellates are also included in the tree. All the sequences used were obtained from GenBank at NCBI. The evolutionary history was inferred using the NJ method. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. Bootstrap test: 1000 replicates. All ambiguous positions were removed for each sequence pair. (B) Condensed tree of (A) showing relationships among taxa. Percent identity values (min-max values amongst those observed in each metazoan group) are indicated in parentheses. † indicates Homo sapiens AIF-1 reference identity value (100%).

Figure 1 .

 1 Figure 1. (A)Neighbor-joining (NJ) optimal tree based on selected metazoan allograft inflammatory factor-1 (AIF-1)-related proteins. The tree was generated using MEGA X including AIF-1 from different species ranging from mammals to porifera. The amino acid sequences of two choanoflagellates are also included in the tree. All the sequences used were obtained from GenBank at NCBI. The evolutionary history was inferred using the NJ method. The evolutionary distances were computed using the Poisson correction method and are in the units of the number of amino acid substitutions per site. Bootstrap test: 1000 replicates. All ambiguous positions were removed for each sequence pair. (B) Condensed tree of (A) showing relationships among taxa. Percent identity values (min-max values amongst those observed in each metazoan group) are indicated in parentheses. † indicates Homo sapiens AIF-1 reference identity value (100%).

Figure 2 .

 2 Figure 2. Multiple alignment among the sequences analyzed in Figure 1 as obtained by Clustal Omega (default parameters). EF-hand, calcium-binding motifs (Efh) are highlighted in grey. Asterisks (*) correspond to single, fully conserved residues. Colons (:) indicate conservation of residues sharing strongly similar properties (equivalent to scoring > 0.5 in the Gonnet PAM 250 matrix). Periods (.) indicate conservation of residues with weakly similar properties (equivalent to scoring ≤ 0.5 and > 0 in the Gonnet PAM 250 matrix).

Figure 2 .

 2 Figure 2. Multiple alignment among the sequences analyzed in Figure 1 as obtained by Clustal Omega (default parameters). EF-hand, calcium-binding motifs (Efh) are highlighted in grey. Asterisks (*) correspond to single, fully conserved residues. Colons (:) indicate conservation of residues sharing strongly similar properties (equivalent to scoring > 0.5 in the Gonnet PAM 250 matrix). Periods (.) indicate conservation of residues with weakly similar properties (equivalent to scoring ≤ 0.5 and > 0 in the Gonnet PAM 250 matrix).

Funding: This research received no external funding.

Conflicts of Interest:

The authors declare no conflict of interest.

Supplementary Materials:

The following are available online at http://www.mdpi.com/2079-7737/9/11/355/s1, Figure S1: Percent Identity Matrix (upper panel) created by Clustal Omega alignment (lower panel). Amino acid sequence alignment of metazoan AIF-1. Multiple sequence alignment was generated using Clustal Omega at https://www.ebi.ac.uk/Tools/msa/clustalo/ using default parameters.

Author Contributions: J.V., T.V., and P.P., writing, review and editing; P.P., supervision. All authors have read and agreed to the published version of the manuscript.


Anthony N Tantillo

Alexander Barcza

Volker Zellmann

Vittorio Basso

Martino Lobue

Nora M Dempsey

Karl G Sandeman
email: karlsandeman@brooklyn.cuny.edu

Hard Ferromagnets as a new Perspective on Materials for Thermomagnetic Power Generation Cycles

Keywords: thermomagnetic, power conversion, magnetocaloric, spin reorientation

We consider the ways in which magnetically hard materials can be used as the working materials in thermomagnetic power generation (TMG) cycles in order to expand the area in the magnetisation vs. applied field (M -H) plane available for energy conversion. There are 3 parts to this Perspective. First, experiments on commercially available hard ferrites reveal that, while these materials are not yet good TMG candidates, hard ferromagnets with higher thermal conductivity and a greater change of magnetization with temperature could outperform existing TMG materials. Second, computational results indicate that biasing a soft magnet with a hard ferromagnet is essentially equivalent to shifting the M -H loop by an amount proportional to the field of the biasing magnet. Work outputs under biased conditions show a substantial improvement over unbiased cycles, but experimental verification is needed. Third, we discuss the rationale for exploring artificial spin reorientation materials as novel TMG working materials.

Introduction

Energy harvesting and pyromagnetic heat engines

The industrial ability to generate power from heat is well established, but it suffers from a lack of widespread adoption, despite the magnitude of available waste heat sources [START_REF] Chan | A review of chemical 605 heat pumps, thermodynamic cycles and thermal energy storage technologies for low grade heat utilisation[END_REF][START_REF] Xu | Perspectives for low-610 temperature waste heat recovery[END_REF]. In 2017, industrial energy use accounted for 37% of the 9.7 gigatonne of oil equivalent (GToe) worldwide total final consumption [START_REF]Key World Energy Statistics 2019[END_REF]. Much of the energy consumed by industry is in thermal processes such as cementmaking and steelmaking and about 20-50% of this is discarded as heat [START_REF] Pellegrino | Energy Use, Loss, and Opportunities Analysis for U.S. Manufacturing and Mining[END_REF].

Many commercial heat energy conversion devices employ an indirect approach, first converting heat to mechanical work, and then using this mechanical energy to drive an electric engine. The most commonplace method is the organic Rankine cycle (ORC), which has an energy generation cost of $1500-3500/kW [START_REF] Cunningham | Waste Heat/Cogen Opportunities in the Cement Industry[END_REF]. However, the relatively low efficiency of the ORC in small-power (kW-scale) settings means that alternative working fluids and cycles are being sought. This search is complicated by the fact that solid-state alternatives to fluid-based heat-to-mechanical indirect energy generation are few, relatively expensive, search into pyromagnetic effects (PMEs), and in doing so, open up a new source of magnetically hard, anisotropic power conversion materials. The pyromagnetic effect is the induction of a magnetization by the action of a change of temperature. It is strong around a magnetic phase tran-35 sition and is the corollary of the more often-studied magnetocaloric effect, the basis for magnetic field-driven cooling [START_REF] Sandeman | Magnetocaloric materials and applications[END_REF]. Both Edison, in 1888 [START_REF] Edison | Pyromagnetic motor[END_REF] and Tesla (in 1889 [START_REF] Tesla | Thermo magnetic motor[END_REF] and 1890 [START_REF] Tesla | Pyromagneto-electric generator[END_REF]) published the first patents on the use of iron near its Curie point as a working material for conversion 40 of heat into electricity but a lack of suitable pyromagnets to cover a range of applications has arguably resulted in only a sporadic research literature until now.

Interest in energy harvesting, combined with the discovery of new magnetocaloric materials that have been [START_REF] Pirc | Rotary point curie magnetic engine[END_REF] proposed as pyromagnets, has resulted in increased attention for the field of TMG [START_REF] Srivastava | The direct conversion of heat to electricity using multiferroic alloys[END_REF][START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF][START_REF] Christiaanse | Proof-of-Concept Static Thermomagnetic Generator Experimental Device[END_REF][START_REF] Almanza | Numerical study of thermomagnetic cycle[END_REF][START_REF] Chun | Thermo-Magneto-Electric Generator Arrays for Active Heat Recovery System[END_REF][START_REF] Gueltig | High-Performance Thermomagnetic Gener-665 ators Based on Heusler Alloy Films[END_REF][START_REF] Waske | Energy harvesting near room temperature using a thermomagnetic generator with a pretzel-like 670 magnetic flux topology[END_REF][START_REF] Dzekan | Can gadolinium compete with La-Fe-Co-Si in a thermomagnetic generator?[END_REF][START_REF] Dzekan | Efficient and affordable thermomagnetic materials for harvesting low grade waste heat[END_REF][START_REF] Ahmim | Self-oscillation and heat management in a LaFeSi based thermomagnetic generator[END_REF]. One method for using the pyromagnetic effect in TMG is to surround a heated ferromagnet with a coil. When the ferromagnet becomes paramagnetic above its Curie temperature, the changing flux in the external coil results in a current [START_REF] Tesla | Pyromagneto-electric generator[END_REF]. This is direct conversion to electricity of the energy associated with the magnetic field of the pyromagnet. By contrast, indirect methods such as a Curie wheel are also feasible [22]. In a Curie wheel, the active pyromagnetic material is arranged on the circumference of a wheel that is rotated in a variable magnetic field. During rotation, the pyromagnet is heated and cooled by external fluids. The hot fluid is essentially the energy source in this scenario. Cold parts of the pyromagnet ring are more attracted to the high field zones, resulting in a torque and thereby conversion of heat into mechanical energy [22]. A review of the history of thermomagnetic devices can be found in Refs. [START_REF] Kishore | A review on design and performance of thermomagnetic devices[END_REF][START_REF] Kitanovski | Energy Applications of Magnetocaloric Materials[END_REF]. Recent commercial activity in this area includes a Curie-wheel motor which used gadolinium with a minimum temperature span of 20 °C to generate power in the kilowatt range with zero emissions [START_REF] Vida | Thermo Magnetic Motor TMM[END_REF].

As discussed by Hsu et al. [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF], the measure of efficiency, η, of a pyromagnetic heat engine is the ratio of work output (W ) to heat input (Q h ) and has a Carnot limit, η Carnot , defined by the temperature of the heat source, T hot and that of the heat sink, T cold :

η = W Q h (1)
η Carnot = 1 - T cold T hot (2)
η rel = η η Carnot (3)
Here, η rel is defined as the engine efficiency relative to that of the Carnot limit. Low grade waste heat (which the US Department of Energy defines as heat supplied at temperatures below 230°C [START_REF] Johnson | Waste Heat Recovery[END_REF]) is sufficiently abundant as to make it the prime target for technology development, despite its reduced Carnot efficiency according to Equation 2. Pyromagnetic heat engines are expected to function at up to 55% of Carnot efficiency at temperatures a few hundred degrees Kelvin above room temperature [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF][START_REF] Brillouin | Thermomagnetic generator[END_REF]. By contrast, the efficiency of typical TEGs has not exceeded 20% of the Carnot value [START_REF] Kishore | A Review on Low-Grade Thermal Energy 625 Harvesting: Materials, Methods and Devices[END_REF].

The role of pyromagnetic material properties

The efficiency of a pyromagnetic heat engine is ultimately the property of a given device. However, the maximum efficiency of any device can be derived from material properties, depending on the thermodynamic cycle employed. The starting point for determining the potential pyromagnetic output of a magnet is the examination of a thermomagnetic cycle of the "working material" such as the one shown schematically in Figure 1(a). The magnetization, M , of a conventional ferromagnetic pyromagnet will decrease as a function of temperature (∂M/∂T < 0). It is also possible for an unconventional pyromagnet, in which ∂M/∂T is instead positive, to be used and the thermodynamic cycle to be reversed. However, we will focus 100 on the more common conventional effects.

In Figure 1(a), the isothermal magnetisation of the pyromagnet is shown as a function of magnetic field at two temperatures, T hot and T cold , which are those of the heat source and heat sink, respectively. In the example cycle, 105 the material begins at point (1) in a low temperature, high magnetization state at a low applied field value. The magnetic field is increased isothermally up to point [START_REF] Xu | Perspectives for low-610 temperature waste heat recovery[END_REF]. Then, the material is heated at a constant magnetic field, dropping its magnetization to point [START_REF]Key World Energy Statistics 2019[END_REF]. From point (3) to 110 point (4), the field is decreased at constant temperature, bringing the material to a low magnetization, high temperature state. The last step in the cycle is to cool the material in a fixed field, to return to the starting position [START_REF] Chan | A review of chemical 605 heat pumps, thermodynamic cycles and thermal energy storage technologies for low grade heat utilisation[END_REF]. The work output, W from the pyromagnetic work-115 ing material is given by the area bounded by the magnetic isotherms at T hot and at T cold [START_REF] Solomon | Improving the performance of a thermomagnetic generator by cycling the magnetic field[END_REF]:

W = -µ 0 H dM = µ 0 M dH, ( 4
)
where H is the magnetic field applied. This area (shaded in the M vs H plots in Figures 1(a) and (b)) should be 120 maximized.

While it is not a focus of our Perspective, we complete this subsection with a statement on the cycle efficiency. From Equation 1, we need to calculate the heat input to the engine, Q h . From conservation of energy:

125 Q h = Q c + W , (5)
where Q c is the heat rejected by the engine to the cold sink. As explained by Solomon, the Q c term comprises two contributions, when heat flows into the cold sink.

In our cycle, these are in steps (4)→( 1) and ( 1)→(2) .

130

During step (1)→( 2), an amount of heat T cold ∆S flows into the cold sink, resulting from changing the magnetic field applied to the working material at fixed temperature T cold . During step (4)→(1), the working material is cooled from T hot to T cold in a finite field. Therefore, the 135 amount of heat passed to the cold sink during that step is

T hot T cold C p (T, H 1 )dT where C p (T, H 1 )
is the heat capacity of the active pyromagnetic material at constant pressure and magnetic field H 1 . In this nomenclature, we may write the efficiency of a pyromagnetic engine as: Ni-Mn-Ga Perm. Linear Self-actuating 5 µm Ni-Mn-Ga film on CuZn oscillating towards/away from a heated magnet; 84 Hz resonant operation with average power density ∼118 mW cm -3 when the periodic temperature variation in the TMG is 3 K. [START_REF] Coray | Fully Operational Prototype of a 1 kW Thermo-Magnetic Motor for Generating Electricity from[END_REF][START_REF] Vida | Thermo Magnetic Motor TMM[END_REF] (A)

140 η = µ 0 M dH T hot T cold C p (T, H 1 )dT + T cold ∆S + µ 0 M dH . (6
Demonstrated that TMG efficiency is higher with La-Fe-Co-Si rather than Gd as the working material, due to the symmetrical M (T ) curve of the former. [START_REF] Ahmim | Self-oscillation and heat management in a LaFeSi based thermomagnetic generator[END_REF] (A) La(Fe,Si)13H Perm. Linear Improved self-oscillating device, using flexing of a piezoelectric buzzer, on which the active material is mounted, to generate electricity. Average power density ∼240 µW cm -3 and positive (4.2 µW) net power output when ∆Tres ∼35 K.

Table 1: Details of a selection of TMG prototype developments found in patents (P), in academic articles (A) or in reports (R). We focus on: the type of magnetic working material, magnetic field source used (or proposed) and the type of design. In the latter case, we use the TMG prototype categorization of Kitanovski [START_REF] Kitanovski | Energy Applications of Magnetocaloric Materials[END_REF]: passive rotary (P.R.), static induction or linear. Static induction devices have a further genus class (0,1,2 or 3) given, according to the terminology developed by Waske et al. [START_REF] Waske | Energy harvesting near room temperature using a thermomagnetic generator with a pretzel-like 670 magnetic flux topology[END_REF]. The last column lists any other features of significance, including ∆Tres, the temperature difference between the hot and cold reservoirs. We note that most working materials are magnetically soft; the 1977 TMG prototype of Ohkoshi et al. [START_REF] Ohkoshi | Rotational-type spin reorientation in Nd 1-x DyxCo 5 and its application to thermomagnetic generator[END_REF] based on spin reorientation was the first to use a magnetically hard (anisotropic) working material.

Hard Ferromagnets In Past TMG Studies

The role of hard ferromagnets in previous TMG research and patent literature has primarily been to provide the applied magnetic field required to convert heat into work. In Table 1 we list a selection of the key advances in past TMG prototype development from the late 18th century to the present day, highlighting in particular the choice of working material, magnetic field source, and heat source (employed or anticipated, in the case of a design proposal). Many of these are also discussed in Refs [START_REF] Kishore | A review on design and performance of thermomagnetic devices[END_REF] and/or [START_REF] Kitanovski | Energy Applications of Magnetocaloric Materials[END_REF]. As can be seen from this table, hard ferromagnets dominate the field source column, with magnetic anisotropy in the working material only being explored in the scientific literature in the work of Ohkoshi in 1976 and 1977 [START_REF] Ohkoshi | A Proposal of Application of Spin Reorientation Phenomenon to the Thermomagnetic Power Generation[END_REF][START_REF] Ohkoshi | Rotational-type spin reorientation in Nd 1-x DyxCo 5 and its application to thermomagnetic generator[END_REF] and 30 years later in the work of Carman and co-workers [START_REF] Wetzlar | Leveraging anisotropy to enhance multiferroic transduction[END_REF][START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF][START_REF] Wetzlar | A unifying metric for comparing thermomagnetic transduction utilizing magnetic entropy[END_REF]. While not the focus of our work, several other trends are of note: (1) the preferential use of permanent magnets in later years, particularly after high-strength permanent magnets made of materials such as Nd-Fe-B became available; (2) that recent work has divided into (sub-watt) energy harvesters and larger, multi-watt systems; and (3) the increased study of passive (self-actuating) small-scale devices in the last 10 years.

Single Domain Soft Ferromagnets, Hard Ferromagnets and Spin Reorientation

This Perspective's focus on anisotropic, hard ferromagnets is strongly motivated by work conducted at UCLA by Greg Carman and coworkers [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF][START_REF] Wetzlar | Leveraging anisotropy to enhance multiferroic transduction[END_REF][START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF]. In 2011, Hsu et al. concluded that single domain gadolinium could yield a thermodynamic cycle with higher efficiency than is possible with polycrystalline, multi-domain crystals [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF]. The reason for the hypothesis was that a magnetically soft material with no domain walls can retain a magnetization after the removal of an applied magnetic field. This finite remanance increases the area bounded by the hot and cold isotherms in Figure 1(a). The group pursued the idea in nm-scale Gd thin films [START_REF] Wetzlar | Leveraging anisotropy to enhance multiferroic transduction[END_REF][START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF]. Ultimately, the Gd nanostructures fabricated had non-uniform crystallinity, crystallographic ordering and some degree of oxidation, factors which contributed to a lack of single domain behavior [START_REF] Wetzlar | Leveraging anisotropy to enhance multiferroic transduction[END_REF][START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF].

The aforementioned previous work on single domain magnets has led us to consider the use of magnetically hard materials, which also exhibit finite magnetization at zero field, for thermal energy conversion. We therefore present three new perspectives on the use of hard ferromagnets in a TMG cycle. In the first two instances, we utilise the properties of established hard ferromagnets to measure or model their potential impact on a TMG cycle. In the third case, we offer a novel route to the construction of anisotropic ferromagnets tailored to use as TMG working materials.

The first perspective is the use of a hard ferromagnet as the working material. As demonstrated below via experimental results, the aim is to use two, rather than one, quadrant of the magnetisation vs. field (M -H) loop, as depicted in Figure 1(b). This aim is similar to the deployment of positive and negative electric fields in a study by Basso et al. on the electrocaloric effect in the ferroelectric 200 copolymer P(VDF-TrFE) [START_REF] Basso | Doubling the electrocaloric cooling of poled ferroelectric materials by bipolar cycling[END_REF].

The second perspective is the use of a hard ferromagnet to provide a biasing field to a magnetically soft working material. The aim here is to achieve a higher work output than is possible without a biasing field while again utilising 205 two quadrants in M -H space. This part of the study will be in the form of a numerical model.

The third perspective is the use of nano-structured hard magnets to provide a novel source of spin reorientation transitions (SRTs). In addition to single domain 210 pyromagnetism, Refs. [START_REF] Wetzlar | Leveraging anisotropy to enhance multiferroic transduction[END_REF] and [START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF] explored SRTs as a source of pyromagnetism, based on the potential of the SRTs to generate an order of magnitude more energy with roughly comparable efficiencies [START_REF] Wetzlar | A unifying metric for comparing thermomagnetic transduction utilizing magnetic entropy[END_REF]. It was found that high-quality sample fabrication presented a significant ob-215 stacle. Synthesized thin film, epitaxial NdCo 5 samples had non-uniform crystal epitaxy which limited their utility for thermal energy applications [START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF][START_REF] Wetzlar | Leveraging anisotropy to enhance multiferroic transduction[END_REF]. Our third perspective, is an alternative idea: an artificial spin-reorientation compound can potentially be formed by combining layers 220 of two magnetic compounds with different easy axis alignments and temperature coefficients of magnetic anisotropy. These artificial compounds may be easier to fabricate while presenting a similar pyromagnetic effect to their thin-film counterparts. Our presentation of this idea is conceptual.

225

The rest of this article is organized as follows. Section 2 details: the materials and experimental methods used our study of hard ferrites as working materials, and the computational methods used to analyze the effect of a bias magnetic field on a magnetically soft working material.

230

Section 3 contains the main results of those two studies and a discussion of the concept of artificial SRTs as pyromagnetic working materials. Conclusions are drawn in Section 4. Experiments. An initial viability test was conducted for each material by measuring isothermal magnetization loops for a wide range of temperatures between -2 T and 2 T, Figure 1: Work outputs (shaded areas) of thermomagnetic cycles, operating between two temperatures, T cold and T hot , and between two applied fields, H 1 and H 2 . A cycle is indicated by the numbers 1-4 in which order a positive work can be extracted from the TMG working material. In (a) a soft ferromagnet (FM) is operated up to temperatures above its Curie point. One quadrant of the M -H is used. In (b), a hard ferromagnet is operated below its Curie point, in positive and negative applied fields. In this case, two quadrants of the M -H loop can be used. In the schematic shown, the temperature coefficient of the coercivity is negative, which is typical for most hard magnets. The physical example presented in Section 3.1 is a magnetically hard ferrite with a positive temperature coefficient of coercivity.

Methods

as shown in Figure 3. Then, the M -H loops were analyzed to select isotherms which yield significant enough changes in magnetization to generate substantial work outputs. For the second set of tests, both materials were cycled between -0.25 T and 0.25 T, although an operational temperature range of 200 K (from T cold = 320 K to T hot = 520 K) was used for Y30BH, whereas a range of 320 K (from T cold = 280 K to T hot = 600 K) was used for HF8/22. Each sample was loaded into the magnetometer, and an initial magnetic field of 2 Tesla was applied at T cold in order to ensure an axial magnetization in the positive zdirection, regardless of sample orientation. Then, the field was set to -0.25 T and the loops proceeded in a clockwise fashion, as enumerated by points (1) through (4) in Fig- ure 4. Each sample was cycled through four loops. The work output for each loop was calculated by numerically integrating the M -H isotherms, as outlined in Hsu et al. [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF].

Soft Ferromagnets Biased By a Hard Magnet

Materials. The interaction of magnetically soft magnetocaloric materials with a hard magnet were simulated using magnetization data from the literature. The 2 magnetocaloric materials in the simulation are: (i) a Ce-substituted, fully hydrogenated La(Fe,Si) 13 sample (nominal composition La 0.82 Ce 0.18 Fe 11.8 Si 1.2 H δ , henceforth LCFS-H), with first order T C = 60°C; and (ii) Gd, with T C = 25°C.

The LCFS-H material was prepared at Vacuumschmelze according to methods described elsewhere [START_REF] Katter | Magnetocaloric properties of La(Fe, Co, Si) 13 bulk material prepared by powder metallurgy[END_REF]. Data for magnetization as a function of temperature (on cooling, from 94°C to 36°C in steps of 0.25°C) in fixed applied fields (µ 0 H up to 1.6 Tesla, in steps of 0.1 Tesla) for LCFS-H were taken at Vacuumschmelze and a subset of the data is shown in Figure 2 whereas data for Gd were extracted from the study by Hsu et al. [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF]. In our interaction simulations, each material was placed next to a magnetically hard Nd-FeB magnet, which was simulated by parameters from Ra-285 dia [70], a magnetostatics package for Mathematica. Each simulation used a temperature range of 50°C, centered on the Curie temperature of the soft material. For La-Fe-Si, the temperature range was from 35°C to 85°C, and for Gd, the temperature range was 0°C to 50°C. A 50°C temper-290 ature range was chosen to enable comparison to Hsu et al. [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF]. In that study, a work output of 13.6 J/kg for polycrystalline Gd was found under an applied field variation from 0 T to 0.3 T and a temperature range of 0°C to 50°C.

295

Simulations. The details of the simulation are as follows. First, we constructed the geometric constraints for the problem by defining the dimensions and relative locations for the hard and soft materials. In both cases, we used a square face measuring 3 mm × 3 mm. The thickness of 300 each material was varied in order to obtain relative size effects. Each material was magnetized through its thickness. Three-dimensional M (H, T ) surface plots of magnetization as a function of temperature and magnetic field were interpolated linearly from the data for LCFS-H and Gd. Radia 305 simulations produced values for the magnetic field due to the NdFeB hard magnet as a function of temperature and the amount of hard magnetic material. The magnetic field values thus obtained were used to select the magnetization value of each soft material at every temperature from the 310 M (H, T ) surface plots.

We conducted two sets of simulations: one for Gd with NdFeB; the other for LCFS-H with NdFeB. In each simulation, we varied the thickness of each material to study relative size effects. The isothermal M -H curves were 315 used to calculate work outputs.

Results and Discussion

Magnetically Hard Working Materials

The first novel perspective in this article is in considering ferrites as magnetically hard TMG working materials.

The purpose is to increase the efficiency associated with the TMG cycle while bringing down the cost of the functional material. The efficiency η of a TMG cycle was given in Equation 6. The integral M dH is the work output from one complete cycle around a M -H loop. In this context, the reasons for selecting ferrites (apart from their cost) are twofold. Firstly, hard ferrites have a positive temperature coefficient of coercivity, meaning the coercivity increases as temperature increases. Using hard ferrites therefore avoids the risk of thermal demagnetization under 330 magnetic fields close to the low-temperature coercive field. Magnetization loops for the two ferrites studied in this work are shown in Figure 3. Secondly, magnetically hard materials have magnetization loops with two positive magnetization quadrants in the magnetisation vs applied field 335 (M -H) plane. This offers a new perspective on TMG: by opening up the possibility to construct two-quadrant thermomagnetic generation cycles, wherein the absolute value of the applied magnetic field is small. Low magnetic fields give small contributions to the magnetically-driven entropy change ∆S [START_REF] Hsu | Thermomagnetic conversion efficiencies for ferromagnetic materials[END_REF].

The results of example magnetization measurements taken by thermomagnetic cycling are shown in Figure 4. These cycles consisted of two isotherms (in which the sample temperature was maintained by the magnetometer) and two iso-field steps. Previous work has demonstrated that the major loops shown in Figure 3 possess a superposition of a (reversible) magnetocaloric effect and an irreversible entropy production [START_REF] Basso | Entropy change and entropy production in barium ferrite[END_REF]. We present cycles for which the applied magnetic field does not substantially exceed the coercive field and therefore assume that the material stays within the reversible regime.

The data shown in Figure 4 were taken for clockwise (CW) cycles (positive work output); data were also taken for counterclockwise (CCW) cycles (negative work output, 355 see Table 2). The CW minor loops shown in Figure 4 possess opposite concavity to that exhibited by the counterclockwise major loops shown in Figure 3. The CCW loops (not shown) exhibited the same concavity at the major loops in Figure 3. In both sets of loops, the increasing 360 field leg of the first cycle (points 1 to 2) is noticeably higher than in the remaining cycles. This is an artifact of the experimental procedure. When the samples were set into the magnetometer, an initial magnetic field of +2.0 T was applied to ensure magnetization in the positive direction, 365 regardless of sample orientation. This imparts an additional magnetic remanence to the sample, which lingers until thermal demagnetization (points 2 to 3 on the labelled graphs).

Table 2 contains the work outputs associated with clock-370 wise (CW) and counterclockwise (CCW) cycles performed on both samples. In addition, loops under cycling between magnetic field values of -0.25 T and 0.25 T, an additional loop was performed on the HF8/22 sample between -0.25 T and +0.5 T. Furthermore, the relative loss of work output 375 upon mutiple cycles is shown as an additional column. The highest performing material tested was Y30BH grade ferrite, with a work output of 9.8 J/kg under cycling between magnetic field values of -0.25 T and 0.25 T and a temperature range from 320 K to 520 K. This work output 380 compares well with that of polycrystalline Gd, which provides 13.6 J/kg under cycling between field values of 0 T to 0.3 T and a temperature range from 273 K to 323 K (or 0°C to 50°C). Although the work outputs are comparable, Gd performs far better under realistic TMG circum-385 stances, which call for operating frequencies on the order of 0.5 Hz to 2 Hz in medium to large-scale devices and which therefore utilise smaller temperature ranges. Hence, further work to optimize hard magnetic materials is required to bring about a new class of magnetically hard, high per-390 formance TMG materials.

From Table 2 we also see that there is a reduction of work output upon multiple cycling. It is noticeable from the 1st to the 2nd cycle, i.e. after the first cycle, in both samples, and by the 2nd cycle the work output 395 is more stable. The initial loss of work output is associated with the additional magnetic remanence mentioned above, and from exposing the sample to a magnetic field in the opposite direction to its delivered magnetization. We see that the loss is greatest for clockwise magnetiza-400 tion loops. However, the physics of the change of work output on cycling deserves further exploration, as relaxation of a magnetic loop towards the anhysteretic curve is a potential risk, if the chosen magnetic cycle lies far away from the anhysteretic curve. Indeed, while the magnetic 405 loops made in our experiments are well controlled and consist of isotherms and iso-field steps, conditions in a device

Sample Grade

Field Range (T) might deviate from this and the impact of such deviations is unknown. Lastly, the physical mechanism underlying the work loss may depend on the choice of hard magnet material, which might affect the viability of alternatives to magnetically hard ferrites.

Soft Ferromagnets Biased By a Hard Magnet

Since the work output, W , for any material in a thermomagnetic power generation cycle can be given by Equation 4, we may alter the construction of the magnetization loops in order to maximize the work output. This can potentially be achieved by using a hard ferromagnet to apply a local biasing magnetic field to a magnetically soft ma-terial. The effect of bias electric or magnetic fields have previously been considered in a few caloric materials. The effect of an internal bias electric field has been examined in an analytical Landau model of an electrocaloric material [START_REF] Ma | Tailoring the Electrocaloric Effect by Internal Bias Fields and Field Protocols[END_REF]. The effect of a biasing external magnetic field 425 has been considered for direct (induction-based) TMG cycles on a Heusler alloy with a first order magnetostructural transition, with the motivation of converting as much magnetic work into electricity as possible [START_REF] Song | Thermodynamics of energy conversion via first order phase transformation in low hysteresis magnetic materials[END_REF]. In this Perspective, our purpose is different from either example: if 430 the magnetic field from the hard magnet is opposite to the magnetic field applied to the entire system, then both positive and negative applied (external) magnetic fields can be used in the TMG cycle. The results of simulations using two such systems are detailed below.

435

Magnetization loops for Gd and LFS under both unbiased and biased conditions can be seen in Figures 5 and6, respectively. To calculate the maximum available work for the materials when they are magnetically biased, a thermodynamic cycle between the largest negative field value 440 that still yields a positive work output and 0.3 T is considered. Also, both TMG materials are cycled through a temperature range of 50°C, centered about the Curie temperature. The benchmark values with which our results are compared are the computed work outputs for unbiased 445 materials under an external applied field which varies from 0 T to 0.3 T when the materials are cycled over a 50°C temperature range about their Curie points. For Gd, the benchmark work output thus computed is 13.6 J/kg, and for LCFS-H, the benchmark value computed is 20.4 J/kg.

450

In both Figure 5 and Figure 6, we see that the impact of the bias field is considerable; the external applied field can now be negative as well as positive, yielding additional utilization of the external magnetic field, just as in the hard ferrite example, but via a different mechanism. In of hard and soft magnetic materials in the system have a significant impact on the results. The maximal work output per total system mass is achieved for a combination of 67% soft TMG magnet and 33% hard biasing magnet 460 for both of the soft materials tested, as presented in Figure 7(a). At this material composition ratio, the biased cycle presents an improvement in the work output compared to the benchmark of each material: a 13% improvement in the case of Gd and 9.8% for LCFS-H. These modest numbers are dramatically improved when one no longer accounts for the mass of the hard magnet; when computing work outputs per mass of soft material alone, the work outputs for composites comprising 67% soft ferromagnet are 22.6 J/kg (Gd) and 33.7 J/kg (LCFS-H). The latter values correspond to a 65% increase over the unbiased cycle. Further numerical details for composites with different soft:hard magnet ratios are given in [START_REF] Tantillo | Optimization of materials for magnetic refrigeration and thermomagnetic power generation[END_REF].

The maximum negative field that can be used while obtaining only positive contributions to the work output is equal to the magnitude of the biasing field from the hard magnet. The latter is plotted as a function of hard magnet fraction in the inset to Figure 7. It is also worth considering how finite size effects affect the hard magnet. It is intuitive that a lower fraction of NdFeB in the composite would lead to a smaller biasing field, but the data presented in the inset to Figure 7 confirm the linear increase of biasing field with increasing hard magnet composition.

Spin Reorientation Transition TMG

To this point of our Perspective, the only mechanism for TMG materials that has been discussed involves a thermally-driven change in the magnitude magnetization vector. Although thermally-driven changes in the magni- tude of the magnetization vector can yield significant TMG work output, it is also worth considering other mechanisms 490 for magnetization changes. Spin reorientation transitions occur in materials which experience a temperature-driven change in the direction of the magnetization vector from one crystallographic axis to another. By changing the projection of the magnetization in a certain direction, it is pos-495 sible to drive a change in magnetic flux, which can then be harvested to do work. A brief review of embodiments of this idea, some promising results in the literature on SRT TMG, and some initial concepts for further development, are presented below.

500

The first published spin-reorientation TMG design was created by Ohkoshi et al. in 1977 [46] after an initial conceptual proposal in the previous year [START_REF] Ohkoshi | A Proposal of Application of Spin Reorientation Phenomenon to the Thermomagnetic Power Generation[END_REF]. Their magnetic "pretzel" consisted of an SRT material (NdCo 5 ), a magnet for the applied field (SmCo 5 ), iron yokes to direct the 505 magnetic flux, and pickup coils to convert flux changes directly into electrical energy. The work output achieved by the pretzel design, in terms of the amount of SRT material, was 1.5 J/cm 3 , which corresponds to 177 J/kg of NdCo 5 . This work output is significantly larger than the 510 values found in previous sections for Gd or LCFS-H, albeit for cycles with different input thermal and magnetic parameters.

Further calculations and analysis by Wetzlar et al. [START_REF] Wetzlar | A unifying metric for comparing thermomagnetic transduction utilizing magnetic entropy[END_REF] confirmed that the available work associated with a TMG 515 cycle for NdCo 5 operated around its SRT could be 4 times larger than that associated with single domain Gd operated around its Curie point, albeit with a different operat-ing temperature ranges in each case (30 K for NdCo 5 and 5 K for Gd). We note that single-domain Gd should have a larger work output than poly-domain Gd due to the finite remanence mentioned above, and so the potential for SRTs to provide large work outputs is considerable.

However, there is a drawback to SRTs in single-phase materials: a sharp SRT is normally only observed in highly textured or single crystalline samples. This can place a restriction on sample morphologies and the scalability of the final device. For example, as noted earlier, NdCo 5 films can be prone to oxidation [START_REF] Carman | Energy harvesting a nano-scale based magneto-thermal-electric element[END_REF]. Therefore, to complete this portion of our Perspective, we briefly outline a concept, the details of which we leave to a separate article [START_REF] Basso | [END_REF]: namely, that SRT could also be accomplished using artificial magnetic composites, which could be significantly easier to find or fabricate than single-crystal SRT materials.

To start, we state a simplified form of the magnetic expanded in even powers of sine, is considered.) If thin layers of materials A and B with different anisotropy constants (K A and K B ) and volume fractions (v A and v B ) are arranged so that the easy axes are perpendicular to one another, the free (anisotropy) energy becomes

f = v A K A sin 2 θ A + v B K B sin 2 ( π 2 -θ B ), (8)
where θ B is measured with respect to the easy axis of material A. If the constants K A and K B have different temperature dependencies, there will be a critical temperature at which the magnetizations of the two materials change from 550 being perpendicular to aligning along a single axis. At this temperature, there is an artificial spin-reorientation transition, which can be used to generate energy from magnetic flux in the same way as with single-crystal materials. Further details will be elaborated in a standalone work [START_REF] Basso | [END_REF].

555

TMG based on artificial SRT is distinct from Curie point TMG in several ways. Firstly, while ∆S, the magnetic field-driven isothermal entropy change depends explicitly on the applied magnetic field in pyromagnets with second order Curie transitions, ∆S is independent of field 560 for SRT-TMG, provided the field is large enough to induce a spin-reorientation transition [START_REF] Ilyn | Magnetocaloric effect in single crystal Nd 2 Co 7[END_REF]. Secondly, if an artificial SRT can be constructed from building blocks which do not themselves contain a SRT, there should be good opportunities for tailoring the final SRT properties by adjusting the thermomagnetic properties of the materials A and B and their relative dimensions. Lastly, shape-dependent demagnetizing factors, magnetoelastic stresses in the material, and the magnitude of the applied magnetic field all play a role in the value of SRT work outputs, making the 570 power output and efficiency of a SRT-based TMG highly tunable [START_REF] Wetzlar | A unifying metric for comparing thermomagnetic transduction utilizing magnetic entropy[END_REF].

Conclusions

We have used experimental data, physical models and conceptual arguments to provide three new perspectives 575 on the use of hard magnetic materials in TMG cycles. Firstly, we have demonstrated that known hard ferromagnetic working material can provide work outputs comparable to that of Gd, albeit using operating temperature spans which are larger than desired. The hard magnet is 580 able to make use of two quadrants in the magnetizationapplied field plane, meaning that an applied field may be reversed and extract further energy. This could help to reduce the magnetic field required in TMG, if a hard magnet can be designed in which the coercivity is more ther-585 mally stable than the magnetization. Secondly, we have modeled the effect of an applied bias magnetic field on a soft ferromagnetic working material, showing again that a two-quadrant M -H loop can be used, reducing the magnetic field need. Lastly, we have outlined the potential 590 for artificial spin reorientation transitions to be explored for TMG applications, by tailoring the properties of hard magnetic films with competing thermal coefficients of magnetic anisotropy.

235 2 . 1 .

 21 Magnetically Hard Working Materials Materials. Two hexaferrite (BaFe 12 O 19 ) samples were studied in a in-house-developed magnetometer at the Neél Institute in Grenoble, France. One is a cylindrical HF8/22 grade magnetically-isotropic ferrite measuring 3 mm in 240 height and 3 mm in diameter, with a reported BH max of 6.5 to 6.8 kJ/m 3 , provided by Distrelec in Switzerland. The other is a cylindrical, anisotropic Y30BH grade, measuring 5 mm in height and 4 mm in diameter, with a reported BH max of 27.5 to 32 kJ/m 3 , provided by AMF 245 Magnetics in Australia.

Figure 2 :

 2 Figure 2: Iso-field magnetic polarization, J, for the LCFS-H sample used in Section 3.2. Data were taken on cooling.

Figure 3 :

 3 Figure 3: Isothermal magnetization loops for the two ferrite samples studied in this work. The Y30BH grade sample is magnetically anisotropic while the HF8/22 grade ferrite is isotropic. Both loops are corrected for demagnetizing fields. We note the similarity of these loop shapes, particularly for the Y30BH sample, to the desired schematic loops in Figure 1(b).

  HF8/22 ferrite: 280 K to 600 K.

Figure 4 :Figure 5 :

 45 Figure 4: Thermomagnetic generation cycles for two hard ferrite samples for applied fields between -0.25 T and 0.25 T. Loops proceed clockwise from (1) to (4) on each graph. The average work outputs (excluding the first cycle) for the cycles shown are (a) 9.8 J/kg and (b) 6.9 J/kg.

Figure 6 :

 6 Figure 6: Examples of simulated magnetisation loops for LCFS-H under no magnetic field biasing (open circles, dashed lines) and under biasing from a NdFeB magnet (solid circles, solid lines). The extra work output available when biasing is present is given by the area between the two blue curves, for moment values greater than zero.

Figure 7 :

 7 Figure 7: Relative size effects as they impact the work output per total system mass and (inset) the biasing field provided by the NdFeB portion of the composite.

  )

	Year	Ref. [53] (R)	Working Not specific	Field Perm.	Type	Comments A report which proposed the feasibility of a 1 kW, 2 Tesla TMG
			material	source		device operating at 5 Hz with a ∆Tres=95 K and a hot reservoir
		[29] (A)	Iron	Perm.	P.R.	First description of a Curie wheel as a "novel magnetic engine". temperature of 120°C.
		[30] (A) [54] (P)	Iron Fe2P-based	Either Either	P.R. + Static Static	A patent on the use of Fe2P-based materials for the direct Edison's first article on TMG, describing a rotating armature powered by heat and a means of converting that rotation into conversion of waste heat to electricity.
		[12] (A)	Heusler	Perm.	Static	electricity. Experimental study of Ni45Co5Mn40Sn10 as a low thermal
		[9] (P)	Iron	Either	P.R. (0)	hysteresis alloy capable of converting heat into electricity by First TMG patent, based on kinetic output from a rotating armature (an aspect of 1887 article). induction.
		[31] (P) [55] (A)	Nickel Gd	Perm. Perm.	P.R.	A design study, comparing the exergetic efficiency of TMG, Demonstrator made for educational purposes, using a nickel arc on a pendulum. First research article to provide a ORC, Stirling and Rankine cycles.
		[14] (A)	(Mn,Fe)2(P,As)	Perm.	Static	thermodynamic analysis. Used 4 (Mn,Fe)2(P,As) compositions in a induction-based TMG
		[10] (P)	Iron	Either	Linear (2)	Functional armature used to harvest energy. with a regenerative Ericsson cycle.
		[11] (P) [56] (A) [32] (P) [57] (A) [22] (P) [33] (P) [34] (P) [58] (A) [35] (P) [36] (P) [59] (A) [27] (A) [60] (A)	Iron Ni-Co-Mn-In (Heusler) Iron Gd Unspecified Unspecified Various suggested Gd,NdCo5 Fe-Ni Fe-Ni Gd N/A	Perm. Perm. Either Perm. E/M E/M Either N/A Perm. Either Perm. Perm.	Static Linear (1) P.R. + Static Linear (3+) P.R. P.R. Static (1) N/A P.R. + Static P.R. Linear Static (1)	Use of a free-standing Ni50.4Co3.7Mn32.8In13.1 Heusler alloy film, First patent of an induction device. oscillating at 200 Hz, to generate electricity directly. Average power density ∼1.6 µW cm -3 when the periodic temperature Device based on second half of 1887 paper, designed to produce variation in the TMG is 10 K. dc or alternating current via induction-based energy recovery. Self-oscillation of a Gd cantilever between a room temperature NdFeB magnet and an ice/water container, using a piezoelectric First patent of a Curie wheel. layer to generate electricity. Patent using current-driven heating of working material. Modeling of a thermomagnetic cycle, showing that spin First patent to suggest materials with different TC ; discussion of reorientation transition in NdCo5 with ∆Tres=31 K could give deleterious effects of thermal hysteresis. rise to an order of magnitude greater work density than Gd Used light to heat Ni-Fe alloys, driving either a Curie wheel or operating with ∆Tres=5 K. inducing a current in a coil. Motion of Gd film converted into electricity via coupling to a First patent to suggest the use of waste heat. piezoelectric cantilever. Thermodynamic analyses of induction-based TMG: proposed Ericsson cycle.
		[37] (A)	Gd	Perm.	Static	Design only: direct (induction-based) conversion of electricity
					(1)	based on 20 • C heat source and Alnico XII permanent, fixed
						magnet.
		[38] (A)	Fe ferrofluid	E/M	P.R.	Thermodynamic analysis of a FF circulation-based TMG
			(FF)			without regeneration. Here, rotation is circulation of the FF
						caused by the presence of the high field region.
		[39] (A)	Fe ferrofluid	E/M	P.R.	Thermodynamic analysis of a FF circulation-based static TMG
						with regeneration.
		[40] (A)	Fe ferrofluid	E/M	P.R.	Proposed modulation of applied magnetic field in FF
						circulation-based TMG to achieve Carnot efficiency.
		[41] (P)	Fe-Ni	Perm.	P.R.	Multiple stators in a ring structure.
		[42, 43]	Fe-Ni	Perm.	P.R.	Ornament, using heat to drive motion.
		(P)
		[44] (A)	Unstated	E/M	P.R.	Single-pole wheel with low output and efficiency; postulated use
			(TC ∼50°C)			of more suitable working materials.
		[45] (P)	Gd	Perm.	P.R.	First Gd-based engine patent, using rotation of thin Gd strips.
		[46] (A)	NdCo5	Perm.	Static	First spin reorientation-based approach; induction using
					(2)	Pretzel-like geometry.
		[47] (P)	Soft ferrite	Perm.	P.R.	Proposed use of counter-torque from an external generator to
						provide smooth rotation.
		[48] (A)	Gd	Perm.	Static	Thermodynamic analysis: demonstrated that thermal
					(1)	regeneration would improve TMG efficiency. Power density also
						calculated for shunts made of Fe and Ho69Fe31.
		[49] (P)	Ni-Fe	Perm.	P.R.	Use of solar heat or flame heat to generate motion via
			(example)			pyromagnetic effect.
		[28] (A)	Gd	E/M	Static	Thermodynamic analysis; showed that cycling the magnetic
					(1)	field improves efficiency of generator.
		[50] (A)	Y2(Fe,Co)17	E/M	Static	Thermodynamic analysis of a static generator with regeneration
					(1)	and a compositionally graded working material.
		[51] (A)	Fe54Ni36Cr10	Perm.	P.R.	3-pole, 5 W output device operating with ∆Tres=84 K.
		[52] (A)	Gd	Perm.	Linear	127 µm-thickness Gd film attached to a spring; inferred
						potential electrical output (via piezoelectric conversion) of
						1.85-3.61 mW cm -3 .

Table 2 :

 2 Summary of magnetization data, experimental parameters, and calculated work outputs for the two hard ferrite samples studied here. Cycles were performed in a clockwise (CW) or counterclockwise (CCW) loop in M -H space. We note the change in the work output apparent after the first cycle (expressed as a percentage change relative to the first loop) and that this change is greatest for CW cycles.

			Temperature	Work	Percentage	Loop Ori-
			Range (K)	Output	Loss of Work	entation
				(J/kg)	Output after
					nth Loop
	Y30BH	-0.25 to 0.25	320 to 520	-10.12	1st: 0.82;	CCW
					2nd: 0.63;
					3rd: 0.004
	Y30BH	-0.25 to 0.25	320 to 520	9.63	1st: 4.19;	CW
					2nd: -0.07;
					3rd: 0.08
	HF8/22	-0.25 to 0.25	280 to 600	7.21	1st: 9.60;	CW
					2nd: 0.56;
					3rd: 0.23
	HF8/22	-0.25 to 0.50	280 to 600	-16.66	1st: 0.70;	CCW
					2nd: 0.42;
					3rd: 0.10

Acknowledgements

595

We are grateful to Yves Deschanels for assistance with magnetometry measurements and Dominique Givord, Laurent Ranno, Morgan Almanza and Frédéric Mazaleyrat for enlightening discussions. This work has benefited from the support of the project HiPerTherMag ANR-18-CE05-0019 600 of the French National Research Agency (ANR). K.G.S. would like to acknowledge the Université Grenoble Alpes for sponsoring his visiting researcher position during 2017.

free energy for a uniaxial magnetic system:

where K A is the temperature-dependent anisotropy constant and θ A is the angle of the easy-axis. (Note that only the first term of the anisotropy energy, which can be