Stéphane Péan
email: stpean@mnhn.fr.

Simon Puaud

Laurent Crépin

Sandrine Prat

Anita Quiles

Johannes Van Der Plicht

Hélène Valladas

Anthony J Stuart

Dorothée G Drucker

Marylène Patou-Mathis

François Lanoë

Aleksandr Yanevich

The middle to upper paleolithic sequence of Buran-Kaya III

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

The Buran-Kaya III rockshelter (Figure 1) provides an exceptional stratigraphic sequence extending from the Middle Paleolithic to the Neolithic [START_REF] Janevic | Buran-Kaya 3 -Neue Angaben zur Kulturgliederung des Jungpaläolithikums der Krim[END_REF][START_REF] Pettitt | Middle and early upper Palaeolithic Crimea: the radiocarbon chronology[END_REF][START_REF] Yanevich | Buran-Kaya culture of the Crimea's gravett[END_REF]Chabai et al. 2004a;[START_REF] Monigal | Introduction to the site of Buran-Kaya III[END_REF][START_REF] Yanevich | Upper Palaeolithic settlements in Buran-Kaya 3 (Crimea, Ukraine): new interdisciplinary researches of the layers 5-2, 6-1 and 6-2[END_REF]). This shelter-cave, overhanging the Burulcha River in the Belogorsk region (Figure 2), was discovered in 1990 by A Yanevich and excavated up to 2001 by a team under the direction of A Yanevich and A Marks, with the participation of V Chabai, Y Demidenko, K Monigal, M Otte, and Y Yamada. New fieldwork was undertaken recently from 2009 to 2011 under the direction of A Yanevich and S Péan. [START_REF] Chabai | The Late Middle and Early Upper Paleolithic in Crimea (Ukraine)[END_REF]The and 2009[START_REF] Lanoë | Étude archéozoologique de la couche 4 de l'abri de Buran-Kaya III (Ukraine): economie de subsistance au Paléolithique final[END_REF] excavations in Buran-Kaya III focused on 9 archaeological layers, which yielded Middle and Upper Paleolithic artifacts (Figure 2): Streletskaya or eastern Szeletian (layer C), Micoquian (layer B), , Gravettian (layers 6-2, 6-1, 5-2), and Final Paleolithic (layer 4). A tenth layer (layer 3) is attributed to the Neolithic.

Buran-Kaya III is a key site for understanding the Middle to Upper Paleolithic transition (Chabai et al. 2004b;[START_REF] Anikovich | Early Upper Paleolithic in Eastern Europe and implications for the dispersal of modern humans[END_REF][START_REF] Demidenko | The Early and Mid-Upper Palaeolithic of the North Black Sea region: an overview[END_REF] because it is characterized by a Middle Paleo-Figure 1 Location of the site of Buran-Kaya III (Crimea, Ukraine) (modified after [START_REF] Prat | The oldest anatomically modern human from far southeast Europe: direct dating, culture and behaviour[END_REF] lithic layer (Micoquian, Kiik-Koba type) situated above an Early Upper Paleolithic layer (Streletskaya or eastern Szeletian) and below 3 layers attributed to the Aurignacian complex. Based on radiocarbon dating of this Micoquian layer, the latest Neanderthal settlements in eastern Europe were dated to 28.5 ka BP (34.4-31.7 ka cal BP; [START_REF] Marks | The Middle to Upper Paleolithic interface at Buran-Kaya-III, Eastern Crimea[END_REF][START_REF] Chabai | The Late Middle and Early Upper Paleolithic in Crimea (Ukraine)[END_REF][START_REF] Chabai | The volcanic winter, Buran-Kaya cannibals and the fate of the last eastern European Neanderthals[END_REF]. Furthermore, direct dating results for a human bone from a Gravettian (sensu lato) layer (6-1) establish a secure presence of anatomically modern humans at 31,900 +240/ 220 BP (36.9-35.5 ka cal BP) in this region [START_REF] Prat | The oldest anatomically modern human from far southeast Europe: direct dating, culture and behaviour[END_REF]. They represent the oldest Upper Paleolithic modern humans from eastern Europe in a well-documented archaeological context. Based on taphonomical observations (cut-marks and distribution of skeletal elements), they also show postmortem treatment of the dead.

Previously, 15 14 C dates were obtained on the Paleolithic layers of Buran-Kaya III (Table 1, Figure 3). These former results were scattered between 36.7 and 10.6 ka BP and were in part inconsistent with the stratigraphic sequence. Especially, 2 dates from the Micoquian layer at 28.5 and 28.8 ka BP (respectively, OxA-6673 and -6674) and 2 dates from the Aurignacian layers at 11.9 and 12.0 ka BP (respectively, were younger than the one obtained for an overlying Gravettian layer at 30.7 ka BP (OxA-6882). Furthermore, these 2 samples attributed to the Aurignacian layers show dates of the Late Glacial period. However, the stratigraphic position of these 2 specimens was shown to be uncertain (Marks and Monigal 2000:221). More generally, "… the dates are in a number of cases inverted relative to the stratigraphy" (Jöris et al. 2011:286). Furthermore, regarding the material found during the 1990, 1994, and 1996-1998 Concerning the human sample published by [START_REF] Higham | Radiocarbon dates from the Oxford AMS system: Archaeometry datelist 32[END_REF], its stratigraphic position is questionable. The previous result (32,790 ± 280 BP, OxA-13302) is considered with care in this article, in view of the associated comments by C Lalueza-Fox, who pointed out that the result should be questioned, as the human material was recorded from a Mesolithic layer. This human skull fragment was also attributed to layer 6-4, associated with the Aurignacian culture (C Lalueza-Fox and T Higham, personal communications). However, no human bone was discovered in this layer during the last fieldwork seasons in 2001 and 2009-2011, although the entire bone assemblage was carefully checked afterwards in the laboratory in Kiev. Therefore, we prefer to consider this date with caution, as unfortunately the stratigraphic position of the specimen is uncertain. [START_REF] Hedges | Radiocarbon dates from the Oxford AMS system: Archaeometry datelist 21[END_REF] (1), [START_REF] Yanevich | BuranKaya III and Skalistiy Rockshelter: two new dates Late Pleistocene sites in the Crimea[END_REF](2), Janevic 1998(3), Marks 1998(4), Pettitt 1998(5), Gerasimenko 2004(6), Monigal 2004 (7), Otte and Noiret 2004 (8), Higham et al. 2007 (9), andChabai 2012 (10). (after Hedges et al. 1996;[START_REF] Yanevich | BuranKaya III and Skalistiy Rockshelter: two new dates Late Pleistocene sites in the Crimea[END_REF][START_REF] Janevic | Buran-Kaya 3 -Neue Angaben zur Kulturgliederung des Jungpaläolithikums der Krim[END_REF][START_REF] Marks | A new Middle to Upper Paleolithic "transitional" assemblage from Buran-Kaya III, Level C, eastern Crimea: a preliminary report[END_REF][START_REF] Pettitt | Middle and early upper Palaeolithic Crimea: the radiocarbon chronology[END_REF][START_REF] Gerasimenko | Vegetational history of BuranKaya III[END_REF][START_REF] Monigal | Introduction to the site of Buran-Kaya III[END_REF][START_REF] Higham | Radiocarbon dates from the Oxford AMS system: Archaeometry datelist 32[END_REF]).

The heterogeneity in previous dating results and the discussion concerning the stratigraphy urged us to have a stratigraphic control of the sequence and to obtain new 14 C dates on bones within the Paleolithic layers of Buran Kaya III. This is reported in this paper. Furthermore, based on new geological, zooarchaeological, and 14 C results from large mammal bones, notably human remains, this study presents a paleoenvironmental and chronological interpretation of the stratigraphic sequence of Buran-Kaya III.

METHODS

Stratigraphic and sedimentological analyses were undertaken to better understand the depositional processes of the sedimentary infilling and the paleoenvironmental context of the human settlements in Buran-Kaya III. Sections were drawn and deposits were described macro-and microscopically in order to distinguish the different sedimentary layers and establish a stratigraphic sequence. The texture (sand, silt, clay), composition (quartz, limestone), and porosity of the sediment matrix were recorded. The elements of the coarse fraction (>2 mm) were also analyzed in terms of composition, shape, and surface preservation.

New zooarchaeological studies were conducted on the entire faunal assemblage of large mammals from layers C, B, 6-5, 6-4, 6-3 (by M Patou-Mathis), 6-2, 6-1 (by L Crépin), 5-2 (by S Péan), and 4 (by F Lanoë), excavated in 2001 and 2009-2011. Paleoecological information was inferred from the distribution of large mammal associations, following the methodology of [START_REF] Griggo | Signification paléoenvironnementale des communautés animales pléistocènes reconnues dans l'abri Suard (Charente) et la grotte de Bois-Ragot (Vienne): Essai de quantification de variables climatiques[END_REF].

Seventeen bones from large mammals, including 2 human remains, were 14 C dated by accelerator mass spectrometry (AMS) (Table 2). As far as possible, we sampled bones that could be anatomically and taxonomically determined, showing no postdepositional alteration, and belonging to the taxon preferentially hunted, the saiga antelope (Saiga tatarica). All 14 C dates were performed on bones as no charcoal was found in the Paleolithic layers. Six bones from the Gravettian layers excavated in 2001 were sampled in 2008 during our first zooarchaeological studies of the material curated in Kiev. Eleven bones were sampled during our excavations (2009)(2010)(2011), directly in every excavated archaeological layer of the stratigraphic sequence, from layer B (Micoquian) to layer 4 (Swiderian). The layer C (Streletskaya or eastern Szeletian) did not yield bone materials suitable for sampling. These specimens were not washed and were quickly packed to limit more handling. Three different laboratories performed 14 C AMS dating. This included cross-dating, applying 3 methods of collagen extraction. At the Center for Isotope Research in Groningen, Netherlands (GrA), the collagen extraction was performed following a protocol adapted from [START_REF] Longin | New method of collagen extraction for radiocarbon dating[END_REF]. At the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) using the Artemis facility of the Laboratoire de Mesure du C-14 (LMC-14, Commissariat à l'Energie Atomique, Saclay/Gif-surYvette, France) (GifA/SacA), bones were prepared following the method proposed by [START_REF] Nelson | A new method for carbon isotopic analysis of protein[END_REF], based on chemical reaction between ninhydrin and the amino-acids of collagen (TisnératLaborde et al. 2003). At the Oxford Radiocarbon Accelerator Unit (ORAU), Oxford, United Kingdom (OxA), bone collagen was extracted employing sequential decalcification, base wash, gelatinization, filtration and ultrafiltration steps to remove low molecular weight contaminants [START_REF] Higham | AMS radiocarbon dating of ancient bone using ultrafiltration[END_REF].

Six bones were cross-dated by Groningen and LSCE laboratories: 4 saiga antelope (Saiga tatarica) long bones from layers B, 6-5, 6-4 and 4; 1 mammal long bone from layer 5-2; and 1 horse (Equus cf. caballus) metatarsal initially attributed to layer 6-2 [START_REF] Prat | The oldest anatomically modern human from far southeast Europe: direct dating, culture and behaviour[END_REF]). This horse specimen was sampled in 2008 before our most recent fieldwork and stratigraphic description, which revealed that the thickness of layer 6-2, where it was found, is smaller than the bone length. Its stratigraphic origin remains questionable indeed and one can suspect that it actually could come from an underlying layer. In addition, 1 saiga antelope (Saiga tatarica) radius from layer 6-1 was cross-dated by Groningen and ORAU.

The chemical composition of the collagen is used to assess the reliability of the isotopic measurements. We determined the preservation of the bone collagen using the C/N atomic ratios, which are classically given to testify the quality of the collagen submitted for 14 C dating (van Klinken 1999). C/N ratios were obtained from the dated collagen or from collagen extracted at the Center for Archaeological Science, University of Tübingen, following a protocol adapted from [START_REF] Longin | New method of collagen extraction for radiocarbon dating[END_REF]. The dates were calibrated using the IntCal09 calibration curves [START_REF] Reimer | IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP[END_REF] using OxCal v 4.2.2 and modeled using a Bayesian approach (Bronk Ramsey 2009a). In addition, the OxCal outlier detection method (Bronk Ramsey 2009b) was performed to identify outlier samples.

RESULTS AND DISCUSSION Stratigraphy and Paleoenvironment

The new sedimentological and stratigraphic studies indicate that there are no reworking processes on the square excavated during the 2001 and 2009-2011 seasons. The layers are in situ and well defined and constrained. The sedimentary infilling of Buran-Kaya III is composed of a sequence of diamicton layers divided into 2 sedimentary units. The coarse and fine carbonate fractions result from the destruction of the limestone wall. The fragments are morphologically identified as frost shattered scales [START_REF] Ferrier | Les dépôts d'entrée de grotte[END_REF]; their edges can be more blunted in the lower part. The grain size and sorting of the matrix, which comes from the plateau, are typical of an eolian deposit [START_REF] Antoine | Les formations loessiques en milieux périglaciaires[END_REF].

The lower unit (C to 6-2) is characterized by limestone fragments with a scarce unsorted sandy silt matrix. The development of frost shattering associated with sandy loam sediments indicates contrasting climatic conditions: cold and wet winters, followed by less severe but still moist periods with runoff into the shelter.

In the upper unit (silt-clay level, 6-1 to 5-2), the coarse fraction is less abundant and the matrix becomes more silty and better sorted. The seasonal contrasts are less pronounced and are dominated by an eolian deposit; the climatic conditions become significantly drier than below. Layer 4 contains more organic matter and coarse fraction. It presents characteristics of contrasting conditions (frost shattering/biological and pedological activities).

The differences in large mammal associations along the stratigraphic sequence, based on new and previously published data [START_REF] Laroulandie | Worked bones from Buran-Kaya III level C and their taphonomic context C[END_REF][START_REF] Patou-Mathis | Archeozoological analysis of large mammal fauna from Buran-Kaya III Layer B[END_REF]; Crépin and Péan in [START_REF] Yanevich | Upper Palaeolithic settlements in Buran-Kaya 3 (Crimea, Ukraine): new interdisciplinary researches of the layers 5-2, 6-1 and 6-2[END_REF][START_REF] Prat | The oldest anatomically modern human from far southeast Europe: direct dating, culture and behaviour[END_REF][START_REF] Lanoë | Étude archéozoologique de la couche 4 de l'abri de Buran-Kaya III (Ukraine): economie de subsistance au Paléolithique final[END_REF], allows us to reconstruct different paleoenvironmental conditions (Figure 4). In all layers, the large mammal assemblages show a predominance of steppe species such as saiga antelopes, horses, red or polar foxes, and hares. The Streletskaya or Eastern Szeletian layer C was deposited in a mixed steppe (saiga antelope and horse) and forest (large and small cervids) paleoenvironment, under a cold and wet but not harsh climate. The Micoquian layer B shows an open and semi-arid steppe (with woolly mammoth and woolly rhinoceros) with a few forested areas (with red deer and mustelids), probably located alongside the river, under harsh climatic conditions, especially during winter. In the lower Aurignacian layer 6-5, the environment was still a cold and dry steppe (no cervid). From the Aurignacian layer 6-4 to the Gravettian layer 6-2, the climate was relatively cold, but not harsh, and relatively dry (not arid in 64, with the presence of red deer) in a general steppe environment (presence of reindeer). In the upper Gravettian layers 6-1 and 5-2, the faunal assemblage shows the most diversified spectrum of the sequence. The climate became drier and colder (presence of reindeer, woolly rhinoceros, and marmot), especially at the top of these layers, with less-contrasting seasons and, at the same time showing evidence of more forested areas around the site (presence of brown bear, wild cat, red deer, wild boar, and mustelids). During the Final Paleolithic (layer 4), the climatic conditions became wet; forested animals (red deer, wild boar, mustelids) were dominant at that time.

The palynological study [START_REF] Gerasimenko | Vegetational history of BuranKaya III[END_REF][START_REF] Gerasimenko | Environmental changes in the Crimean Mountains during the last Interglacialmiddle pleniglacial as recorded by pollen and lithopedology[END_REF] indicates a general steppe environment with alternating dry (xeric conditions in 5-2) and wet periods (6-1 and B). The study of micromammals shows similar evolutionary trends to the other paleoenvironmental data [START_REF] Markova | Small mammal fauna from BuranKaya III[END_REF], with a progressive increase in aridity from the layer C to the driest layer 5-2.

Based on the consistent results from pollen, microfauna, and large mammal studies, the paleoecological context of Buran-Kaya III can, on the whole, be described as an open and steppelike periglacial environment characterized by a reduced tree cover (along the river) under a cold and dry climate. The faunal diversity in Buran-Kaya III could result from the proximity of different environments (steppe, riparian forest, and plateau) around the site.

Radiocarbon Dates

Some 24 14 C dates were obtained on the 17 large mammal bones, 2 of which are human remains, associated with all layers, from layer B (Micoquian) to layer 4 (Swiderian) (Table 3, Figure 5). These results include 7 cross-dated samples. The number of 14 C dates was limited by the absence of charcoal and scarcity of identified faunal material appropriate for dating. The C/N atomic ratios of the 14 tested bone samples for 14 C and stable isotope ( 13 C and 15 N) analyses are between 3.0 and 3.6, i.e. within the acceptable range of 2.9-3.6 expected for wellpreserved collagen [START_REF] Deniro | Postmortem preservation and alteration of in vivo bone collagen isotope ratios in relation to palaeodietary reconstruction[END_REF][START_REF] Ambrose | Preparation and characterization of bone and tooth collagen for isotopic analysis[END_REF]). The collagen samples dated and tested contain 31.7-45.7% carbon and 10.3-16.0% nitrogen. These percentages correspond to chemically nonaltered collagens. Furthermore, the obtained collagen yield values range between 1% and 10% weight, i.e. above the threshold defined by van [START_REF] Van Klinken | Bone collagen quality indicators for palaeodietary and radiocarbon measurements[END_REF]. It should be noted that the horse bone sample (GrA-40485), which provided the lowest collagen yield, could be taken with caution. On the whole, the quality parameter values support the accuracy of the 14 C results.

14 C results obtained from the 7 cross-dated samples and their calibrated deduced dates are shown in Table 4. Bayesian analysis enables the combining of 14 C results performed on the same sample, providing an agreement factor (Ai). For 6 samples, the agreement factor was good enough (Ai > 81) to validate the combination; the deduced combined densities were then used in the modeling.

The results that failed this criterion are those obtained from the Gravettian Saiga tatarica specimen (layer 6-1), dated to 29,640 ± 170 BP (GrA-53942) and 32,200 ± 450 BP (OxA-25669) (Ai = 27). Additionally, due to a very large uncertainty (+2450/ 1880 yr) of the date GrA-48399, we decided to reject the combination of the layer 6-4 and to exclude this date from the interpretation of the results. Therefore, only the other result from the same sample (GifA-11220/SacA-25137) was considered for the following modeled analyses. A red fox tibia of layer 5-2 dates to 24,070 ± 260 BP (GifA-80186/SacA-12265). This is younger than the 2 other dates from this layer (33,790 ± 880 BP [GifA-11222/SacA-25139] and 30,100 +180/ 170 BP ). If we consider the possibility that this fox element could be intrusive, its original stratigraphical position is doubtful. Therefore, this date is not considered in the following modeled analyses.

The remaining 14 C dates were modeled using a Bayesian approach. Samples archaeologically attributed to a same cultural unit were placed in a phase, and the 4 phases (Micoquian, Aurignacian complex, Gravettian complex, Swiderian) were then sequenced according to the stratigraphy (Figure 5). The Bayesian analysis was conducted on a total of 15 samples (including 5 combinations) corresponding to 20 dates.

The Buran-Kaya model was run 3 times to obtain a final model (Figure 5) with a high agreement index for the whole modeling (Amodel = 113). The first run of the model showed that GrA-53939 was an outlier, which was subsequently rejected. The second run (on a total of 14 samples and 19 dates) provided a model with a low validity (Amodel < 60), which meant that the stratigraphic sequence did not agree with the 14 C results. As explained above, the stratigraphic origin of the large horse bone (GrA-40485) from layer 6-2 remains questionable. If we rerun the program after attributing this specimen to layer 6-3 rather than to 6-2, the model is validated with a final agreement index equal to 113. In that case, 14 C results and stratigraphy concur perfectly.

The cultural assemblages show the following calibrated 14 C ages with a 2 range:

• The Micoquian (layer B) settlements are modeled from 44.0 to 40.2 ka cal BP;

• The occupations attributed to the Aurignacian complex (layers 6-5, 6-4, and 6-3) and theGravettian complex (layers 6-2, 6-1, and 5-2) are modeled within a close timespan between 40.4 and 33.5 ka cal BP. • After a chronological gap of about 20,000 yr, the Swiderian layer is modeled from 11.8 to 11.3ka cal BP.

The 14 C date of a human cranial fragment (from layer 6-2) is 32,450 +250/ 230 BP (GrA-50457), calibrated to 37.8-36.5 ka cal BP. This makes the second direct dating of an early modern human remain from Buran-Kaya III; the first was from layer 6-1 [START_REF] Prat | The oldest anatomically modern human from far southeast Europe: direct dating, culture and behaviour[END_REF] [START_REF] Dansgaard | The abrupt termination of the Younger Dryas climate event[END_REF][START_REF] Dansgaard | Evidence for general instability of past climate from a 250 kyr ice-core record[END_REF][START_REF] Dahl-Jensen | Past accumulation rates derived from observed annual layers in the GRIP ice core from Summit, Central Greenland[END_REF][START_REF] Members | Climate instability during the last interglacial period recorded in the GRIP ice core[END_REF][START_REF] Grootes | Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores[END_REF][START_REF] Johnsen | The Eem stable isotope record along the GRIP ice core and its interpretation[END_REF][START_REF] Johnsen | The δ 18 O record along the Greenland Ice Core Project deep ice core and the problem of possible Eemian climatic instability[END_REF][START_REF] Stuiver | The GISP2 18 O climate record of the past 16,500 years and the role of the sun, ocean and volcanoes[END_REF][START_REF] Hammer | The Stratigraphic Dating of the GRIP Ice Core[END_REF][START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF][START_REF] Goñi | Millennial-scale climate variability and vegetation changes during the last glacial: concepts and terminology[END_REF].

Discussion

In contrast to the previously published 14 C dates ( Our results also show that 2 Upper Paleolithic cultural traditions, related to the Aurignacian and Gravettian complexes, were successively present in Crimea in a short timespan between 40.4 and 33.5 ka cal BP. All the Upper Paleolithic settlements (layers 6-5 to 5-2) occurred during a short period between the 2 Heinrich stadials HS 4 and HS 3 (Sánchez Goñi and Harrison 2010). At last, after a gap of about 20,000 yr, the site was totally discarded or no depositional process occurred. The Final Paleolithic populations (Swiderian) settled in Crimea between 11.8 and 11.3 ka cal BP, i.e. during MIS 1.

If we compare all the stratigraphic, 14 C, and paleoecological data, the paleoenvironmental changes along the sequence of Buran-Kaya III fit with the global climatic changes in Europe [START_REF] Arslanov | Climate, Black Sea levels and human settlements in Caucasus Littoral 50,000-9000 BP[END_REF][START_REF] Goñi | Contrasting impacts of Dansgaard-Oeschger events over a western European latitudinal transect modulated by orbital parameters[END_REF][START_REF] Dolukhanov | Ecological crises and early human migrations in the Black Sea area[END_REF][START_REF] Naughton | Wet and dry climatic trend in north-western Iberia within Heinrich events[END_REF][START_REF] Fletcher | Millenial-scale variability during the last glacial in vegetation records from Europe[END_REF], in particular during the Middle/Upper Paleolithic transition [START_REF] Jöris | At the end of the 14 C time scale-the Middle to Upper Paleolithic record of western Eurasia[END_REF][START_REF] Jöris | Radiocarbon dating the Middle to Upper Paleolithic transition: the demise of the last Neanderthals and the first appearance of Anatomically Modern Humans in Europe[END_REF]. The bottom part (layers C and B) is characterized by species from open and steppe-like environment, with a significant number of taxa that are characteristic of forest and shrub areas; this is coherent with the generally quite temperate climatic conditions that developed during Greenland Interstadial (GI) events. The late Micoquian occupations, attributed to Neanderthals, mainly occurred during GI 11-9, i.e. just before or at the beginning of the cold episode HS 4. During the Aurignacian, the climate was relatively cold and dry (but not arid) in a steppe-like environment. The dates indicate that all the Aurignacian settlements probably occurred at late HS 4, which is coherent with the information obtained from the faunal remains. The silt-clay level deposit probably results from a short cold and very dry event, during HS 4 or more likely the Greenland Stadial GS 9. During the deposition of the Gravettian layers, the environment became dry with less-contrasting seasons; the faunal spectrum was more diversified. These settlements occurred under interstadial climatic conditions (such as GI 8 or 7), which became colder and drier (during GS 7 or 6). The Final Paleolithic layer indicates a temperate and wet period with the predominance of forest animals (MIS 1).

CONCLUSION

Figure 2

 2 Figure 2 Stratigraphic section of Buran-Kaya III and plan of the excavations

Figure 4

 4 Figure 4 Paleoenvironmental interpretation of sediments, pollen, and faunal remains from Buran-Kaya III

Figure 5

 5 Figure 5 Bayesian modeling of the new 14 C dates for the Buran-Kaya III sequence, produced using OxCal v 4.2.2 (Bronk Ramsey 2009a,b) based on the IntCal09 calibration data set (Reimer et al. 2009), in the global climatic context (δ 18 O GRIP curve,[START_REF] Dansgaard | The abrupt termination of the Younger Dryas climate event[END_REF][START_REF] Dansgaard | Evidence for general instability of past climate from a 250 kyr ice-core record[END_REF][START_REF] Dahl-Jensen | Past accumulation rates derived from observed annual layers in the GRIP ice core from Summit, Central Greenland[END_REF][START_REF] Members | Climate instability during the last interglacial period recorded in the GRIP ice core[END_REF][START_REF] Grootes | Comparison of oxygen isotope records from the GISP2 and GRIP Greenland ice cores[END_REF][START_REF] Johnsen | The Eem stable isotope record along the GRIP ice core and its interpretation[END_REF][START_REF] Johnsen | The δ 18 O record along the Greenland Ice Core Project deep ice core and the problem of possible Eemian climatic instability[END_REF][START_REF] Stuiver | The GISP2 18 O climate record of the past 16,500 years and the role of the sun, ocean and volcanoes[END_REF][START_REF] Hammer | The Stratigraphic Dating of the GRIP Ice Core[END_REF][START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic δ 18 O records[END_REF][START_REF] Goñi | Millennial-scale climate variability and vegetation changes during the last glacial: concepts and terminology[END_REF].

  This study allows us to propose a new chronostratigraphical and paleoenvironmental context for Buran-Kaya III. Our results provide an early chronological framework for the Middle to Upper Paleolithic period and a new chronological context for the demise of Late Neanderthal and arrival of early modern humans in Europe. Our results reinforce an early presence of Early Upper Paleolithic industry (Streletskaya or eastern Szeletian) in eastern Europe. Moreover, Buran-Kaya III is the only site that presents a peculiar stratigraphic position of an Early Upper Paleolithic layer (associated with bone industry) underlying a Middle Paleolithic layer. The new 14 C dates from the Micoquian layers of Buran-Kaya III cast doubt on the existence of Neanderthal refugial survival in Crimea after 40 ka cal BP. Furthermore, our findings fill an important gap in the study of modern human settlements in eastern Europe. The human fossils from layers 6-1[START_REF] Prat | The oldest anatomically modern human from far southeast Europe: direct dating, culture and behaviour[END_REF]) and 6-2 directly dated to 36 ka cal BP represent the oldest direct evidence of the presence of modern humans in far southeastern Europe.In Buran-Kaya III, the Early Upper Paleolithic (Streletskaya or eastern Szeletian) and Late Middle Paleolithic settlements occurred before HS 4, or during early HS 4 for the Middle Paleolithic ones, in a mixed steppe and forested environment, under a cold and semi-arid climate. The first settlements attributed to the Aurignacian complex probably occurred during late HS 4, in a drier environment, without forested areas. During the Middle to Upper Paleolithic transition, the human settlements in Buran-Kaya III seem to have been quite continuous with no significant gap. The settlements attributed to the Aurignacian and Gravettian complexes occurred between HS 4 and HS 3, during a succession of short warm and cold episodes.Regardless of the paleoclimatic context, the Paleolithic faunal spectrum in Buran-Kaya III remained typical of an open and steppe-like periglacial environment, with more or less forested areas. The specific location of the rockshelter on the ridges of the Crimean Mountains, in a transition zone between steppes and mountains, where different biotopes existed (steppe, riparian forest, and plateau), could have been an important reason for the continuity of human settlements.

Table 1

 1 Previously published 14 C ages of the Paleolithic layers of Buran-Kaya III, after

Table 2

 2 New samples selected for 14 C AMS dating from Buran-Kaya III. New samples selected for 14 C AMS dating from Buran-Kaya III. (Continued)

	Year of
	excav. Square Nr	Layer Culture	Lab code	Species	Anatomical part
	2010	9Z	sieving 4	Swiderian OxA-25670	Saiga tatarica Phalanx 2
	2009	9Z	53			GrA-50461	Saiga tatarica Long bone
						GifA-11219/SacA-25135
	2001	11Б -	5-2 Gravettian GifA-80186/SacA-12265 Vulpes vulpes Left tibia
	2009 10Z	5	GrA-47316	Mammal Long bone
						GifA-11222/SacA-25139
	2010 9Z	1054	6-1	GrA-53942	Saiga tatarica	Left radius
						OxA-25669
	2001	9A	-			GifA-10021/SacA-19018 Cervus elaphus Right metacarpal
	2001 10Б -GifA-11216/SacA-25133 Saiga tatarica Left hemi-mandible 2001 10Б N6 GrA-37938 Homo
	sapiens Cranial fragment
	2001 10Б	N27	6-2		GrA-50457	Homo sapiens Cranial fragment
	2009 9Z	636			GrA-50460	Saiga tatarica Right metacarpal
	Table 2 Year of
	excav. Square Nr	Layer Culture	Lab code	Species	Anatomical part
	2001	9Б	-	6-3	Aurignacian GrA-40485	Equus cf. ca-Right metatarsal
						GifA-80181/SacA 12260 ballus
	2009	9Z	697			GifA-11221/SacA-25138 Saiga tatarica Long bone

Table 3

 3 New 14 C dates of the Buran-Kaya III sequence. Datings performed on the same sample are framed together. The dates were calibrated using the software OxCal v 4.2.2 (Bronk Ramsey 2009a,b) based on the IntCal09 calibration data set (Reimer et al. 2009). The largest values of error (+ or ) are used for calibration. Homo sapiens 31,900 +240/-220 3.3 36,930-35,503 GrA-50460 Longin 6-2 S. tatarica 29,440 +190/-180 3.3 34,643-33,486 GrA-50457 Longin H. sapiens 32,450 +250/-230 3.1 37,831-36,450 Table3New 14 C dates of the Buran-Kaya III sequence. Datings performed on the same sample are framed together. The dates were calibrated using the software OxCal v 4.2.2 (Bronk Ramsey 2009a,b) based on the IntCal09 calibration data set[START_REF] Reimer | IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP[END_REF]. The largest values of error (+ or ) are used for calibration. nd: no data available. *: lowest collagen yield value (1% weight) of the sample set.(Continued)

	Lab code	Pretreatment Layer Culture	Species	14 C BP	C/N cal BP (95.4%)
	OxA-25670 Ultrafiltration 4	Swiderian Saiga tatarica 10,040 ± 45	3.4 11,765-11,321
	GrA-50461 Longin			S. tatarica	10,010 ± 60	nd 11,756-11,266
	GifA-11219/ Ninhydrin SacA-25135		10,050 ± 70	11,958-11,285
	GrA-47316 Longin	5-2 Gravettian Mammal	30,100 +180/-170 3.6 35,060-
						34,523
	GifA-11222/ Ninhydrin				33,790 ± 880	40,862-36,723
	SacA-25139
	GifA-80186/ Ninhydrin			Vulpes vulpes 24,070 ± 260	3.5 29,484-28,347
	SacA-12265
	GrA-53942 Longin 6-1 S. tatarica 29,640 +170/-160 3.3 34,481-33,168 OxA-25669 Ultrafiltration 32,200 ±
	450 38,430-35,505
	GifA-10021/ Ninhydrin			Cervus elaphus 31,320 ± 820	3.3 38,357-34,582
	SacA-19018
	GifA-11216/ Ninhydrin			S. tatarica	31,530 ± 670	3.4 37,735-34,749
	SacA-25133

nd: no data available. *: lowest collagen yield value (1% weight) of the sample set.

Table 4

 4 Intercomparisons of new calibrated 14 C dates for the Buran-Kaya III sequence, using the IntCal09 calibration curves[START_REF] Reimer | IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP[END_REF] OxCal online, Bronk Ramsey 2009a). Intercomparisons of new calibrated 14 C dates for the Buran-Kaya III sequence, using the IntCal09 calibration curves[START_REF] Reimer | IntCal09 and Marine09 radiocarbon age calibration curves, 0-50,000 years cal BP[END_REF] OxCal online, Bronk Ramsey 2009a). (Continued)

		Layer				Combination	R_Combined	Calibrated
	Lab code		Culture Species	14 C BP	name	(BP)	range (95.4%) Ai
	GrA-50461	4	Swide-Saiga ta-10,010 ± 60	Combine 4	10,027 ± 46 11,757-11,313 99
	GifA-11219/		rian	tarica	10,050 ± 70
	SacA-25135
	GrA-47316	5-2 Grave-Mammal 30,100 ± 180 Combine 5-2 30,386 ± 178 35,095-34,515 81
	GifA-11222/		ttian		33,790 ± 880
	SacA-25139
	GrA-53942	6-1		S. tatar-29,640 ± 170 Combine 6-1 30,120 ± 161 34,781-33,730 27
	OxA-25669			ica	32,200 ± 450
	Table 4 La-				Combination R_Combined Calibrated
	Lab code	yer Culture Species	14 C BP	name	(BP)	range (95.4%) Ai
	GrA-40485	6-3 Aurig-Equus cf. 34,050 ± 260 Combine	34,120 ± 251 40,078-38,508 98
	GifA-80181/		nacian caballus 34,910 ± 950 "Horse"
	SacA 12260
	GrA-48399	6-4		Saiga	31,250 ± 2450	Combine 6-4 33,199 ± 789 40,408-36,393 98
	GifA-11220/			tatarica	33,350 ± 830
	SacA-25137
	GrA-47318	6-5		Saiga ta-32,800 ± 230 Combine 6-5 33,025 ± 227 38,423-36,721 92
	GifA-11217/			tarica	35,900 ± 1100
	SacA-25134
	GrA-47319	B	Mico-Saiga ta-35,590 ± 290 Combine B 35,649 ± 282 41,536-40,166 98
	GifA-11218/		quian	tarica	36,500 ± 1200
	SacA-25135

  . Both specimens represent, together with Peştera cu Oase, Romania (Oase 1: 34,290 +970/ 870[GrA-22810];[START_REF] Trinkaus | An early modern human from the Peştera cu Oase, Romania[END_REF], the earliest occurrence of anatomically modern humans in eastern Europe; furthermore, the modern human remains from Buran-Kaya III are the most eastern ones (east of 34°E longitude).

Table 1 )

 1 , our new set of 24 14 C dates from 3 different laboratories shows a high consistency with the stratigraphy. Six cross-datings, out of the 7 performed, show a good reproducibility of the 14 C results.Our results support an early chronological framework for the Middle to Upper Paleolithic period in southeastern Europe. They show an old presence of Early Upper Paleolithic (Streletskaya or eastern Szeletian) populations in Crimea, older than 40.0 ka cal BP. This confirms a previous dating of 44.4-38.9 ka cal BP (36.7 ± 1500 ka BP, OxA-6868) of the Streletskaya or eastern Szeletian layer (C) in Buran-Kaya III[START_REF] Marks | A new Middle to Upper Paleolithic "transitional" assemblage from Buran-Kaya III, Level C, eastern Crimea: a preliminary report[END_REF]. This date was obtained on a bone industry artifact ("bone rod";[START_REF] Marks | A new Middle to Upper Paleolithic "transitional" assemblage from Buran-Kaya III, Level C, eastern Crimea: a preliminary report[END_REF]). It provided early evidence of this type of transitional industry.The results indicate a timespan of 44.0-40.2 ka cal BP for the presence of Late Middle Paleolithic populations in Crimea, which is much older than was previously proposed (28.5 ± 460 ka BP [OxA6674] and 28.8 ± 460 ka BP [OxA-6673], i.e. 34.6-31.7 ka cal BP,[START_REF] Marks | The Middle to Upper Paleolithic interface at Buran-Kaya-III, Eastern Crimea[END_REF][START_REF] Chabai | The Late Middle and Early Upper Paleolithic in Crimea (Ukraine)[END_REF]). Thus, our new 14 C dates for the Micoquian layers of Buran-Kaya III cast doubt on the existence of Neanderthal refugial survival in Crimea after 40 ka cal BP. The same conclusion has recently been reached at other Neanderthal refugial survivals in southern Spain (Zaffaraya;Wood et al. 2013), northern Caucasus (Mezmaiskaya Cave;[START_REF] Pinhasi | Revised age of late Neanderthal occupation and the end of the Middle Paleolithic in the northern Caucasus[END_REF], and southern Caucasus (Ortvale Kdle;[START_REF] Adler | Dating the demise: Neanderthal extinction and the establishment of modern humans in the southern Caucasus[END_REF]).

ACKNOWLEDGMENTS

We thank the National Academy of Sciences of Ukraine for permission to excavate at BuranKaya III and all the team members of the 2001 and 2009-2011 excavation seasons. We are grateful to A Lister (Oxford 14 C dates as part of a NERC-funded Megafaunal extinction project with A J Stuart), E Kaltnecker (LSCE/UVSQ, bone sample preparation), C Moreau (LMC-14, 14 C dating on Artemis), and T Higham (chemical information on a dated specimen in ORAU). The analyses were supported by the ANR "Mammouths" Research Program No. ANR-05-JCJC-0240-01 of the French National Research Agency (Agence Nationale de la Recherche), the ATM Program "Relations Sociétés-Nature dans le long terme" of the Muséum National d'Histoire Naturelle (MNHN, Paris), and the UPR 2147 of the Centre National de la Recherche Scientifique (CNRS). The MNHN and CNRS funded the 2009-2011 field seasons. Current field and lab research about Buran-Kaya archaeological sites are supported by the MNHN, the CNRS, the Fondation Fyssen (Paris), and the French Ministère des Affaires étrangères et européennes.


Alberto Bollero
email: alberto.bollero@imdea.org

Volker Neu

Vincent Baltz

David Serantes

José Luis

F Cuñado

Javier Pedrosa

Ester M Palmero

Marietta Seifert

Bernard Diény

Rafael P Del Real

Bernard Dieny

Manuel Vázquez

Oksana Chubykalo-Fesenko

Julio Camarero

An extraordinary chiral exchange-bias phenomenon: engineering the sign of the bias field in orthogonal bilayers by a magnetically switchable response mechanism

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Magnetic materials are playing a key role in our technological development and they are present in many technological sectors: energy, transport, aerospace, biomedicine, spintronics… 1 This latter area keeps indeed a close relation with all the others since mass storage media and magnetoresistive sensors find important functionalities in many technological control and monitoring devices. Paradoxically, these devices are based on a phenomenonthe so-called exchange-bias (EB)-which was discovered in 1956, 2 and whose physical origin remains poorly understood at present. 3,4 The rapid technological development and miniaturization of devices make necessary to advance in the knowledge and control of the interfacial effects responsible of the EB phenomenon to discover novel effects with added functionalities. 3 EB is frequently used in magnetic random access memories and magnetoresistive read heads to pin the magnetization of a reference layer. Setting EB is usually achieved by cooling a ferromagnetic (F)-antiferromagnetic (AF) bilayer under an applied field through the Néel temperature (TN) of the AF. 5 The EB phenomenon typically manifests itself as a unidirectional field shift (EB field) as well as a broadening of the hysteresis loop, i.e. an increased coercivity. Both coercivity and EB field can be controlled by fine-tuning the F anisotropy, the F-AF coupling and the angle of the applied magnetic field as shown in continuous and nanostructured films. 6,7 Usually a positive cooling field results in a shift of the F hysteresis loop toward negative fields (i.e., negative bias field). However, there are also reports showing the possibility of achieving a positive bias field. Nogués et al. 8 demonstrated the possibility of setting a positive exchange bias in FeF2 (AF)-Fe (F) bilayers by field cooling of the system in large magnetic fields. 8 This effect was attributed to a combination of an AF exchange at the AF-F interface and a F coupling of the AF surface spins to the cooling field above TN. A subsequent study on this same system, additionally including CoO (AF)/Co (F), done by Miltényi et al. 9 (and including coauthors of the previous work) and another research work carried out by Gökemeijer and Chien 10 on CoO (AF)/Ni81Fe19 (F) showed the possibility of tuning the sign of the bias field by cooling the system in zero field. 9 The post-annealing (field-or zero field-cooling) treatment typically needed to set EB has been recently shown to be avoidable in specifically designed systems such as FeCo/IrMn bilayers via a structural phase transition. Please do not adjust margins

Please do not adjust margins

The possibility of inducing EB-like effects has also been demonstrated in recent years in systems consisting of two coupled F materials with orthogonal anisotropies: [Pt/Co] multilayer (with out-of-plane anisotropy)-NiFe (with in-plane anisotropy). [12][13][14] A shift along the field axis in the in-plane hysteresis loop of the permalloy (NiFe) was obtained without necessity of a field cooling procedure. In a similar manner to the mechanism behind the EB phenomenon observed in the TbCo (ferrimagnetic)/NiFe system, 15,16 the EB field is a consequence of the coupling between NiFe and a net in-plane magnetization of the [Pt/Co] multilayer. Navas et al. 17 and Nguyen et al. 18 observed similar behavior in two F/F systems consisting of CoCrPt (out-of-plane easy axis)-Ni (in-plane) and [Co/Pd]5 (out-ofplane)-NiFe (in-plane), respectively. Experimental results in combination with micromagnetic simulations have shown that the loop shift magnitude in orthogonally coupled [Pt/Co]/NiFe is strongly dependent on the strength of the out-of-plane anisotropy of the [Pt/Co] multilayer. 19 This conclusion opened the door to explore the possibility of managing an effective tuning of the EB field in orthogonally coupled bilayers by proper choice of a hard magnet system with a strong out-of-plane anisotropy. That makes perpendicular epitaxial hexagonal SmCo5 an excellent candidate to be used in EB magnetically hard-soft bilayers as the hard magnet since it exhibits the largest uniaxial magnetocrystalline anisotropy of Ku=14 MJ/m 3 at room temperature as a bulk phase, 20,21 thus guaranteeing a stable out-of-plane magnetization in the hard layer when reversing the magnetization of the soft magnetic layer on top.

By comparison with the relatively large number of studies dealing with Sm-Co films with in-plane magnetic anisotropy, [22][23][START_REF] Fullerton | Please do not adjust margins[END_REF][25][26][27] those focused on out-of-plane Sm-Co films are rather scarce. 28,29 The fabrication of epitaxial Sm-Co films with out-of-plane anisotropy is of interest as part of the research and technological effort done on developing highly anisotropic materials for perpendicular recording media. The strong perpendicular anisotropy in these devices guarantees the stability of the stored information against thermal switching of the magnetization.

The present study combines a hard magnetic phase (SmCo5) and a soft magnetic phase (CoFeB). This combination of hard/soft bilayers provides model systems for studying the exchange-spring coupling. 30 Insight into the magnetization reversal in these bilayer systems can provide guidance for the optimization of metal/permanent magnet exchange-coupled composites. 30,31 In this work we will focus on the possibility of setting an extraordinary exchange bias-like effect -under specific conditions-when having a bilayer of this type with orthogonal magnetic anisotropies: SmCo5 (out-of-plane) and CoFeB (in-plane) separated by a spacer.

Experimental methods

Fabrication of the SmCo5 films by Pulsed Laser Deposition

SmCo5 films have been prepared on a heated substrate by UHV pulsed laser deposition (PLD) from elemental Ru, Sm, Co, and Cr targets in on-axis geometry (KrF excimer laser, 248 nm wavelength, 100 mJ energy per pulse, base pressure <5x10 -9 mbar). The SmCo5 films were prepared at 700°C on Al2O3 (0001) substrates using a Ru buffer layer (20 nm) and a Cr cover layer (2.5-11 nm) as an oxidation protection (Fig. 1a). Prior to film preparation, the deposition rates of the individual targets were measured with an Inficon XTM/2 rate monitor.

Growth of the CoFeB film by sputtering

Sputtering of an amorphous Co62Fe26B12 (3 nm) film on a Pt buffer layer was done with oblique incidence of the atomic flux in order to induce an in-plane uniaxial anisotropy. 32 This structure was capped by an additional protective Pt layer (Fig. 1d). All the CoFeB-based samples were grown at room temperature under a 0.25 Pa Ar pressure with deposition rates of about 0.08 nm s -1 .

Fabrication of the

Magneto-Optical Kerr Effect (MOKE) measurements

The MOKE system used in this study combines simultaneous acquisition of rotations of polarization and changes of reflectivity for a given sample orientation with respect to a variable external magnetic field, allowing full angular studies. 33 The sample was placed on a eucentric goniometer head to ensure a fixed plane of reflection upon sample rotation. The whole head could be rotated in the complete angular range 0-360° by a stepping motor in steps of 0.5°. A 5 mW HeNe laser (λ = 632 nm) was used. A Glen-Thompson polarizer with an extinction coefficient 1x10 -5 addressed the undefined polarization of the laser. Lenses were used to focus the light beam onto the sample as well as to focus the divergent beam after reflection. A λ/2 retarder, set to 22.5 of the optical axes, and a Wollaston prism with extinction coefficient 1x10 -5 were used. The intensities of the reflected waves were measured by two fast photodiodes incorporated into amplification electronics. The v-MOKE measurements were performed at room temperature with a 10 Hz triangular-shaped magnetic field ramp and a maximum field strength of 60 mT. For each angular condition the signal was averaged during 1 min (recording of over 600 loops at each angular condition). This setup provided sensitivity better than 1 rad and 10 -6 for Kerr rotation angles and reflectivity changes, respectively.

Results and discussion

Design of the multilayer structure

We have designed a multilayer system consisting of a hard magnet epitaxial SmCo5 film with a strong perpendicular anisotropy 28 and a

Please do not adjust margins Please do not adjust margins soft magnetic amorphous Co62Fe26B12 film with in-plane anisotropy interacting through a spacer. Fernández-Pacheco et al. 34 have reported a chiral bias in a system consisting of two coupled ultrathin magnetic layers made of Co and CoFeB. In our study, we demonstrate the possibility of obtaining a chiral EB effect by changing the orientation of the magnetization (upwards/downwards) of the SmCo5 layer. Well beyond that effect, our work presents the first experimental evidence of the possibility of controlling isothermally not only the magnitude but also the sign of the bias field in an unprecedented manner. The bias controlling mechanism is based on pinning exclusively one of the branches of the hysteresis loop (either the ascending or descending one at will) while displacing the second one along the field axis. This is managed by simple variation of the orientation of an external magnetic field applied parallel to the film surface (in-plane field) and with no need of any post-depositional treatment. Variation of the thickness of the spacer allows tuning the strength of the magnetostatic coupling between the soft and the hard magnetic layers, i.e. studying the proximity effect of both layers.

Inducing magnetic anisotropy in the ferromagnetic hard and soft layers

Prior to fabrication of the complete SmCo5/spacer/CoFeB structure, reference samples consisting on SmCo5 and CoFeB have been grown to check for the quality of the individual films.

A 30 nm thick SmCo5 epitaxial film was prepared by pulsed laser deposition (PLD) on Al2O3(0001) substrate using a Ru (20 nm) buffer layer. 28 The film structure was completed with the deposition of a Cr layer, with two different thickness values of 2.5 and 11 nm. The complete multilayer structure was Al2O3(0001)/Ru(20nm)/ SmCo5(30nm)/Cr(2.5-11nm) as schematically shown in Fig. 1a. Fig. 1c shows the quality of the perpendicular SmCo5 film with the c-axis perpendicular to the film plane (schematics in Fig. 1b). It is worth remarking the absence of a Cu buffer layer during the growth, by comparison with some previous works, [35][36][37][38] which has been avoided to maintain the high uniaxial anisotropy of the pure SmCo5 phase. 28 It must be considered that the use of a Cu buffer layer is beneficial to decrease the crystallization temperature, increase the stability of the SmCo5 phase and promote the out-of-plane orientation, but on the other hand it decreases the intrinsic anisotropy of the SmCo5 phase. Further details on the growth of the SmCo5 film used in this work have been published elsewhere. 28 A large uniaxial magnetocrystalline anisotropy constant of about Ku=7.5 MJ/m 3 was determined for SmCo5 at room temperature in the out-of-plane configuration, resulting in a coercivity above 1 T (see Fig. 1c). Sputtering of an amorphous CoFeB (3 nm) film on a Pt buffer layer was done with oblique incidence (Figs. 1d,e) of the atomic flux in order to induce an in-plane uniaxial anisotropy. 39 This structure was capped by an additional protective Pt layer. Fig. 1f shows successful inducement of a preferred magnetization (easy axis) direction in the CoFeB film. Please do not adjust margins Please do not adjust margins

Effect of the exchange-bias phenomenon on the coercivity of the orthogonally-coupled bilayers

Once verified the magnetic quality of the magnetically hard (SmCo5) and soft (CoFeB) films, the complete multilayer structure was prepared by growing first the SmCo5 structure shown in Fig. 1a followed by transfer of the sample to the sputtering chamber to complete the system by growing the Pt(1.8nm)/CoFeB(3nm)/Pt(1.8nm) multilayer. The Cr layer protected the SmCo5 against oxidation during ex-situ transfer from the PLD to the sputtering chambers. A detail of the complete SmCo5/spacer/CoFeB multilayer structure is shown in Fig. 2, comprising Al2O3(0001)/Ru(20nm)/SmCo5(30nm)/spacer(tspacer)/ CoFeB(3nm)/Pt(1.8nm) with a total thickness of the spacer tspacer = 4.3 and 12.8 nm. The spacer is based on Cr(tCr)/Pt(1.8nm) (with tCr = 2.5 and 11 nm, respectively). A direct consequence of using a spacer between the two ferromagnetic layers is the possibility of applying relatively low in-plane fields to study systematically the angular dependence of the magnetization reversal in CoFeB by Magneto-Optical Kerr Effect (MOKE). This has been done by rotating the sample in a full angular range (0-360°) in presence of an external in-plane magnetic field with a maximum strength of 60 mT.

The EB phenomenon typically results in a broadening of the hysteresis loop, i.e. enhanced coercivity, and a unidirectional field shift. In order to show the effect on coercivity due to the magnetostatic coupling between the magnetically soft and hard layers, Fig. 2 shows the hysteresis loops after proper centering along the field axis measured by MOKE for Al2O3(0001)/Ru(20nm)/SmCo5(30nm)/spacer(tspacer)/CoFeB(3nm)/Pt (1.8 nm) with tspacer = 4.3 and 12.8 nm and, for the aim of comparison, the hysteresis loop of the CoFeB reference film [Si/SiO2/Pt(1.8nm)/CoFeB(3nm)/Pt(1.8nm)] grown with oblique incidence. This representation allows observing an enhanced coercivity of 8.5 mT for the multilayer containing a thinner spacer (4.3 nm), i.e. CoFeB closer to the SmCo5 layer, by comparison with 2.6 mT measured for the multilayer comprising a thicker spacer (12.8 nm). This latter value is still higher than the coercivity (1.5 mT) measured for the single CoFeB reference layer and shows the effect that the magnetostatic coupling of CoFeB to SmCo5 has on increasing coercivity. A thinner spacer results in an enhanced coercivity due to a stronger coupling between the SmCo5 and CoFeB layers, which makes necessary application of a stronger external field to reverse the magnetization of the CoFeB layer.

The angular evolution of the coercivity has been studied by applying an external in-plane magnetic field to the multilayer system while rotating in a full 0-360° angular range (see Fig. 3). An angle of the external applied field H = 0° has been chosen as that corresponding to the application of the field parallel to the easy axis direction of the Si/SiO2/CoFeB reference sample (Figs. 1d-f), i.e. in absence of the influence exerted by the magnetically hard SmCo5 film. When using a thinner spacer, i.e. decreasing the separation distance between the hard and the soft magnetic layers, the maximum of coercivity of the combined SmCo5/spacer/CoFeB system points 90° rotated with respect to the easy axis of the single CoFeB film (Fig. 3a). Similar observation related to the rotation of the easy axis direction has been previously done by Camarero et al. 39 for two F layers (Ni80Fe20 and Co) coupled through a NiO layer.

In that system an in-plane uniaxial anisotropy was induced on the Co layer resulting in an in-plane perpendicular coupling between both layers, with the hard axis of Co corresponding to an easy axis for Ni80Fe20. The authors demonstrated that this effect was the result of the combination between interfacial roughness and the small value of the effective NiO anisotropy. 39 The mechanism behind the effect observed in the system under study might be related to the homogeneous spiraling spin structure considered in Slonczewski's model 40 or, mostlikely based on the analogies between the two ferromagnet/spacer/ferromagnet systems, to the formation of partial spirals in the spacer layer going from one ferromagnet interface to the other. 39 Further work is in progress to discriminate the precise mechanism behind the observed effect. Indeed, and additionally supporting the given argument, Fig. 3b shows that an increased thickness of the spacer tends to reorient the maximum of coercivity of the SmCo5/spacer/CoFeB system towards the uniaxial anisotropy induced during the growth of the CoFeB layer. The angular evolution measured for the CoFeB layer (Fig. 3c) agrees well with that of a thin film with a well-defined uniaxial anisotropy. 33 This may be understood considering that the strength of the magnetostatic coupling between both soft and hard magnetic layers is diminished with increasing the separation layer between both of them. On this basis, the maximum coercivity for a sufficiently thick spacer is foreseen to lie along H = 0° since for such situation the CoFeB layer will not be subjected to the magne-

Please do not adjust margins

Please do not adjust margins tostatic field of the SmCo5 layer, and thus will follow the uniaxial anisotropy that was induced during its growth (analogous configuration to that shown in Fig. 3c). The diminished saturation field of the CoFeB layer with increasing the thickness of the spacer (Fig. 2) also corroborates the weaken coupling. In order to understand the requirements needed for appearance of this phenomenon, two different types of samples were additionally prepared: isotropic CoFeB layer grown under vertical incidence onto magnetized SmCo5 with a separating spacer (Supplemental Fig. S2); and anisotropic CoFeB layer (oblique incidence) onto demagnetized SmCo5 film with a separating spacer (Supplemental Fig. S3). In both cases, when measuring the angular evolution of coercivity, both multilayer SmCo5/spacer/CoFeB samples were isotropic. Accordingly, and as a first conclusion, the competition between the uniaxial anisotropy induced in the CoFeB layer and the dipolar field created by the out-of-plane magnetization of the SmCo5 layer is mandatory to guarantee the appearance of the reported phenomenon.

Novel modulation of the bias field in the orthogonally coupled bilayers

We proceed in the following to study the influence of the spacer thickness, i.e. modulation of the CoFeB-SmCo5 coupling, on the EBlike phenomenon. Fig. 4 shows representative hysteresis loops measured with the external magnetic field applied at different inplane angles, H, and corresponding to the angular regions (I-IV) shaded in Fig. 3. Figs. 4a,b relate to the multilayer system comprising the thinnest spacer in this study. First important remark is that no EB effect should be expected in this system based on the mutually orthogonal orientation of both magnetic layers (CoFeB oriented in-plane, and SmCo5 oriented out-of-plane). The explanation to address the appearance of the EB-like phenomenon in this system comes from the formation of an in-plane magnetization component in the SmCo5 film, which results in a coupling effect between the in-plane magnetization of the CoFeB and such an in-plane component of the SmCo5 film. Previously reported systems comprising orthogonally coupled F bilayers, [12][13][14]17 showed that the EB field was a consequence of an artificially created domain configuration with an effective in-plane magnetic moment in the hard magnetic layer at the interface, thus leading to the coupling between the magnetic moments of both soft and hard layers. Experimental results in combination with micromagnetic simulations 19 showed that the magnetic configuration responsible of the measured loop shift was the result of the exchange-coupling between the magnetization of the soft magnetic layer and the magnetization of vortex cores, which formed in the domain walls separating upwards and downwards magnetized domains in the hard magnetic multilayer. This cannot be the explanation behind the phenomenon observed in the system under study because the hard magnetic layer (epitaxially grown SmCo5) is in a single-domain state, which in this particular case invalidates the argument referred to the formation of any closure domains at the interface. However, a small misfit with the substrate and/or interfacial roughness 39 cannot be disregarded and would result in a tilt of the out-of-plane magnetization in SmCo5 and thus in the appearance of an in-plane component responsible of the coupling between CoFeB and SmCo5. Indeed, the large lattice misfit of 8% between SmCo5 and the Ru buffer layer is expected to be accommodated by misfit dislocations, which will result in a granular film morphology with a roughness that can achieve 2-3 nm, as previously reported by authors of this work for a comparable SmCo5 thin film (28 nm). 28 28 ) and its extremely high uniaxial anisotropy. However, it is the tilt that the dipolar field of the SmCo5 film will experience when growing the soft magnetic layer on top of it (moreover with a spacer between them), which might play the main role behind the origin of the bias effect here reported. Anh Nguyen et al. 41 showed for a similar F (out-of-plane)/F (in-plane) system, and consisting of [Co/Pd]n-FeNi, an increased tilt of the magnetization of the FeNi layer with increasing the thickness of this magnetically soft layer. In particular, they reported a threshold of the NiFe thickness (> 5nm) below which magnetic force microscopy (MFM) images were essentially featureless (i.e., a large single remanent domain with predominant out-of-plane magnetization) and above which contrast variations appeared in the MFM images suggesting some tilting of the magnetization away from the film normal. 41 In the system under study we are able of tuning the tilt of the magnetization in the CoFeB layer by varying the thickness of the spacer between the hard and soft films. In other words, an increased thickness of the spacer between the magnetically hard and soft layers will weak the strength of the dipolar field at the position of the soft magnetic layer and thus will have an increased in-plane component. Accordingly, and in a similar manner to the study reported on [Co/Pd]n-FeNi, we have found contrast variations in remanent state MFM images for the two SmCo5/spacer/CoFeB systems, suggesting a tilt in the magnetization of the CoFeB film (Supplemental Fig. S4).

It is observed that application of an in-plane external field in the 0-90° angular range (region I, Fig. 3a) results in approximately pinning of the descending branch of the hysteresis loops, while the ascending branch evolves to higher positive field values with increasing the angle. This effect results in a transition from a negative to positive bias field going through zero (i.e., no EB effect). Application of the in-plane external magnetic field in a 180-270° angular range (region II, Fig. 3a) shows the same effect but with opposite evolution of the two branches of the hysteresis loop, i.e. approximate pinning of the ascending branch while the descending one evolving to higher negative field values with increasing the angle (Fig. 4b). As a result, a transition from positive to negative bias field may be managed in this system by simple variation of the orientation of the in-plane applied magnetic field. Compared to the coercivity of the hysteresis loop, the described effect is significantly more pronounced when increasing the separation between the CoFeB and the SmCo5 layers, as it is shown in Figs. 4c,d. Again, and taking Fig. 4c as an example (since Fig. 4d is the symmetrical equivalence), the bias field can be tuned from negative to positive values going through zero bias. This result suggests that the interplay between the uniaxial anisotropy of the CoFeB layer and the unidirectional anisotropy (EB-like phenomenon) is responsible for both the amplitude of the bias field and the unusual change in its sign from negative to positive, and vice versa, depending on the in-plane incidence angle of the externally applied magnetic field. sample with the thinnest and the thickest spacer (Figs. 5a,b). Independently of the thickness of the spacer layer, the loop shift has 360° symmetry while coercivity has 180° symmetry, which is the well-known result of the unidirectional nature of the exchange bias phenomenon. 33,42 However, with increasing the thickness of the spacer, the magnetostatic coupling between the CoFeB and the SmCo5 layers is diminished and, accordingly, the uniaxial anisotropy of CoFeB begins to play a relevant role as it may be observed by the presence of two maxima (minima) in the angular region H = 0-180° (H = 180-360°). As a consequence, the cosine angular dependence of the bias field, 42 related to the unidirectional anisotropy, typically observed in EB systems, becomes frustrated by an increased influence of the uniaxial anisotropy of the CoFeB layer (Fig. 5b).

Chiral exchange-bias phenomenon

An additional multilayer structure was grown in the same fashion as detailed for the previous two samples to proof that a bias field can be induced in an orthogonally coupled SmCo5(out-ofplane)/spacer/CoFeB(in-plane) system independently of using a material different to Chrome (Cr) as constituent of the spacer. With this aim a sample with a spacer comprising Tantalum (Ta) instead of Cr was grown, specifically with the following structure: Al2O3(0001)/Ru(20nm)/SmCo5(30nm)/Ta(3nm)/Pt(1.8nm)/CoFeB(3n m-oblique)/Pt(1.8nm). Fig. 6a shows a room temperature MOKE hysteresis loop measured with the external in-plane magnetic field applied along H = 90°, where a shift of the hysteresis loop towards negative field values is clearly distinguishable. Consequently, and independently of using Cr or Ta, a bias field is induced in this orthogonally coupled system. In a second step, and in order to proof the control that the orientation of the magnetization of the SmCo5 layer exerts on the EB-like effect phenomenon, we decided to switch the magnetization orientation in the SmCo5 layer from upwards to downwards by application of an external magnetic field of 2 T perpendicular to the film surface as schematically illustrated in Figs. 6a,b, respectively. The effect of switching the magnetization state in SmCo5 (from upwards to downwards) on the sign of the bias field (from negative to positive) is straightforward as observed from a direct comparison of the recorded hysteresis loops (H = 90°) shown in Fig. 6. Coercive and bias fields take the same values in both situations, respectively, although the sign of the bias field is reversed when magnetizing the SmCo5 layer in opposite sense. This result points to the main role played by the dipolar field created by the SmCo5 layer in defining the initial sign of the bias field in the SmCo5-CoFeB system.

Conclusions

In summary, exceptional exchange bias-like effects have been observed in a system consisting of a hard magnet epitaxial SmCo5 film with out-of-plane anisotropy in interaction with a soft magnetic amorphous CoFeB film with in-plane anisotropy through a spacer.

By contrast with conventional EB films, this system shows extraordinary features induced by application of an external This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins magnetic field and, by contrast with typical EB systems, with no need of any post-growth annealing treatment:

(i) Possibility of tuning not only the magnitude, but also the sign of the bias field (with no need of any post-annealing treatment). This is achieved in a manner not reported to date for any exchange bias system: pinning exclusively one of the hysteresis loop branches (either the ascending or the descending branch) while simultaneously displacing the second branch along the field axis. As a consequence the bias field can be modulated from negative to positive values going through zero (i.e., no bias). This mechanism is reversible and the choice of the pinned branch is done by selecting the orientation of an external magnetic field applied parallel to the film surface.

(ii) Possibility of frustrating the cosine angular dependence of the bias field by modifying the magnetostatic coupling strength through variation of the thickness of a spacer between the CoFeB (in-plane) and the SmCo5 (out-of-plane) layers.

(iii) Chiral exchange-bias phenomenon: switching the orientation of the magnetization (upwards/downwards) in the SmCo5 film allows for fixing the initial sign of the bias field in the SmCo5/spacer/CoFeB system.

A systematic study done by changing the magnetization states of both hard and soft magnetic layers has confirmed that the observed effects are due to the interplay between the uniaxial anisotropy of the soft magnetic layer and the bias field imprinted in the soft layer by the magnetostatic field provided by the hard layer. We have proven that the combination of the uniaxial anisotropy induced in the CoFeB layer and the dipolar field created by the out-of-plane magnetized SmCo5 layer is mandatory to guarantee the appearance of this novel phenomenon. Variation of the thickness of the CoFeB layer might be an additional factor to modulate the bias field reported in this study. The observed effect is of interest for applications in the development of magnetic read-heads and magnetic sensors as it allows for a controlled modulation of both the bias field strength and the sign in a manner not reported to date, thus opening the path to a new generation of spintronic devices.

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Fig. 1 .

 1 Fig. 1. Reference SmCo5 and CoFeB multilayers: (a) Al2O3(0001)/Ru(20nm)/SmCo5(30nm)/Cr(2.5-11nm) structure grown by PLD with the c -axis perpendicular to the film surface as schematically shown in (b); (c) hysteresis loops measured with the field applied perpendicular (out-of-plane) and parallel (in-plane) to the surface. (d) Si/SiO2/Pt(1.8nm)/CoFeB(3nm)/Pt(1.8nm) structure with the CoFeB layer grown by sputtering with oblique incidence (schematically shown in (e)) in order to induce a uniaxial anisotropy, responsible of setting a preferential magnetization (easy axis) direction; (f) hysteresis loops measured with the applied field along the easy (defined as H=0°) and hard (H=90°) axis directions of the CoFeB film.

Fig. 2 .

 2 Fig. 2. Centered in-plane hysteresis loops measured by MOKE for CoFeB reference sample [Si/SiO2/Pt(1.8nm)/CoFeB(3nm)/Pt(1.8nm)] grown with oblique incidence (solid black line); Al2O3(0001)/Ru(20nm)/SmCo5(30nm)/ spacer(tspacer)/CoFeB(3nm)/Pt(1.8 nm) with tspacer = 4.3 nm (close symbols) and 12.8 nm (open symbols). Spacer comprises Cr(tCr)/Pt(1.8nm) [with tCr = 2.5 and 11 nm, respectively]. Schematic representation of the complete multilayer structure is displayed in inset.

Fig. 3 . 6 |

 36 Fig. 3. Angular evolution (polar-plot representation) of coercivity and schematic representation of the corresponding multilayer structure (bottom part) for: (a) SmCo5(30nm)/spacer(4.3nm)/CoFeB(3nm); (b) SmCo5(30nm)/spacer(12.8nm)/CoFeB(3nm); and (c) CoFeB(3 nm) reference film (equivalently represented by an infinite thickness of the spacer). H = 0° corresponds to application of the external magnetic field parallel to the easy axis direction of the Si/SiO2/CoFeB reference sample as indicated in (c). Representative hysteresis loops for shaded regions (I and II; III and IV) are shown in Fig. 4.

  Figs. 5a,b show the complete angular evolution of the bias field and the coercivity for the two samples under study, with indication of the angular regions from which the selected hysteresis loops shown in Fig. 4 were extracted. Another interesting observation comes from the different behavior of the bias field when comparing the

Fig. 4 .

 4 Fig. 4. In-plane hysteresis loops measured by MOKE with magnetic field applied at different in-plane angles, H, and corresponding to colored regions I-II and III-IV in Figs. 3a,b, respectively, for: (a) and (b) SmCo5(30nm)/spacer(4.3nm)/CoFeB(3nm); and (c) and (d) SmCo5(30nm)/spacer(12.8nm)/CoFeB(3nm).H = 0° corresponds to application of the external magnetic field parallel to the easy axis direction of the Si/SiO2/CoFeB reference sample. Arrows are visual guides to show the continuous increase in coercivity when varying H while maintaining one of the loop branches pinned.

Fig. 5 .

 5 Fig. 5. Angular evolution of the loop shift (close symbols) and coercivity (open) vs field angle, H, for SmCo5(30nm)/spacer/CoFeB(3nm) with a thickness of the spacer (a) 4.3 nm and (b) 12.8 nm. Shaded regions illustrate regions I-II and III-IV indicated in Figs. 3a,b, respectively. Angle H = 0° corresponds to application of the external field parallel to the easy axis direction of the Si/SiO2/CoFeB reference sample. Dash line is a visual guide indicative of H = 180°.

Fig. 6 .

 6 Fig. 6. In-plane hysteresis loops measured by MOKE for Al2O3(0001)/Ru/SmCo5(30nm)/Ta(3nm)/Pt(1.8nm)/CoFeB(3nm)/Pt(1.8nm) after application of an out-of-plane magnetic field of 2 T upwards (a) and downwards (b), respectively. A symmetric break of ±2 mT has been introduced in the field axis to allow for a better comparison. Images illustrate schematic representation of the multilayer structure and the two different orientations of the magnetization in the SmCo5 layer.

SmCo5/spacer/CoFeB multilayer structure

	Once verified the magnetic quality of the magnetically hard (SmCo5)
	and soft (CoFeB) films, the complete multilayer structure was
	prepared by growing first the SmCo5 structure followed by transfer
	of the sample to the sputtering chamber to complete the system by
	growing the Pt(1.8nm)/CoFeB(3nm)/Pt(1.8nm) multilayer. The
	parameters used for the growth of the SmCo5 and the CoFeB
	structures were identical to those previously described. The Cr layer
	protected the SmCo5 against oxidation during ex-situ transfer from
	the PLD to the sputtering chambers. The SmCo5/spacer/CoFeB
	multilayer structures consisted of: Al2O3(0001)/Ru(20nm)/
	SmCo5(30nm)/spacer(tspacer)/CoFeB(3nm)/Pt(1.8nm) with a total
	thickness of the spacer tspacer = 4.3 and 12.8 nm. The spacer was
	based on Cr(tCr)/Pt(1.8nm) (with tCr = 2.5 and 11 nm, respectively).

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Acknowledgements

This research was supported by the Joint German-Spanish Actions Programme (DAAD and Fundación Universidad.es via Ref. 57050243), the Spanish Ministerio de Economía y Competitividad (MINECO) through "SIESPER" (MAT2011-25598) Project, and the Regional Government of Madrid through NANOMAGCOST (P2018/NMT-4321) project. D.S. acknowledges financial support from Xunta de Galicia under the postdoctoral program I2C Plan (Modalidade B). IMDEA Nanoscience is supported by the 'Severo Ochoa' Programme for Centres of Excellence in R&D, MINECO [grant number SEV-2016-0686].

Conflicts of interest

There are no conflicts to declare.