Pierre Bourhis

Gianluca Demartini

Shady Elbassuoni

Emile Hoareau

Raghav Rao

H Rao

Ethical Challenges in the Future of Work

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The rise of online piecework platforms such as Amazon Mechanical Turk1 and Uber2 are profoundly changing the way we conduct work. These platforms typically link requesters to workers from all over the world in a scalable manner, providing the opportunity to accomplish work in an unprecedented way. However, while these transformations offer numerous opportunities to both stakeholders, they also bring threats especially for workers, who are more vulnerable against requesters' demands and platforms' setups. The traditional work environment is highly regulated through law to avoid power imbalance and thus abuses from the more powerful actor, the employer, can be avoided. Online work platforms remain, at this time, less subject to strong regulations, which could then foster unfair situations. Several studies have already pointed out the potential drawbacks of the practices of online work platforms especially towards workers [START_REF] Kittur | The future of crowd work[END_REF]. The different roles and activities offered by online work platforms (e.g., requesters, contract workers, micro-tasking, etc.) make the creation of an ethical and fair work environment challenging. Indeed, while the main role of a platform is to allocate workers to available jobs, it also needs to ensure work payments and the security of both requesters and workers.

In the future, work facilitated by online platforms may become unfair or harmful in the absence of a strong and deep ethical reflection. An ethical approach in this context is defined as stakeholders in the "loop of work" doing the "right things" based upon rationally justifiable standards. It is important to ensure that any artifact in the loop should not be used in ways that can harm people, the environment, and society. As such, each artifact would have to follow certain legal, regulatory, and ethical frameworks. It is important to ask questions about the relationship between humans and computational entities in terms of what are the implications of replacing and supplementing humans with machines, and also try to foresee both utopian and dystopian future-of-work scenarios in order to be prepared for both an optimistic future as well as a pessimistic future. The role of work platform in the relationship between the job providers and the workers has to be better defined and regulated as compared to the current situation. Indeed, platforms could play several roles: putting in relation requesters and workers, transferring payments between them, ensuring a virtual platform where to do work. These roles have to be defined precisely and need to follow relevant legal obligations. The notion of work contract in this context becomes complex and often not well defined, as it puts in relation three parties (i.e., job providers, workers, and platforms) and in different contexts (e.g., online and offline). Requesters, workers, and platforms can also be in different countries and the question of which work legislation should apply is complex.

In this paper, we point out that practices of online work platforms may raise ethical issues which must be handled to aspire for a sustainable Future of Work (FoW). We focus on issues related to privacy, accountability, fairness (with regard to compensation and job allocation for instance), transparency and explainability. We offer a detailed view of each of them, presenting existing research and suggestions about future directions.

We first detail them, presenting how they appear today and how they are addressed by existing scientific literature. Then, we suggest a path forward, for each of them. We focus on ethics in this case, as it would apply to the stakeholders consisting of the "worker", the "requester" and the "platform". The rise of online work platforms raises several major ethical issues: (i) privacy/access control -ensuring that the use of the personal data is not used for harmful purposes; loss of personal information can result in negative consequences which could have an effect on physical and mental health and hence access control becomes an important tool for the protection of privacy, (ii) accountability -identification of outcomes and procedures and association with the appropriate entities, with the goal of not violating system or organizational policies and rules for compliance, (iii) fairness -in terms of equity or equality of decision outcomes such as compensation to workers, (iv) transparency of decision making processes including rejected job applications, decision on task assignment, and on reward allocation, (v) and explainability of processes involved. These last two focus on the degree of justification and truthfulness regarding explanations and information provided in the process of work as well as the rewards and the assignments.

Related Work

Several researchers have started to explore online work platforms especially in the context of crowdsourcing. Crowdsourcing involves the distribution of tasks to various people, often across the world, via the Internet. The strength of crowdsourcing is that a variety of skills are instantly available and work can be conducted online in any language. From the point of view of these piecework workers, while some may be working at hours when they are not in their official work capacity, in order to make some extra money, for others, like Uber drivers, the work may actually be a full time job. In this section we review some of the related work regarding work condition and ethics of crowdsourcing.

Deng et al. [START_REF] Deng | The duality of empowerment and marginalization in microtask crowdsourcing: Giving voice to the less powerful through value sensitive design[END_REF] explore microtask crowdsourcing as perceived by crowd workers, revealing their values as a means of informing the design of such platforms. Analyzing detailed narratives of 210 crowd workers participating in Amazon's Mechanical Turk, they uncovered a set of nine values they share: access, autonomy, fairness, transparency, communication, security, accountability, making an impact, and dignity. They found that these values are implicated in four crowdsourcing structures: compensation, governance, technology, and microtask. Two contrasting perceptions-empowerment and marginalization-coexist, forming a duality of microtask crowdsourcing. Their study heightens awareness of worker marginalization in microtask crowdsourcing, and offers guidelines for improving crowdsourcing practice. Specifically, they offer recommendations regarding the ethical use of crowd workers (including for academic research), and call for improving platform design for greater worker empowerment.

Adda et al. [START_REF] Adda | Crowdsourcing for Speech: Economic, Legal and Ethical analysis[END_REF] discussed the issue of compensation (monetary or otherwise) for completed tasks on crowdsourcing platforms. They also discussed the ethical and legal issues raised when considering work on crowdsourcing platforms as labor in the legal sense. They used crowdsourcing for under-resourced languages as a case study to exemplify the different issues they discussed. Finally, they proposed some specific solutions for researchers who wish to use crowdsourcing in an ethical manner.

Gellman [START_REF] Gellman | Crowdsourcing, citizen science, and the law: legal issues affecting federal agencies[END_REF] reviewed legal and regulatory issues that federal agencies face when they engage in citizen science and crowdsourcing activities. His report identified relevant issues that most federal agencies must consider, reviewed the legal standards, suggested ways that agencies can comply with or lawfully evade requirements, and discussed practical approaches that can ease the path for federal citizen science and crowdsourcing projects, including procedural activities, cooperative actions, legislative changes, and regulatory adjustment.

Brey [START_REF] Brey | Disclosive computer ethics[END_REF] provided a critique of mainstream computer ethics and argued for the importance of a complementary approach called disclosive computer ethics, which is concerned with the moral deciphering of embedded values and norms in computer systems, applications and practices. Also, four key values were proposed as starting points for disclosive studies in computer ethics: justice, autonomy, democracy and privacy. Finally, it was argued that research in disclosive computer ethics should be multi-level and interdisciplinary, distinguishing between a disclosure level, a theoretical level, and an application level.

Schmidt [START_REF] Schmidt | The good, the bad and the ugly: Why crowdsourcing needs ethics[END_REF] discussed some of the ethical implications of crowdsourcing in general and of contest-based crowd design in particular, especially in regard to the question of fair payment. He established four different categories of crowdsourcing with separate ethical challenges and argues for the crowd work industry to develop a code of ethics from within, in order to counter the exploitation and abuse that it often enables. In [START_REF] Salehi | We are dynamo: Overcoming stalling and friction in collective action for crowd workers[END_REF] authors proposed and tested such a bottom-up approach to defining a code of practice for crowdsourcing platform use which resulted in a set of guidelines for requesters to provide fair work conditions to Mechanical Turk workers.

Williamson [START_REF] Williamson | On the ethics of crowdsourced research[END_REF] examined the ethics of crowdsourcing in social science research, with reference to her own experience using Amazon's Mechanical Turk. She pointed out that many people work long hours completing surveys and other tasks for very low wages, relying on those incomes to meet their basic needs. She resented her own experience of interviewing Mechanical Turk participants about their sources of income, and she offered recommendations to individual researchers, social science departments, and journal editors regarding the more ethical use of crowdsourcing. In [START_REF] Hara | A data-driven analysis of workers' earnings on amazon mechanical turk[END_REF], authors present a data-driven study of crowd worker wages showing how, on average, Mechanical Turk workers earn about $2 per hour while only 4% earns more than the US federal minimum wage.

Ford et al. [START_REF] Ford | Crowdsourcing: A new way of employing non-employees[END_REF] reviewed the various ways organizations employ non employees to overcome human resource limitations. They then focused on crowdsourcing as a novel source of external labor. After presenting key questions that every organization considering the use of crowdsourcing must address, they offered specific recommendations for those organizations who choose to employ a crowd to meet their needs.

Saxton et al. [START_REF] Saxton | Rules of crowdsourcing: Models, issues, and systems of control[END_REF] provided a practical yet rigorous definition of crowdsourcing that incorporates crowds, outsourcing, and social web technologies. They then analyzed 103 well-known crowdsourcing websites using content analysis methods and the hermeneutic reading principle. Based on their analysis, they developed a "taxonomic theory" of crowdsourcing by organizing the empirical variants in nine distinct forms of crowdsourcing models. They also discussed key issues and directions, concentrating on the notion of managerial control systems.

Garber et al. [START_REF] Graber | Internet-based crowdsourcing and research ethics: the case for irb review[END_REF] demonstrated that the crowdsourcing model of research has the potential to cause harm to participants, manipulates the participant into continued participation, and uses participants as experimental subjects. They concluded that protocols relying on this model require institutional review board (IRB) scrutiny.

Harris [START_REF] Harris | Dirty deeds done dirt cheap: a darker side to crowdsourcing[END_REF] explored the potential for which crowdsourcing may be used to bypass commonly-established ethical standards for personal or professional gain. Adda et al. [START_REF] Adda | Crowdsourcing for language resource development: Critical analysis of amazon mechanical turk overpowering use[END_REF] demonstrated that the situation in crowdsourcing is far from being ideal, be it from the point of view of quality, price, workers' status or ethics. Their goal was threefold: 1-to inform researchers, so that they can make their own choices with all the elements of such reflection in mind, 2-to ask for help from funding agencies and scientific associations, and develop alternatives, 3-to propose practical and organizational solutions in order to improve new language resources development, while limiting the risks of ethical and legal issues without letting go of price or quality.

In this paper we present a summary of the current and envisioned future forms of work taking a stand on the ethical challenges that will need to be faced, including issues of transparency, fairness, and bias.

Open Issues in Current Platform-based Work

Empowering workers and protecting their rights and privacy should be at the heart of the Future of Work (FoW). This is a critical challenge as, while work platforms have a global reach, policies and regulations remain local for the most part. Advances in cybersecurity can be used to address privacy and access control mechanisms to guarantee that the right actors have visibility of the right data. Platforms should provide different privacy settings and be transparent about what data about workers is exposed and to whom. Requesters should be transparent about what the purpose of work is, and how the work outcome will be utilized. They should also be able to protect their confidential information when needed and to protect the copyrights and intellectual property of the work done by workers. Fair compensation for workers, including base payments, bonuses, benefits and insurance should be guaranteed and regulated by law. Workers should have the freedom to choose the compensation type they deem acceptable. Finally, job allocation should be transparent, fair and explainable by design. Worker's sensitive attributes that might bias the job allocation process should be protected. Auditing mechanisms to ensure compliance with fair, transparent and explainable job allocation and compensation need to be developed and adopted. Even though these ethical approaches are well known in the context of traditional work settings, these are more complex when applied through virtual platforms such as crowdsourcing. Workers are not considered as employees of the platforms but rather as self-employed workers that are paid for a one-off service. The question of which type of relation exists between workers and platforms is complex as different countries and states legislate differently on this issue. For example, California would consider drivers of Uber as employees. Platforms have also some other legal obligations such as the security of workers or requesters. For example, Uber has been banned from London because it was not able to guarantee the security of the drivers and customers.

In terms of fairness, an interdisciplinary approach will be required to develop novel methods to assess and quantify algorithmic fairness in job allocation practices. For example, looking at bias trade-offs between fullyalgorithmic vs human-in-the-loop job allocation methods where algorithmic bias could lead to different issues as compared to implicit bias in humans. This will also result in higher levels of algorithmic transparency for job allocation where decisions should be easy to explain independently of whether they have been made by humans or by AI models. Processes and procedures should be in place to specify how to best address unfair cases, e.g., by means of additional rewards for workers or novel/better job opportunities. We also envision novel methods to make job allocation distribution (i.e., the long tail effect where few workers complete most of the available jobs) and time spent on jobs more transparent to workers and external actors like compliance agents. For example, visual analytics dashboards that communicate to workers how much time they spent and how much money they have earned on a platform, with warnings on risks for addiction or on unfair payments, or how transparent the requesters are with regard to the rules and procedures regarding compensation, for instance, are important in the FoW space. The power of the platform to delete the account of a worker or to prioritize particular workers over others creates difference in the work relations among the platform. These decisions should not only be explained, but processes should be put in place for workers and work providers to contest these unilateral platform decisions.

To summarize, in most of online work platforms, workers are currently poorly protected from the pressure of platforms and work providers, thus creating a power imbalance that puts workers in an unfavourable condition with limited or no contractual power. This situation raises a growing number of ethical issues related to privacy/access control, fairness and compensation.

First, the level of privacy is currently quite low since workers do not have control over their own data and their final work output. As IP regulations are non clear or almost non-existent, transfer of property rights is currently in favour of requesters which use the results/outputs of the crowd as they wish. Moreover, data relating to workers (personal information and online behaviour, for instance) are under the complete control of the platform and of the requesters.

Second, workers' compensation is limited. In some circumstances, workers are ready to work for free as they find some intrinsic motivation to do the job (pleasure to do a fun task, satisfaction to take part in a social/humanitarian/scientific project, willingness to be recognized by peers or the need to develop human and social capital). However, voluntary work also raises a number of issues: how to recruit enough volunteers? How to ensure work quality? How to mobilize them for a specific task within a specific time frame? Most of the time, workers are compensated by means of task-based payments. Here also, a huge number of issues emerge regarding this compensation mechanism. Task-based payment often leads to very low income, which is typically under the official minimum wage of the worker's country as the workers cannot negotiate their compensation. In addition, workers do not have the usual work benefits they would typically have in traditional work such as luncheon vouchers and health insurance, for instance. Finally, workers have no social security as they do not get paid if they are unable to work for any reasons (disease, lack of skills, less job offers, etc.).

Third, job platform offer a great opportunity for people who cannot work in a physical workplace to work anyway online. However, it remains, at the same time, a place where discrimination is currently maintained and perpetuated.

Requirements for the Future of Work

Workers' privacy should be at the heart of the future of work. Platforms should provide different privacy options and be transparent about what worker data is exposed and to whom. Access control for requesters should also be supported, for instance by having workers sign non-disclosure agreements for sensitive work. Finally, requesters should be transparent about what the work is for, and how the work outcome will be utilized and who owns what.

Compensation for workers should be fair and regulated by law. More training for workers should be provided, as this is mutually beneficial for workers and requesters and leads to a higher worker retention rate. Transparent work contracts that clearly indicate conditions of work should also be available. Base payments and bonuses should be provided for paid work as well as work protection and insurance for volunteers. Workers should have the freedom to choose among different compensation types (either paid, free or platform credits, for instance).

Finally, job allocation should be transparent and fair. Work platforms should explain to workers why or why not they have been allocated certain jobs. The job allocation algorithms should be open and explainable by design. Access control on sensitive attributes that might bias the job allocation process should be supported. Auditing mechanisms to ensure compliance with fair, transparent and explainable job allocation should be in place. Regulations to guarantee compliance with fairness requirements should be implemented.

The Universal Declaration of Human Rights (UDHR) (www.un.org/en/universal-declaration-human-rights/) points to the inherent dignity of human beings as the foundation for freedom, justice and peace in the world. It would be important to revisit the UDHR to develop fundamental principles of consistent and fair crowdsourcing practices for the future.

Impact on Data Engineering Research

Research on Privacy and Access Control

In terms of privacy and access control, required changes for FoW platforms will trigger advances in research on cybersecurity including topics on privacy management and access control. This will include designing novel and usable privacy control mechanisms for all involved actors where workers can decide how much information to disclose to the platforms at different points in time and for different jobs, requesters can have data confidentiality guarantees in place where confidential data and business processes can be safely managed even if exposed to external on-demand workers. For example, NDA workers will be trusted not to disclose confidential information they may encounter while completing jobs with well defined legal implications in case of non compliance.

Together with advances in the computational field, we envision a catch up of the regulatory framework around the future job market where international law research will need to deal with challenges of conflicting regulations across national boundaries for cases in which the different actors involved are covered by different legal systems. From an ethical point of view, research practices will need to adapt to make sure experimenters disclose information to participating subjects on how they are being involved in an experiment and for which purpose their data is being used (e.g., informed consent).

In terms of compensation, we envision a more structured approach where researchers will be required to follow standard guidelines on how to reward participating subjects. This may include standards for monetary rewards (e.g., official price list per task type) as well as standards on how to manage volunteer participation (e.g., researchers being required to follow up with participants to disseminate the results of their research conducted thanks to volunteer contributions).

Research on Fairness

Fairness is an ethical concept which is addressed by numerous fields and perspectives. Roughly defined, fairness is the idea that an individual should obtain what it deserves. In work platforms, the issue of fairness mainly concerns the compensation system and job allocation process.

In terms of fairness, an interdisciplinary approach will be required to develop novel methods to assess and quantify algorithmic fairness in job allocation practices. For example, looking at bias trade-offs between fullyalgorithmic vs human-in-the-loop job allocation approaches where algorithmic bias could be different from implicit bias in humans. It would also be necessary to be cognizant of selection bias, where the data inputs to the algorithms are not representative of a population and could result in conclusions that would favor certain groups over others. Further, it would be necessary to look out for unintended promotion of biases where feedback loops perpetuate bias in results.

By compensation, we refer to what is obtained by the worker in exchange for their work. In some cases, workers are willing to work without financial compensation in the extent that they find some intrinsic motivation to do the job, for instance, pleasure to do a funny task, satisfaction to take part to a social, humanitarian or scientific project, development of human and social capital. Most of the time, workers are compensated by a task-based payment, which raises a huge number of fairness concerns. One of the most prominent issue is the low income associated to task-based payments. As the workers cannot negotiate their compensation, they are often paid under the official minimum wage of their country. In addition, workers cannot benefit of usual advantages granted to traditional workers as health insurance, for example. Lastly, workers lacks of job security since if they are unable work for any reasons (illness, lack of skills, no job offer), they would not get paid.

As the unfairness of compensation is the most visible abuse of current paid crowdsourcing, we advocate for the development of strong regulation through national and international law. From an ethical point of view, we propose to implement for each kind of task, a base payment estimated by automatic calculations or negotiated by stakeholders. This minimum payment could then be supplemented by bonuses based on work performance. In addition, we claim that the main compensation type (i.e. financial compensation) is severely limited as compared to the wide range of compensation types potentially adopted. We therefore suggest to extended the kind of compensation also including training, bonuses, credit for insurance or social protection, or any other convenience item on the platform. Workers could hence choose the kind of compensation they prefer in accordance with theses goals or actual situation. As far as voluntary work is concerned, we recommend the mandatory integration of an insurance scheme which could prevent the occurrence of mental health issues caused by the job (e.g., due to online content moderation tasks).

Research on Algorithmic Transparency

This will also result in higher levels of algorithmic transparency for job allocation where job allocation decisions should be easy to explain independently of whether they have been made by humans, algorithms, or by a combination of those. The other side of the coin is to develop methods to assess algorithmic unfairness [START_REF] Speicher | A unified approach to quantifying algorithmic unfairness: Measuring individual &group unfairness via inequality indices[END_REF]. While it may be easy for a human to determine what is fair or unfair based on fundamental ethical principles, understanding the level of fairness or unfairness is important in order to make decisions. Quite obviously there would be several situations where one would have to take the most fair or least unfair decision. This also calls for an additional research direction -methods to fix unfair decisions. In cases in which job allocation or compensation decisions have been judged to be unfair by a compliance verification step, processes should be in place to specify how to best address the situation, e.g., by means of additional rewards for workers or novel/better job opportunities. We also envision novel methods to make job allocation distribution (i.e., the long tail effect where few workers complete most of the available jobs) and time spent on jobs more transparent to workers and external actors like compliance agents. For example, visual analytics dashboards that communicate to workers how much time they spent and how much money they have earned on a platform with warnings on risks for addiction or unfair payments.

While, to-date, the literature has discussed the human-in-the-loop, soon, researchers will need to be studying society-in-the-loop (SITL) [START_REF] Rahwan | Society-in-the-loop: programming the algorithmic social contract[END_REF] methods. Innovations need the wisdom of the crowd, in fact, the collaboration of the crowd with algorithms is expected to be the future [START_REF] Malone | How human-computer'superminds' are redefining the future of work[END_REF]. Rahwan [START_REF] Rahwan | Society-in-the-loop: programming the algorithmic social contract[END_REF] points out that, to move from humanin-the-loop to SITL, would be necessary in the case to have mechanisms for negotiating the values of various stakeholders, bringing in the social contract. This results in a whole new level of complexity, since often, aspects of the social contract are implicit rather than explicit and are embedded in social norms bringing in issues of ethics. Further, there is the issue of how to resolve trade-offs between security and privacy, or various aspects of fairness while at the same time considering unbiased inputs to algorithms through algorithmic regulations.

An interesting evolution in the context of FoW is the use of smart contracts which are developed to design and manage virtually contracts. They are used to establish contract between different participants and the rules of this contract are ensured by methods currently based on block chains. The ability to express a formal contract that could be transparent for the different participants and be ensured by some clear and automatic mechanisms would simply and clarify the interactions between workers and requesters.

Impact on Work Platforms

As ethical issues of FoW lies heavily on governance mechanisms, work platform will be impacted at several levels. Depending on the evolution of law, platform transformations will be voluntary or mandatory.

Ensuring privacy and increasing access control means providing all users (worker and requesters) with the ability to modulate their privacy parameters. They must be able to decide what kind of information can be disclosed and to whom. For instance, the user account may include a section called "privacy parameters" where users have access to information related to them (personal information, work history, log files, performance measure, and so on) and for each users could switch a button to express their willingness to disclose it. Since users' behaviours are tracked and monitored, they need to know what kind of data is being collected, for what reasons, how is is treated, where it is stored and when it gets deleted. Regarding the platform, theses needs implies to provide users a space where they can get informed (e.g., a section in the user account). Information relating to the treatment of behaviour's data could also be included in the general policy which should be read and accepted by any users.

As fairness issues lie on the algorithms used by platforms to select workers, assign jobs, and evaluate outputs, platforms should be accountable for them. Many mechanisms to restrict the use of highly-unfair algorithms are available: disclosing them to the public, legal authorities or third parties, compliance to law, and frequent auditing. Thanks to the future developments in algorithms auditing research [START_REF] Sandvig | Auditing algorithms: Research methods for detecting discrimination on internet platforms. Data and discrimination: converting critical concerns into productive inquiry[END_REF], Fow should rely on sophisticated auditing techniques and tools. Platform development would be impacted since they will have to be more rigorous when designing algorithms, integrating fairness while building algorithms, and accept frequent auditing processes. Depending on the development of law and scientific research on fairness measures and scales, compliance to standards may be formally required. Support to requesters and workers is another driver of fairness. Platforms are in the best position to do so. Today, some platforms already offer their support for designing tasks, defining the reward, and controlling quality. However, this service is rare and most of the time it does not consider fairness. In addition, platforms often take the requesters side in case of conflict with workers as they are seen as the customer. In the Fow, platforms should become more active and concerned about requesters needs as well as about the quality of requester-worker relationships. They can, for example, develop algorithms in order to model the utility function of the workers, be moderating and listening to both sides during conflict resolution, helping to fix performance criteria for requesters, and provide opportunities for improvement to workers. A greater monitoring of the platform activities may lead to avoiding mistakes, abuses, and unfair decisions.

Fow platforms should integrate a sophisticated compensation system. First, the different kinds of compensation can be expanded to include training, bonuses, insurance, and others. Training appears as a compensation type that may be beneficial to all stakeholders: For the platform, it is a good way to obtain better crowd retention rates; requesters then would have access to highly skilled workers; and workers could earn better wages. Insurance could prevent workers being harmed by the work itself, especially in the case of voluntary jobs. Taxes may be charged to platforms, in order to fund compliance activities and worker insurance. With this system, platforms could then offer the opportunity to workers to decide how they want to be compensated. To go further, they could design a recommendation system which does a matchmaking between work/workers/kind of compensations. At the second level, when focusing on the monetary compensation, FoW will include specific support mechanism for setting rewards to prevent unfairness (e.g., checking the average wage balance across genders). Helping requesters to fix a reward which matches the difficulty and the length of the task would be a good way to ensure that workers will receive a decent payment. Guidelines for worker compensation can be also be made available by platforms. Ideally, fair Fow platform will deny jobs with a very low monetary compensation. However, as requesters are the main customers of these platforms, it is more realistic to envision a negotiation process unless strict regulations are put in place.

As discussed in this study, FoW platforms have to face complex challenges as they are designed to serve different purposes: their main role is to match workers and requesters but they also provide payments to the workers, guarantee the security of workers and requesters, provide the technical support for running the tasks. If the matchmaking challenge has got a lot of attention by the research community, the other issues are often forgotten even though they have important ethical aspects. The current mix of the different roles in work platforms creates high complexity in managing access control over the data as well as challenges related to fairness and explainability. We believe that a clearer distinction of the different roles in platforms and possibly a better separation of them could lead to more ethical work processes.

https://www.mturk.com/

https://www.uber.com/

Conclusions

FoW will require overhauling the design and engineering of online job platforms to enable the collection, storage, retrieval, analysis, and mining of a wide array of human data across different types of technology-driven work. A fair bit of engineering and testing will be needed to ensure the development of scalable and portable platforms and the integration of multi-stakeholder goals in efficient and effective ways.

We call the data engineering community to consider working on the upcoming FoW research challenges as data will be the key enabler of the FoW. While doing this, we urge the community to also consider the ethical dimension of innovative solutions to make sure that aspects of privacy, accountability, fairness, and transparency/explainability are embedded by design in such data-driven solutions for FoW platforms.


Rachid Cheddadi

Fausto Sarmiento

Alain Hambuckers

Majda Nourelbait

Anne-Marie Lézine

Pierre Taberlet

Francesco Ficetola

Alexandra-Jane Henrot

Kangyou Huang

Louis François

Frédéric Boyer

Mark Bush

Matthieu Carré

Ali Rhoujjati

Zhuo Zheng

Stephen Jay Gould

Multidisciplinary approaches for conservation issues

The essence of Darwinism lies in a single phrase: natural selection is the creative force of evolutionary change. No one denies that natural selection will play a negative role in eliminating the unfit. Darwinian theories require that it create the fit as well.

Introduction

In the current global warming context, mountain tree species need to adapt locally within their modern range [START_REF] Sexton | Evolution and Ecology of Species Range Limits[END_REF][START_REF] Aitken | Adaptation, migration or extirpation: climate change outcomes for tree populations[END_REF][START_REF] Hoffmann | Climate change and evolutionary adaptation[END_REF] or migrate [START_REF] Malcolm | Global warming and extinctions of endemic species from biodiversity hotspots[END_REF][START_REF] Williams | Projected distributions of novel and disappearing climates by 2100 AD[END_REF]) to more climatically suitable areas. If these two options cannot be accomplished then at best the species will persist in a restricted range as small population(s) [START_REF] Araújo | Selecting areas for species persistence using occurrence data[END_REF][START_REF] Keppel | The capacity of refugia for conservation planning under climate change[END_REF] under a suitable local (micro)climate or might turn extinct. Mountain species in Europe are considered highly sensitive to climate change with 60% of them threatened with complete extinction by 2080 [START_REF] Thuiller | Climate change threats to plant diversity in Europe[END_REF][START_REF] Thomas | Extinction risk from climate change[END_REF]). The involved mechanisms seem complex. [START_REF] Steinbauer | Accelerated increase in plant species richness on mountain summits is linked to warming[END_REF] demonstrated that mountain summits became enriched in species at an accelerating pace during the last decade and that the new colonizers show traits which are typical of lower altitudes species. They suggested that this phenomenon will later result in the impoverishment of the initial diversity by competitive replacement. At the global scale, observed changes indicate a sustained decline in cold mountain plant communities in response to the recent warming while the climatic impact seems more pronounced in areas with a marked increase in temperature [START_REF] Gottfried | Continent-wide response of mountain vegetation to climate change[END_REF]. The International Union for Conservation of Nature (IUCN) reported about 800 extinctions in species over the last 500 years, with more extinctions that have probably not been inventoried. Species extinction is a natural process that one may observe in the geological records (Darwin 1872). However, in the natural process a species may persist for few millions of years [START_REF] Raup | Periodicity of extinctions in the geologic past[END_REF].

Fossil records contain information on how plant species have responded to past climate changes over the geological periods. Spatial data syntheses allow us to depict where species have persisted during past periods when climates were not suitable [START_REF] Bennett | What do we mean by 'refugia[END_REF] and how they expanded their range to more suitable habitats [START_REF] Jackson | Responses of plant populations and communities to environmental changes of the late Quaternary[END_REF] when climates became more favorable. Fossil records also allow us to depict periods of times when species became extinct [START_REF] Magri | Quaternary disappearance of tree taxa from Southern Europe: Timing and trends[END_REF]. Expected rapid climate changes during the next century will likely induce noticeable changes in the species ranges and will impact their potential extinction rates [START_REF] Thomas | Extinction risk from climate change[END_REF][START_REF] Thuiller | Climate change threats to plant diversity in Europe[END_REF]. The modern challenge is that the velocity of the ongoing global climate change exceeds the known migration rates of the many plant species composing major global ecosystems [START_REF] Loarie | The velocity of climate change[END_REF][START_REF] Dobrowski | The climate velocity of the contiguous United States during the 20th century[END_REF][START_REF] Corlett | Will plant movements keep up with climate change?[END_REF].

The expansion/regression and adaptation/extinction processes which have taken thousands of years may occur within the next century. This is why scientists are exploring different options to preserve those species which may not be able to neither migrate to more suitable habitats nor adapt locally in such short period of time [START_REF] Aitken | Adaptation, migration or extirpation: climate change outcomes for tree populations[END_REF].

The objective of this chapter is to show that the integration of different but complementary disciplines such as palaeoecology, genetics and species modelling can provide us with pertinent information on the environmental changes which have impacted plant species across time and therefore, can contribute to setting better strategy for preserving species. Genetics allow us, for instance, to evaluate the infra-specific diversity between and among populations (Petit et al. 1998) and the overall capacity of a species to cope with strong environmental changes (or not). Meanwhile, vegetation model simulations may provide useful information on the geographical range changes through time and help predict future suitable areas for the species persistence [START_REF] Pearman | Niche dynamics in space and time[END_REF][START_REF] Maiorano | Building the niche through time: Using 13,000 years of data to predict the effects of climate change on three tree species in Europe[END_REF]. Palaeoecological data allows us to evaluate the past relationships between plant species and their contemporaneous climate which can contribute to a better management of the future plant/environment relationship [START_REF] Gillson | From myopia to clarity: Sharpening the focus of ecosystem management through the lens of palaeoecology[END_REF][START_REF] Petit | Forests of the Past: A Window to Future Changes[END_REF][START_REF] Willis | How can a knowledge of the past help to conserve the future? Biodiversity conservation and the relevance of long-term ecological studies[END_REF]). These disciplines are of particular interest to the conservation of mountain tree species which often represent a great value to the ecosystem services they provide [START_REF] Schröter | Ecosystem service supply and vulnerability to global change in Europe[END_REF]. The mountain landscapes of Morocco have been declared by UNESCO as a critical habitat for conservation, justifying the recent inclusion of the Atlas Cedar Biosphere Reserve [START_REF] Sarmiento | Sustainability and the Biosphere Reserve: A compromise between Biodiversity, Conservation and Farmscape Transformation[END_REF].

A multidisciplinary approach for a better species conservation?

One of the goals of conservation is to prevent species from becoming extinct. Thus, one of the key conservation aims is to identify the threatened species, evaluate the degree of the threat, identify putative regions where the species may be preserved, and consider the environmental conditions that are suitable for the survival of the species. Thus, a scientific-based management strategy for preserving species requires the integration of several disciplines such as ecology, climatology, genetics, and modeling. In order to be pertinent and efficient at managing species under any different climate from that of today, we need this integrated multidisciplinary approach within a historic data frame.

Palaeoecology is the study of the interactions between species and their environment, including climate, over geological timescales. The study of these interactions provides a scientific basis for evaluating a species capacity to cope with climate fluctuations, its capacity for migration or the suitable areas for its potential occurrence. Thus, palaeoecology can provide sound interpretations that can be directly used for species conservation. Fossil pollen grains are one of the most used biological proxies in vegetation palaeoecology for reconstructing past plant-environment relationships. Their excellent preservation in sediments is due to sporopollenin, one of the most resistant biological substance to degradation, which constitutes their ornamented and recognizable outer wall. The highest number of fossil pollen records available cover the last 20,000 years which is a period of time that covers the last glacial maximum and its transition to the Holocene warm period where the human population massively colonized the planet. Fossil pollen records contain information on the occurrences of all the local species through time. A data set of well-dated fossil records allows for the spatial reconstruction of past species occurrences at different time periods and spatial scales [START_REF] Brewer | The spread of deciduous Quercus throughout Europe since the last glacial period[END_REF]. Using these spatial past occurrences across time, one can evaluate the evolution of the species range over time and the velocity of the spread [START_REF] Cheddadi | Revisiting tree-migration rates: Abies alba (Mill.), a case study[END_REF][START_REF] Huntley | How Plants Respond to Climate Change: Migration Rates, Individualism and the Consequences for Plant Communities[END_REF]. These estimates may provide us with information on whether a species may cope with a climate warming scenario or not.

Fossil pollen data sets can also provide essential information on the past ecosystems composition and dynamics [START_REF] Overpeck | Climate-induced changes in forest disturbance and vegetation[END_REF]), the species richness through time [START_REF] Svenning | Ice age legacies in the geographical distribution of tree species richness in Europe[END_REF], the competition between species [START_REF] Clark | Stability of forest biodiversity[END_REF], the abundances of the species composing the succession of ecosystems through time [START_REF] Bennett | Continuing the debate on the role of Quaternary environmental change for macroevolution[END_REF], the prevalence of a species over the others, the expansion and extinction of species [START_REF] Jackson | Late Quaternary extinction of a tree species in eastern North America[END_REF] when climate changes and so on. All these past ecological parameters are essential for vegetation modelers and conservation managers. Quantifying ecological variables can be directly applicable to conservation managers to (1) identify whether there is a risk of extinction, (2) develop realistic plans for conservation in suitable areas, and (3) to identify appropriate management tools for maintaining or, if at all possible, restore the ad hoc ecological settings. Basing conservation decisions on a long-term ecological perspective will undoubtedly be of a more robust approach than only on contemporary observations [START_REF] Willis | How can a knowledge of the past help to conserve the future? Biodiversity conservation and the relevance of long-term ecological studies[END_REF]) and the inclusion of a long-term ecological perspective can provide a more scientifically defensible basis for conservation decisions than the one based only on contemporary records.

Vegetation models are required to predict future species distributions and potential suitable areas and habitats for their conservation [START_REF] Pearman | Niche dynamics in space and time[END_REF][START_REF] Sykes | A bioclimatic model for the potential distributions of north European tree species under present and future climates[END_REF][START_REF] Prentice | A global biome model based on plant physiology and dominance, soil properties and climate[END_REF]). However, reconstructing past ecological parameters of a species from fossil data sets is essential for validating and improving the vegetation models [START_REF] Maiorano | Building the niche through time: Using 13,000 years of data to predict the effects of climate change on three tree species in Europe[END_REF][START_REF] Cheddadi | Temperature range shifts for three european tree species over the last 10,000 years[END_REF]. There are basically two types of models describing the geographical distribution of plant species: the ecological nichebased models (ENMs) and the process-based dynamic vegetation models (DVMs). ENMs are empirical relationships between plant species occurrences, or more rarely abundances, and spatial climate datasets (e.g. [START_REF] Guisan | Predicting species distribution: offering more than simple habitat models[END_REF]. ENMs don't take into account the atmospheric CO 2 concentration which has a direct impact on the plant tolerance to water stress. DVMs are more complex tools that combine, in a chronological process schemes and feedbacks, both inputs and outputs of sub-models. Beside species distributions, DVMs compute productivity and other ecological variables (e.g. [START_REF] Dury | Responses of European forest ecosystems to 21 st century climate: assessing changes in interannual variability and fire intensity[END_REF][START_REF] Snell | Using dynamic vegetation models to simulate plant range shifts[END_REF] ). The sub-models are designed to compute environmental conditions at plant level (e.g. water availability, CO2 concentration inside the leaves, photon flux etc.) from environmental data (including climate) and physiological processes describing plant functions and allowing growth and development (e.g. stomatal aperture, fixed carbon allocation. So far, DVMs are rather limited by the lack of specific information required for each species to be simulated (i.e. plats traits).

More recently, environmental DNA (eDNA), a powerful past environmental proxy, has been developed and is experiencing an unprecedented boom for reconstructing past plant occurrences from the fossil records [START_REF] Taberlet | Environmental DNA[END_REF][START_REF] Willerslev | Fifty thousand years of Arctic vegetation and megafaunal diet[END_REF]. eDNA already allows us to identify much more organisms (plant and animals) in a fossil record, that have lived in the past ecosystem [START_REF] Giguet-Covex | Long livestock farming history and human landscape shaping revealed by lake sediment DNA[END_REF][START_REF] Pansu | Reconstructing long-term human impacts on plant communities: An ecological approach based on lake sediment DNA[END_REF] which represents an essential information for conservation managers on the habitat suitability and the co-existence of plants/animals species within a specific environment. The ongoing development of eDNA is expected to provide even more accurate and efficient information on past plants occurrences and their direct environment.

Conservation of the Atlas cedars in Morocco

State of the art

Cedrus atlantica (Atlas cedar) is a North African mountain conifer tree that may withstand strong droughts [START_REF] Aussenac | Influence de la sécheresse sur la croissance et la photosynthèse du cèdre[END_REF][START_REF] Aussenac | Comportement hydrique estival de Cedrus atlantica Manetti, Quercus ilex L. et Quercus pubescens Willd. et de divers pins dans le Mont Ventoux[END_REF] that are higher than any European temperate coeval mountain tree species. However, the aridification trend recorded over the last century has already impacted its range with a migration of its lower viable elevation limit by about 200m in the Middle Atlas [START_REF] Rhanem | Aridification du climat régional et remontée de la limite inférieure du cèdre de l'Atlas (Cedrus atlantica Manetti) aux confins de la plaine de Midelt (Maroc)[END_REF]) and a substantial reduction of its range in the Rif Mountains [START_REF] Cheddadi | Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco[END_REF]. The increase in winter temperatures [START_REF] Ezzahiri | Synthèse de quelques résultats sur la régénération naturelle du cèdre de l'Atlas au Moyen Atlas (Maroc)[END_REF] and the recent successive years of soil droughts [START_REF] Ladjal | Drought effects on hydraulic conductivity and xylem vulnerability to embolism in diverse species and provenances of Mediterranean cedars[END_REF] seem to affect its growth as well. Tree-ring data has allowed us to observe the negative impact of the recent recurrent droughts and temperature increase on its growth [START_REF] Linares | Biogeography and evolution of Abies (Pinaceae) in the Mediterranean Basin: the roles of long-term climatic change and glacial refugia[END_REF]. The Atlas cedar is now considered an endangered species by the IUCN (http://www.iucnredlist.org). Forest managers in Morocco are now protecting some populations with fences and replanting seedlings around existing populations which show a strong trend in population decline. Indeed, locally, grazing, poaching, and inadequate management still hinder regeneration (Navarro-Cerrillo et al. 2013) while replanting is not 100% successful.

What is the approach?

In order to evaluate the impact of past environmental changes on the Moroccan Atlas cedar and contribute to its conservation we need to identify the areas of concern with potentially threatened populations and propose a strategy based on both the scientific facts and a realistic strategy for conserving the species over the long term. Our approach is based on reconstructing past climates and species occurrences from fossil records, simulating its past ranges using a vegetation model and evaluating the species genetic diversity through an exhaustive genetic survey of the modern populations. The target area is located in the northernmost part of the Atlas cedar distribution in Morocco. We collected several geological corings from the Rif mountains at different altitudes and distances from the modern Atlas cedar forests. Then we sampled the modern populations for their DNA study before finally using a DVM to simulate the evolution of the Atlas cedar productivity and range in the Rif mountains continuously over 50 years between 1960 and 2010.

What does the data say?

We know from fossil pollen records that Cedrus sp. was present in the Mediterranean borderlands since at least 2.6 Ma [START_REF] Magri | Quaternary disappearance of tree taxa from Southern Europe: Timing and trends[END_REF] including Northern Morocco during the Pleistocene [START_REF] Feddi | Histoire plio-pléistocène des écosystèmes végétaux de Méditerranée sud-occidentale: apport de l'analyse pollinique de deux sondages en mer d'Alboran[END_REF]. How did the species persist throughout several climatic cycles with marked alternating glacial and interglacial climates?

The fossil record

Fossil records allow us to track species occurrences in space and time. However, there are several physical limitations in arid areas, such as North Africa, to the accumulation of sediments and the preservation of fossil biological environmental proxies in continuous records encompassing more than the last glacial period and the Holocene. As a matter of fact, there are very few records encompassing more than the last 20,000 years. These available geological records contain vegetation and climate proxies that help us reconstruct past species occurrences and past climate variability. The fossil proxies help reconstruct and then interpret the relationship between species and their contemporaneous climate over the time period covered by the record.

The pollen records available in the Rif Mountains indicate that Atlas cedar populations were present at much lower altitudes than today, near the Mediterranean Sea [START_REF] Linstädter | Chronostratigraphy, site formation processes and pollen record of Ifri n'Etsedda, NE Morocco[END_REF][START_REF] Zapata | Holocene environmental change and human impact in NE Morocco: Palaeobotanical evidence from Ifri Oudadane[END_REF]. Today, Atlas cedar occurs at altitudes higher than 1400m above sea level (asl) in a more reduced and fragmented range than during the last glacial period at lower elevations (figure 1). Based on several fossil records from the Rif Mountains, we can observe that the Atlas cedar had a much more extended range around 6000 BP with extensive populations down to 800 masl or probably at lower elevations. The 600 meter migration upwards to the modern 1400 masl took place over the past thousand years after 6000 BP. These expansion/regression spatial movements during the Holocene allow us to evaluate the response velocity of the Atlas cedar to the climate forcing that may be inferred from the climate proxies.

The climate reconstructions (figure 2) that we have performed from the pollen records show that winter temperatures were lower between 7500 and 5700 BP than today. This time span was also the wettest within the Holocene time period with a more marked increase of precipitation during spring than in the winter season. This cool and wet time interval corresponds to the most extended geographical range of Atlas cedar in the Rif Mountains (figure 2). These past climate data tend to suggest that even if the Atlas cedar is rather well-adapted to drier climates than many other conifer species, it may show a more sustained growth and expansion of its range under a wetter and colder climate than today. These past climate interpretations are coherent with the genetic data and confirm that the species expanded its range during the last glacial period, more probably at lower altitudes towards the Mediterranean Sea [START_REF] Linstädter | Chronostratigraphy, site formation processes and pollen record of Ifri n'Etsedda, NE Morocco[END_REF][START_REF] Zapata | Holocene environmental change and human impact in NE Morocco: Palaeobotanical evidence from Ifri Oudadane[END_REF] where some moisture may be available even during cold periods.

The modern genetic evidence

The modern genetic data reveal very low divergence between populations and do not support a recolonization process from isolated population(s) (figure 3) but rather a persistence of at least one large population which has fragmented in the Moroccan mountains by extinction at lower elevations. This process prevented from a marked genetic drift and lead to the modern low genetic differentiation in the whole range of the species and maintained the occurrence of all haplotypes in all populations. Indeed, previous genetic surveys carried out on the modern populations of Atlas cedar in Morocco show that there is no clear geographical diversity structure [START_REF] Terrab | Genetic diversity and population structure in natural populations of Moroccan Atlas Cedar (Cedrus atlantica; Pinaceae) determined with CPSSR markers[END_REF][START_REF] Cheddadi | Putative glacial refugia of Cedrus atlantica deduced from Quaternary pollen records and modern genetic diversity[END_REF]) which tends to suggest that since pollen does not disperse far from its originating Atlas cedar tree [START_REF] Wright | Pollen dispersion of some forest trees[END_REF][START_REF] Hajar | Vegetation changes during the late Pleistocene and Holocene in Lebanon: a pollen record from the Bekaa Valley[END_REF], the most likely explanation is that Moroccan populations are the result of a massive colonization event, without founder effect. According to the ecological requirements of the species, such colonization happened during cold periods when the potential distribution area was probably continuous.

The model simulations

DVMs may help identifying potential suitable areas and evaluating the risk of extinction of a species under different climate scenarios. To assess the impact of the past climate changes on cedar populations, we run a model simulation continuously over the past 50 years, between 1960 and 2010 (figure 4). Our model simulation [START_REF] Cheddadi | Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco[END_REF] provided three major information: (1) that the range of Atlas cedar in northern Morocco decreased by about 75%, (2) that those populations located at the easternmost part of the range are heavily threatened by the decrease of the water availability, and (3) that populations at lower elevation are strongly impacted which lead over the past few decades to an upward lift of the lower limit of the distribution by about 200 meters. The latter is coherent with observed data in the Middle Atlas [START_REF] Rhanem | Aridification du climat régional et remontée de la limite inférieure du cèdre de l'Atlas (Cedrus atlantica Manetti) aux confins de la plaine de Midelt (Maroc)[END_REF].

What do we do with this scientific data?

Today, Atlas cedar populations in Northern Morocco occur in quite restricted and fragmented areas. The main populations are located in Jbel Kelti, Talassemtane National Park, Jbel Tiziren, Targuist, Oursane and Tidighine and they occupy less than 12k ha. The populations in Jbel Bouhachem have strongly declined with many that turned extinct over the past few decades. There remains only one very reduced population that is almost close to extinction. Our model simulations show that the occupied range was about four times higher (40k ha) in 1960-70 than today which is coherent with the IUCN status of Cedrus atlantica which states that the species is "endangered with a decreasing population trend" (http://www.iucnredlist.org). The fossil data have recorded an even more extensive range 6000 years ago and the genetic survey confirm the presence of few and very extended populations in Morocco since there is very weak genetic structure between the isolated modern population.

Thus, based on our multidisciplinary and multi-scale approach, we consider the modern populations of Atlas cedar in the Rif Mountains as relic populations that are persisting today in microrefugial areas which offer a habitat that has a suitable microclimate. However, these populations are quite distant from each other which prevents the gene flow between them. Such genetic isolation may lead to a collapse of all these remnant populations on a more or less short term.

To maximize the chances of persistence for the Atlas cedar in its endemic range we may propose (1) as a basic rule, to protect all the remaining microrefugial areas from any human disturbance, mostly seedlings at the upper limit of each population since the species requires cool and wet conditions which may be still available at higher elevations; (2) to evaluate quantitatively the microclimate suitability in each microrefugial area with a dedicated system for climate monitoring; (3) eventually, to transplant seedlings into new areas that are unoccupied today by the Atlas cedar but which have been identified by the model simulations as potentially suitable new microrefugial areas under different climate scenarios; and (4) since the modern populations are too remote to have any gene exchanges, we could also consider some gene exchanges between them which could improve heterozygosity and potentially the capacity of the successful hybridized generation to adapt locally.

Conclusions

The aim of this work is to highlight the importance of combining information from different but complementary disciplines within a multi-scale approach for building a reasonable scientific basis that may contribute to species conservation. This approach is being developed within the VULPES project (www.vulpesproject.com) for other species in South America, Tropical Africa and China. The case study chosen here is the threatened species Cedrus atlantica in Northern Africa which persists as few remaining and geographically isolated populations. The combined data that we have collected and analyzed tend to indicate that the areas where the Atlas cedar populations are persisting today should be considered as microrefugia with a microclimate that is still cooler and wetter than the average climate in Northern Morocco where snow still occurs, although not every year, during winter (figure 5).

The future persistence of the remaining Atlas cedar populations in the Rif Mountains will depend on the long term natural climate stability of these microrefugia and the management strategies. Generally, one may observe that the human impact is not as strong in the Rif mountains as in the Middle Atlas because the remaining populations are not so easily accessible such as in Jbel Kelti, Talassemtane National Park or Jbel Tiziren.

Acknowledgments

This work is a contribution to the Belmont Forum funded project VULPES (Project ID: ANR-15-MASC-0003). RC thanks very much Matthew Greene, independent journalist in Morocco, for his comments on the manuscript and the English editing.


Liangcai Xu

Xiaokang Li

Linchao Ding

Kamran Behnia

Zengwei Zhu

come

Planar Hall effect caused by the memory of antiferromagnetic domain walls in Mn 3 Ge

Planar Hall effect caused by the memory of antiferromagnetic domain walls in Mn 3 Ge Liangcai Xu 1,2 , Xiaokang Li 1 , Linchao Ding 1 , Kamran Behnia 2 , Zengwei Zhu * ,1 In Mn 3 X (X=Sn, Ge) antiferromagnets domain walls are thick and remarkably complex because of the non-collinear arrangement of spins in each domain. A planar Hall effect (PHE), an electric field perpendicular to the applied current but parallel to the applied magnetic field, was recently observed inside the hysteresis loop of Mn 3 Sn. The sign of the PHE displayed a memory tuned by the prior orientation of the magnetic field and its history. We present a study of PHE in Mn 3 Ge extended from room temperature down to 2 K and show that this memory effect can be manipulated by either magnetic field or thermal cycling. We show that the memory can be wiped out if the prior magnetic field exceeds 0.8 T or when the temperature exceeds T N . We also find a detectable difference between the amplitude of PHE with zero-field and field thermal cycling. The ratio between the PHE and the anomalous Hall effect (AHE) decreases slightly as temperature is increased from 2 K to T N , tracks the temperature dependence of magnetization. This erasable memory effect may be used for data storage.

The discovery of a large room-temperature anomalous Hall effect(AHE) in non-collinear antiferromagnetic Mn 3 X (X = Sn, Ge) [START_REF] Nakatsuji | Large anomalous Hall effect in a non-collinear antiferromagnet at room temperature[END_REF][START_REF] Nayak | Large anomalousHall effect driven by a nonvanishing Berry curvature in the noncolinear antiferromagnet Mn 3 Ge[END_REF][START_REF] Kiyohara | Giant anomalous Hall effect in the chiral antiferromagnet Mn 3 Ge[END_REF] was theoretically anticipated [START_REF] Chen | Anomalous Hall effect arising from noncollinear antiferromagnetism[END_REF][START_REF] Kübler | Non-collinear antiferromagnets and the anomalous Hall effect[END_REF] . The non-trivial Berry curvature of the electron wave-function [START_REF] Xiao | Berry phase effects on electronic properties[END_REF][START_REF] Nagaosa | Anomalous Hall effect[END_REF] is widely believed to be the origin of this phenomenon. It stimulated numerous experimental studies and was followed by the discovery of other topological magnets [START_REF] Yin | Quantum-limit chern topological magnetism in TbMn 6 Sn 6[END_REF][START_REF] Morali | Fermi-arc diversity on surface terminations of the magnetic Weyl semimetal Co 3 Sn 2 S 2[END_REF][START_REF] Liu | Magnetic Weyl semimetal phase in a Kagomé crystal[END_REF][START_REF] Belopolski | Discovery of topological Weyl fermion lines and drumhead surface states in a room temperature magnet[END_REF][START_REF] Ding | Intrinsic anomalous Nernst effect amplified by disorder in a half-metallic semimetal[END_REF] .

Other counterparts of the AHE were observed in the Mn 3 X family. the list includes the anomalous Nernst effect [START_REF] Li | Anomalous Nernst and Righi-Leduc effects in Mn 3 Sn: Berry curvature and entropy flow[END_REF][START_REF] Ikhlas | Large anomalous Nernst effect at room temperature in a chiral antiferromagnet[END_REF][START_REF] Xu | Finite-temperature violation of the anomalous transverse Wiedemann-Franz law[END_REF] , the anomalous thermal Hall effect [START_REF] Li | Anomalous Nernst and Righi-Leduc effects in Mn 3 Sn: Berry curvature and entropy flow[END_REF][START_REF] Xu | Finite-temperature violation of the anomalous transverse Wiedemann-Franz law[END_REF][START_REF] Sugii | Anomalous thermal Hall effect in the topological antiferromagnetic state[END_REF] , a large Kerr effect [START_REF] Higo | Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal[END_REF] as well as high-frequency AHE [START_REF] Matsuda | Room-temperature terahertz anomalous Hall effect in Weyl antiferromagnet Mn 3 Sn thin films[END_REF] .The Mn 3 X family provide also candidates for antiferromagnetic spintronic devices, which are attractive due to the absence of perturbing stray field and their fast response [START_REF] Núñez | Theory of spin torques and giant magnetoresistance in antiferromagnetic metals[END_REF][START_REF] Gomonay | Spintronics of antiferromagnetic systems (Review Article)[END_REF][21] . There is a prospect for high-speed operation of the devices. They can reach THz range, which is three orders of magnitude faster than those made of ferromagnets [START_REF] Jungwirth | The multiple directions of antiferromagnetic spintronics[END_REF] . Thus, the applications of the devices may extend to the areas like terahertz information technologies or artificial neural networks [START_REF] Jungwirth | The multiple directions of antiferromagnetic spintronics[END_REF] . Non-collinear antiferromagnets host the abovementioned AHE, and possibly a topological Hall effect due to the chirality of their spin texture [START_REF] Sürgers | Large topological hall effect in the non-collinear phase of an antiferromagnet[END_REF][START_REF] Li | Momentum-space and real-space Berry curvatures in Mn 3 Sn[END_REF] . This could allow the generation of spin polarized charge current [START_REF] Železný | Spin-polarized current in noncollinear antiferromagnets[END_REF] and other applications.

Theoretical studies have associated the magnetic texture with cluster octupoles [START_REF] Suzuki | Cluster multipole theory for anomalous Hall effect in antiferromagnets[END_REF] and found that its topological defects, including the walls separating magnetic domains have non-trivial properties [START_REF] Liu | Anomalous Hall effect and topological defects in antiferromagnetic Weyl semimetals: Mn 3 Sn/Ge[END_REF] . The antiferromagnetic structure of Mn 3 X can be characterized by the cluster octupole moments for the D 6h irreducible representations. [START_REF] Suzuki | Cluster multipole theory for anomalous Hall effect in antiferromagnets[END_REF] These domain walls were scrutinized in a number of experiments [START_REF] Higo | Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal[END_REF][START_REF] Li | Momentum-space and real-space Berry curvatures in Mn 3 Sn[END_REF][START_REF] Li | Chiral domain walls of Mn 3 Sn and their memory[END_REF] . In particular, in presence of domain walls, a topological Hall effect, presumably arising from the real-space Berry curvature, was observed first in Mn 3 Sn crystals [START_REF] Li | Momentum-space and real-space Berry curvatures in Mn 3 Sn[END_REF] and subsequently in thin films [START_REF] Taylor | Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet Mn 3 Sn[END_REF] .

The subject of the present paper is the planar Hall effect (PHE). Such a signal (i.e. an electric field, which resides in the same plane with magnetic field and electric current, but is normal to the latter) was recently reported in Mn 3 Sn 29 . Together with its thermoelectric counterpart (i.e. a planar Nernst effect) and off-diagonal magnetization, the PHE emerges in a narrow magnetic field where domains of opposite polarity co-exist [START_REF] Li | Chiral domain walls of Mn 3 Sn and their memory[END_REF] . Its sign in Mn 3 Sn was found to have a memory depending on the rotational history of magnetic field in respect with the specific chirality of domain walls. This constitutes a type of memory formation for recording a direction [START_REF] Keim | Memory formation in matter[END_REF] . The previous study on Mn 3 Sn 29 was restricted to room temperature. Here, we present a study of the PHE and the memory effect in Mn 3 Ge in a wide temperature range. In contrast to Mn 3 Sn, where the triangular spin order is destroyed below ∼50 K [START_REF] Nakatsuji | Large anomalous Hall effect in a non-collinear antiferromagnet at room temperature[END_REF][START_REF] Ohmori | Spin structure and weak ferromagnetism of Mn 3 Sn[END_REF] , in Mn 3 Ge one can follow the AHE and associated phenomena down to 2 K 2,3 and even below 1 K [START_REF] Xu | Finite-temperature violation of the anomalous transverse Wiedemann-Franz law[END_REF] . Moreover, the lower Néel temperature (T N = 370 K [START_REF] Xu | Finite-temperature violation of the anomalous transverse Wiedemann-Franz law[END_REF] ) makes it possible to go above the ordering temperature by sweeping to 400 K. We find that both the PHE and its memory exist in Mn 3 Ge. At room temperature, the PHE/AHE ratio is somewhat smaller than what was found in Mn 3 Sn, presumably due to a difference in the width of the hysteresis loop and coercive field scales. The PHE/AHE ratio presents a temperature dependence, which roughly mirrors the temperature dependence of magnetization. By performing a thermal cycle across T N , we show that the memory can be erased by warming the sample above its ordering temperature. We also quantified the threshold of erasure for prior magnetic field required to wipe out the PHE [START_REF] Li | Chiral domain walls of Mn 3 Sn and their memory[END_REF] .

The single crystals were grown by Bridgman-Stockbarger technique [START_REF] Xu | Finite-temperature violation of the anomalous transverse Wiedemann-Franz law[END_REF] . They were subsequently cut into approximate dimensions of 1 × 2 × 0.5mm 3 with a wire saw. [START_REF] Li | Chiral domain walls of Mn 3 Sn and their memory[END_REF] . Each arrow represents an octupole moment [START_REF] Suzuki | Cluster multipole theory for anomalous Hall effect in antiferromagnets[END_REF][START_REF] Sugimoto | Electrical nucleation, displacement, and detection of antiferromagnetic domain walls in the chiral antiferromagnet Mn 3 Sn[END_REF] .

 0 H (T)  PHE yz ( cm)  0 H (T) I z V y I z V y d) I z V y I z V y a) b) c) e) f) Rotate  0 H // x  0 H // -x  0 H // y
field is swept. In regime I, there is a single domain induced by magnetic field. Above a threshold field µ 0 H 0 , new domains with opposite polarity nucleate and regime II starts. At sufficiently large field, the whole system becomes single domain but with spins oriented in opposite orientation compared to regime I. The amplitude and field dependence of ρ yz is almost similar to what was previously reported 2,3 .

To measure PHE, the magnetic field was rotated from x to y (By PHE, we mean the symmetric part of the measured signal). Then, the magnetic field (kept oriented along y) was swept from negative to positive values, using the same electrodes for current (along z) and voltage (along y). The measured signal was therefore a version of the so-called planar Hall effect, because the three vectors (the electric current, the electric field and the magnetic field) were residing in the same plane. To assure high accuracy PHE signal, the tilted angle between the magnetic field and the current-electric field was kept less than 2 degrees. We define the magnetic field at which the rotation took place as the prior magnetic field (µ 0 H p ).

Since the PHE is very sensitive to the field orientation, the field angle is to be carefully tuned. The measured signal has two components which are even and odd as a function of magnetic field. The asymmetric part is the residual AHE arising from imperfect alignment, but the symmetric part (even in magnetic field) can only be produced by a genuine PHE [START_REF] Li | Chiral domain walls of Mn 3 Sn and their memory[END_REF] . This signal emerges only in regime II. In this field window, the two domains with opposite polarities compensate the transverse response of each other. The measured electric field is produced by the domain wall whose spin configuration is perpendicular to the two domains sandwiching it (See Fig. 1 e,f). The sign of this electric field is set by the chirality of the domain wall. As seen in Fig. 1 c) and d), two distinct types of PHE curves similar in amplitude but opposite in sign can be obtained. What controls the sign is the history of the magnetic field prior to sweeping field. The prior field orientation determines the sign of the PHE. The domain wall separating two domains with 0 and π (the angle is defined as between the x-axis and a pair of parallel spins in one unit cell [START_REF] Suzuki | Cluster multipole theory for anomalous Hall effect in antiferromagnets[END_REF][START_REF] Liu | Anomalous Hall effect and topological defects in antiferromagnetic Weyl semimetals: Mn 3 Sn/Ge[END_REF] ) can have two chiralities. If the prior rotation was from x to y, then the wall will correspond to a clockwise rotation of spins (+π/2 chirality). On the other hand, when the prior rotation is from -x to y, it will correspond to an anti-clockwise rotation of spins ( -π/2 chirality). In Fig. 1 f) and g), each arrow represents the effective moment of each cluster octupole [START_REF] Suzuki | Cluster multipole theory for anomalous Hall effect in antiferromagnets[END_REF][START_REF] Sugimoto | Electrical nucleation, displacement, and detection of antiferromagnetic domain walls in the chiral antiferromagnet Mn 3 Sn[END_REF] .

Since the moment rotates from the center to the periphery the domain wall in the middle has a net moment perpendicular to the moment of the two domains sandwiching it.

The amplitude of PHE depends on the strength of the prior magnetic field, as detailed in Fig. 2. The raw data is shown in In field-thermal-cycling(FTC), the sample was warmed from T meas. to cycling temperature(T cycl. ) and cooled down to T meas. and then measurement was performed. In the zero-field-thermal-cycling(ZFTC), the warm-up to T cycl. and the cool-down to T meas. were performed in absence of magnetic field. b,c,d,e) show the FTC planar Hall effect with different cycling temperatures. Panel f.g.h.i) show the ZFTC residual planar Hall effect. All measurements were performed at T meas. = 300K. In both cases, the planar Hall effect gradually decreases with the approach to T N . But it drops faster in the FTC process than in the ZFTC process.

panel a). Similar to the case of Mn 3 Sn 29 , the larger the prior magnetic field, the lower the magnitude of PHE. Moreover, there is a difference between the evolution of the symmetric and the asymmetric components of the PHE. The asymmetric component, which is odd in magnetic field does not depend on the magnitude of the prior magnetic field (see panel c) ). The symmetric component, the genuine PHE, which is even in magnetic field decreases as the the prior field is increases from 0.2 T to 0.8 T (panel c)). Its amplitude decreases roughly proportional to the inverse of the prior field (panel (d)). This is compatible with a scenario in which the population of minority domains surviving after a field cycle determine the dominant chirality of domain walls. The larger the field, the lower the population of surviving minority domains, and the smaller the amplitude of the PHE signal. We found that the prior mag-

M ( e m u / g ) -ρ m a x P H E ( µΩc m ) T c y c l . ( K ) P H E ( F T C ) P H E ( Z F T C ) M ( T )
T m e a s . = 3 0 0 K b )

ρ P H E y z , ρ A H E y z ( µΩ c m ) A H E P H E a ) P H E / A H E T ( K ) P H E / A H E FIG. 4.
Evolution with temperature: a) The blue and the red curves represent the evloution of the FTC and ZFTC PHE as a function of cycling temperature. The PHE in the ZFTC process survives at temperatures slightly exceeding the Néel temperature, as observed by the M(T) curve (dashed line). b) The temperature dependence of the AHE (red), PHE (blue) and their ratio (green). netic field required to suppress the PHE in Mn 3 Ge is much smaller than that in Mn 3 Sn at same temperature. This is compatible with the fact that the hysteresis loop is wider and the coercive fields larger in Mn 3 Sn. Now we turn our attention to the memory erasure by temperature cycling. The Néel temperature in Mn 3 Sn is as high as 420 K, beyond what can be accessed to by a popular commercial instrument , namely Quantum Design's PPMS. On the other hand, the Néel temperature in Mn 3 Ge is 370 K and accessible. This allowed us to check that the PHE is wiped out when the sample is warmed above the ordering temperature.

Two measurement procedures were employed to obtain the data presented in Fig. 3 a,b). They are defined in the top panels of the figure. In the first procedure, dubbed field temperature cycling (FTC), the sample is warmed up to T cycl. from 300 K keeping µ 0 H = 0.2T along y and then cooled down to back to 300 K and then PHE was measured by sweeping the field from -0.2 T to +0.2 T and back to -0.2 T . In the zero field temperature cycling (ZFTC) process, the magnetic field was set to zero during the warm-up to T cycl. and cool-down to 300 K. Af-terwards a PHE measurement was performed by sweeping the magnetic field in an identical fashion to the FTC process.

The PHE data obtained following these two ways of temperature cycling is displayed in Fig. 3. The left panels show the FTC process and the right panels show the ZFTC process with several selected cycling temperatures. All measurements were performed at 300 K. Fig. 3b,f) show the PHE at the T meas. = 300 K, peaking at 0.2 µΩ cm. When the cycling temperature is up to T cycl. = 360 K, the peak value of residual PHE decreases to 0.1 µΩ cm in the FTC(see Fig. 3c)) while it is 0.18 µΩ cm in the ZFTC (Fig. 3g)). With further increasing T cycl. to 370 K which is close to the T N and cooling back to T meas. , only a slight residual PHE remains in Fig. 3d). Remarkably, the PHE is barely changed in the ZFTC in the Fig. 3h) while the cycling temperature approaches T N . The PHE disappears when T cycl is higher than T N in the FTC. For comparison, it can sustain beyond the T N for ZFTC process, and it still has a finite value when T cycl. = 380 K, slightly, but significantly, larger than T N (see Fig. 3i)).

The evolution of the amplitude of the PHE measured at T = 300 K after thermal cycling is shown as a function of T cycl. in Fig. 4a). The temperature dependence of magnetization is also shown. Within experimental margin, the magnitude of field-cycled PHE and M(T ) curve have a similar evolution. On the other hand, the amplitude of PHE after a ZFTC process is more robust and survives even when T cycl. > T N . This indicates that thermal fluctuations above T N are strong enough to play a role in smearing out the chirality of magnetic domain walls, Indeed, warming beyond T N is expected to destroy the antiferromagnetic structure.

The temperature dependence of AHE and the peak value of the symmetric component are compared in Fig. 4b). Both show a broad peak around 150 K, and become vanishingly small at the Néel temperature T N of 370 K. The anomalous Hall conductivity, which can be extracted by combining Hall and residual resistivity is almost flat at low temperature [START_REF] Xu | Finite-temperature violation of the anomalous transverse Wiedemann-Franz law[END_REF] . The temperature dependence of PHE is some what different. The evolution of the ratio of PHE/AHE with temperature is shown in Fig. 1(d)). It steadily increases with decreasing temperature. Its peak amplitude is about 0.2, which is half of what was observed in Mn 3 Sn 29 . This is presumably related to the fact that the hysteresis loop is wider and the coercive fields larger in the tin-based compound.

The manipulation of this PHE and memory effect by field and temperature cycling could be used in data storage. The PHE has two states (positive and negative) at room temperature, depending on the field history. By heating the device slightly higher than T N , can erase the direction information or binary information. So it could act as information store devices, realizing writing and erasing operations, while the AHE only depending on field orientation cannot fulfill the task. The rotating field process can be created by two perpendicular fields produced by two small coils in proximity to the device, so actual rotation of device or field is avoided. However, the coercive field in the Mn 3 Ge is still large at 15 mT and the T N are still high. If doping or other methods can reduces the T N and coercive field, the device could be realized.

In summary, we observed the planar Hall effect in non-collinear antiferromagnetic Kagome Mn 3 Ge and its memory effect down to 2 K. The ratio of PHE/AHE and the prior field to suppress the PHE are smaller than those in Mn 3 Sn. Moreover, we erase this memory effect by temperature cycling through its Néel temperature with or without field, and find the sustain of PHE even the cycling temperature is higher than T N . This memory effect and memory erasing in PHE will be a new potential application in data storage device.

( 1 )

 1 Wuhan National High Magnetic Field Center and school of physics, Huazhong University of Science and Technology, Wuhan 430074,China (2) Laboratoire de Physique et d'Etude des Matériaux (CNRS), ESPCI Paris, PSL Research University, 75005 Paris, France (Dated: 5 November 2020)

Fig. 1 a

 1 Fig.1 a) and c)) shows the measurement procedure to obtain the AHE and PHE. The electric current was applied along z[0001] and the voltage was measured along y[01 10]. The AHE was recorded by orienting the magnetic field along x[2 11 0], and sweeping it from negative to positive values(Fig.1 a)) and/or vice versa(Fig.1 b)). As seen in the Fig.1 a), three different regimes (I, II, III) replace each other as the

FIG. 1 .

 1 FIG.1. Anomalous Hall effect and planar Hall effect at room temperature a) and b) Anomalous Hall effect (AHE) with field sweeping from µ 0 H//x → µ 0 H//x and x → -x. Curves with transparent symbols refer to the opposite sweeping direction. c) Planar Hall effect (PHE) with an prior field rotated from the final state of a). d) PHE with an initial field rotated from the final state of b). Note that the magnetic field has been rotated from out-of-plane to in-plane to induce chirality of the antiferromagnetic domain walls. The sign of PHE is set by the history of prior orientation before rotation. e) and f) show two domain wall chiralities[START_REF] Li | Chiral domain walls of Mn 3 Sn and their memory[END_REF] . Each arrow represents an octupole moment[START_REF] Suzuki | Cluster multipole theory for anomalous Hall effect in antiferromagnets[END_REF][START_REF] Sugimoto | Electrical nucleation, displacement, and detection of antiferromagnetic domain walls in the chiral antiferromagnet Mn 3 Sn[END_REF] .

FIG. 2 .

 2 FIG. 2. Dependence of the planar Hall effect on the amplitude of the prior field: (a) Raw PHE data after rotating the field from +π/2 to π. (b) The asymmetric component of the measured signal. (c) The symmetric component. d) The peak amplitude of the two components of PHE as a function of the amplitude of the prior magnetic field. The symmetric part follows 1/µ 0 H, while the asymmetric part is almost flat, indicating that is a residual AHE signal.

TFIG. 3 .

 3 FIG.3. The evolution of planar Hall effect with thermal cycling a) Two distinct procedures. In field-thermal-cycling(FTC), the sample was warmed from T meas. to cycling temperature(T cycl. ) and cooled down to T meas. and then measurement was performed. In the zero-field-thermal-cycling(ZFTC), the warm-up to T cycl. and the cool-down to T meas. were performed in absence of magnetic field. b,c,d,e) show the FTC planar Hall effect with different cycling temperatures. Panel f.g.h.i) show the ZFTC residual planar Hall effect. All measurements were performed at T meas. = 300K. In both cases, the planar Hall effect gradually decreases with the approach to T N . But it drops faster in the FTC process than in the ZFTC process.

Acknowledgements-This work was supported by the National Key Research and Development Program of China (Grant no. 2016YFA0401704) and the National Science Foundation of China (Grant nos.51861135104 and 11574097). In France, it was supported by the Agence Nationale de la Recherche (ANR-18-CE92-0020-01; ANR-19-CE30-0014-04) and by Jeunes Equipes de l Institut de Physique du Collège de France. L. X. acknowledges a PhD scholarship by the China Scholarship Council(CSC).

Date Availability-The data that support the findings of this study are available from the corresponding author upon reasonable request * zengwei.zhu@hust.edu.cn