ECONOMIC MIGRATIONS, HOUSEHOLDS AND PATRIARCHY IN AFRICA AND SOUTHERN EUROPE

Véronique PETIT1 and Yves CHARBIT2 Migration, whether internal or international, is classically analysed in migration theories as a behaviour associated with social change, professional mobility, technological innovation and, more globally, modernity, the migrants being presented as entrepreneurs gifted with specific psychosocial characteristics. On the contrary, patriarchy is defined as a type of conservative social organisation; a certain number of institutions and practices based on law, custom and religion aimed at ensuring its perpetuation. For example, in West Africa, patriarchy functions to the advantage of the oldest men while the twofold seniority of gender and age allows them to control both women (through polygamy) and younger men (control over access to land), thus all the resources linked to production and reproduction. Opposition between migration and patriarchy appears radical and a source of conflicts: the changes brought about by the mobility of members of society hitherto subject to patriarchal authority undermines the latter and leads to backlashes from those who wield power in the society of origin. The reality is undoubtedly more complex, as the interrelations between migrations and patriarchy may or may not be a source of conflict, given the complexity inherent to both the domain of international migrations (historical narrative, evolutions of flows, measurement problems) and that of patriarchy (world diversity of family and societal organisations). For instance forced migration of refugees are governed by political factors largely exogenous to strictly patriarchal rationales. Indeed, forced mobility has very real impacts of on social organisations, on kinship and thus on patriarchy, but this is another subject. Besides, even assuming migration is a factor of change of patriarchal systems, it is evidently not the only one. Economic and political changes, new information technologies, the increased level of education, in which all favour the rise of individualism, can occur without any migration flows.

A few words are needed to explain three basic choices I made. First, this interdisciplinary paper deals with the structure and functioning of households and only as far as they are affected by economic migration. I have chosen the household as a dependant variable because even when the migrant is an individual the household he belongs to played a major part in the process of migration. As Faist put it, the meso level is crucial [START_REF] Faist | The crucial meso-level[END_REF]. Second, examples are drawn from various countries located in southern Europe, northern and sub-Saharan Africa to show how the diversity of local contexts shapes the broad relation between migration and patriarchy. Third, the period under consideration is roughly the second half of the XXth century (1950's-2000's), when emigration flows were rooted in societies in which patriarchy had only partially been affected by the impact of globalisation. Referring to two partly intertwined paradigms, gender and social development, I start from patriarchy seen as an ideal model and briefly sketch the control it is believed to exert on demographic behaviours, notably migration (I). From a strictly demographic standpoint, migration leads women to become the heads of households in the absence of their husbands. The "sex ratio argument", largely verified empirically, is nevertheless quite debatable as a real explanation (II). Economic data are called at the bar to explore the internal functioning of these households. In particular, since migration is a possible response to poverty, are households managed by women more vulnerable than others and can one speak of the "feminisation of poverty" in Senegal (III)? Sociologically, massive emigration took place from the 1950's in the countries surrounding the Mediterranean Sea and the induced dynamics of intra-family relations have been amply documented (IV). Last, the migrations of the Dogon (Mali) reveals the anthropological consequences of the double seniority of gender and age on households (V). I conclude with some epistemological criticism of the much too simplistic opposition between migration and patriarchy (VI).

PATRIARCHY: THE IDEAL MODEL AND DEMOGRAPHIC BEHAVIOURS

The authority of patriarchy can be analysed as an ideal model that weighs on nearly all demographic behaviours. Decisions related to marriage (choice of spouse, place of residence of the new couple) are made in the framework of strategies of alliance in which the family's interests take precedence over the aspirations of young adults old enough to be married. Once the marriage is concluded, the new couple remains under the authority of the elders, and the horizontal solidarity between the spouses is much weaker than the vertical solidarity of the lineage. As for marriage and reproduction, since controlling the female body is a prerogative essential for ensuring male authority, women are totally bereft of decision-making power regarding fertility and contraception. This applies for the management of health in general, notably recourse to traditional healers or to "marabouts" chosen as a function of the patriarch's belief.

Neither education nor the pursuit of studies are decided as a function of the individual fates of the children, but once again according to what is perceived as being in the interests of the family. Girls are usually sacrificed to the benefit of boys, since in the patriarchal system their future is limited to childbearing. But not all the boys are necessarily educated, as the head of the family may decide to privilege the education of the one boy deemed to have the greatest potential, with the others being rapidly withdrawn from schooling to participate in the economic tasks of the economic unit represented by the family. In sub-Saharan Africa, the patriarch also decides on the placement of one or more children outside the family and sometimes for very long periods, for reasons of apprenticeship, religious education, and to contribute economically to the household. More generally, in poorly mechanised rural economies, family labour is vital for production, above all in self-subsistence farming, and the use of female and child labour without monetary remuneration is the rule. However, this control by men over this economically crucial resource has certain limitations, notably the possibility for women to cultivate separate fields whose crops belong to them; and the management by the wife, often grouped with the wives of other families organised in tontines, of a specific segment of economic activity (for example, selling the produce).

This ideal model must however be nuanced as changes have occurred in family structures and family organisation, and in particular the roles and links of kinship. Regarding sub-Saharan Africa, works have shown that the economic crisis that occurred after 1980 altered the bonds of solidarity between individuals and damaged the myth of the large, generous African family, confronted with a fall in the income obtained from solidarity within a kinship system that has become more selective and chosen [START_REF] Dime | Remise en cause, reconfiguration ou recomposition ? Des solidarités familiales à l'épreuve de la précarité à Dakar[END_REF][START_REF] Marie | Communauté, individualisme, communautarisme : hypothèses anthropologiques sur quelques paradoxes africains[END_REF][START_REF] Leimdorfer | L'Afrique des individus : itinéraires citadins dans l'Afrique contemporaine[END_REF][START_REF] Calves | Présentation : les processus d'individualisation « à l'africaine[END_REF]. The same caution is necessary, as we shall see, when examining the case of migration and the relation between patriarchy, migration and one of the characteristics of family structures, the fact that the head of the family is a woman.

WOMEN HEADS OF HOUSEHOLD AND THE ARGUMENT OF THE SEX RATIO

Let us briefly recall the archetypal case of Botswana where international emigration grew swiftly, following the country's independence in 1966, for the most part tending towards South Africa. In 1982, 54% of international migrants went to work in the mines of South Africa, of which 83% were between 15 and 34 years old. Already in the censuses of 1940, 1964 and 1971, the proportion of absent men was about 25% versus 4% for women. In 1982, the average sex ratio for the whole population was 82 men for 100 women. The rural exodus to the cities also affected mostly men (59%). As a result, 36% of heads of all Botswanan households were women, and 8% of these households included no adult men. 91% of women heads of household declared that their only resource was the money sent to them by emigrant men (69% internal migrants and 22% international migrants), the remaining 9% lived on pensions and various forms of aid (Census 1982, table 14.1). 83% of de facto women heads of household were married. In a nutshell, to be head of a household was a direct consequence of her husband's emigration. This irrefutable relation between the migration of men and the frequency of households headed by women had long ago been theorized. The "sex ratio" argument was systematised by Antony Marino in 1970, although it had been mooted as far back as 1955 by Georges Roberts in a little known article on Barbados, by Michael. G. Smith in 1961 regarding the Cariacou and by Keith Otterbein in 1965. In brief, the argument explains the high proportions of women heads of household by the shortage of men in the population. For example, in Barbados, the emigration of men has led to the feminisation of labour: "Women found themselves at the head of household units in one way or another, so that they had to seek jobs to satisfy their needs and those of their children" (Roberts 1955: 278-279).

But is patriarchal functioning really being called into question? Two major criticisms arise. First, the explanatory strength of the sex ratio argument is questionable; second it suffers from a major bias in research work. In a scathing response to Otterbein", William Goode mocked "this great intuition that if men were absent more women would be at the head of households " (1967: 226). It does not appear useful here to recount the controversy, though it should be underlined that Goode's objection missed the nature of the argument. A subtler opinion was expressed by M.G. Smith in his analysis of the situation at Carriacou. Since there was a severe shortage of men (649 men for 1000 women) the women had the choice of emigrating, of remaining their whole lives in chaste celibacy, or of accepting to be single mothers. He did not see the outcome as the result of a deliberate choice, but as the adaptation to a constraint which, through time, ended by becoming institutionalised (1961: 466-467).

It is inevitable that men have extra-residential unions with women for the system to function. In addition, Marino added that with such a shortage of men, the levels of relatively high fertility in the Commonwealth Caribbean could be maintained only thanks to de facto polygamy (1970: 167). Last but not least, migration did not weaken patriarchy. The paradox is only apparent; migration contributed to strengthening it, since the scarcity of men bolstered their power over women. In short, the crucial explanatory variable is the system of unions, in which the demographic dimension is not central, in comparison to values and norms. In other contexts, with the same sex ratio, women could very well be socially obliged to remain alone. Such was the case in south Europe and North Africa, as we shall see.

The second objection stemmed from a research bias that had long prevailed despite facts that were perfectly well-known. Since 1947, the share of women in migratory flows has fluctuated between 47 and 49%. Therefore, it is apparent that nearly one international migrant in two is a woman. But in addition to this global figure, the sex ratio varies considerably according to region (women represented 42% of migrants in Asia in 2016 versus 52.4% in Europe and 51.2% in North America, for example). Certain were highly feminised (migrations linked to care for example, Filipino women to Spain, Caribbean to the UK, the flow from Cape Verde), while others were dominated by men (Algerian migrants in France at the beginning of the 1950s), or more balanced. This demographic composition can be explained by different factors: the freedom of movement given by each society to its members as a function of their status (linked to gender and age in particular), the attractiveness of certain sectors providing jobs, strengthened by migratory policies aimed at selecting certain migrants in the receiving country or by policies aimed at establishing certain occupations in the country of departure as a function of the gendered segmentation of job markets.

For a long time, the migrant was always perceived as a male in both common sense and scientific representations. Migrating women were only viewed as accompanying the male migrants, as "comigrants" incapable of decision, autonomy, economic activity and benefiting from family reunification policies. Scarcely visible socially, still less economically, and given little political consideration whereas they contributed to demography and population dynamics, migrant women were for long minimised not only in quantitative statistical demographic and economic works, but also in qualitative socio-anthropological ones. Nonetheless, since the beginning of the 1990s this androcentric vision of migration has been subject to deconstruction and called into question [START_REF] Zlotnik | Preliminary report of the United Nations Expert Group Meeting on the feminization of international migration[END_REF][START_REF] Green | Repenser les migrations[END_REF]Zlotnik 2003;[START_REF] Dumitru | Existe-t-il une féminisation de la migration internationale ?[END_REF][START_REF] Dumitru | Féminisation de la migration qualifiée : les raisons d'une invisibilité[END_REF]. Castles et al. 2014) pointed out five trends of this new migratory mode: globalisation, acceleration, differentiation, the politicisation of migrations and their feminisation. More than a statistical evolution that would have uncovered the existence of a large proportion of migrants in migratory flows, this process of feminisation of migrations is above all a change in the perception of researchers regarding the profiles of migrants, their roles, investments in the migratory experience and the diversity of situations [START_REF] Catarino | Femmes, genre, migration et mobilités[END_REF][START_REF] Catarino | Femmes, genre, migration et mobilités[END_REF]). Recent research questioned the effects of the migration of women on family roles, maternity and paternity, the care of children in transnational families, and the autonomy and emancipation of women due to mobility [START_REF] Casas | Migration, genre et foyers transnationaux : un état de la bibliographie » in Les cahiers du CREDEF[END_REF][START_REF] Mondain | Les effets de l'émigration masculine sur les femmes et leur autonomie : entre maintien et transformation des rapports sociaux de sexe traditionnels au Sénégal[END_REF]Lesclingland and Hertrich 2017). Finally, the original factor of the functioning of patriarchal systems being called into question by migrant women who escape patriarchal domination has been totally obscured by research. However, from the standpoint of the structure of households in the strict meaning, the emergence of households managed by women in the countries of origin remains a useful quantitative indicator, as will be seen in the case of Senegal.

THE DEMOGRAPHY AND ECONOMY OF FEMALE HEADED HOUSEHOLDS IN SENEGAL

In Senegal, as elsewhere in sub-Saharan Africa, a growing proportion of households are headed by women; it increased from 19.4% in 2001 to 27.4% in 2011. 3 Can women heads of households be personally characterised by a socio-demographic profile that portrays vulnerability? What are the other factors of vulnerability of households led by women? Finally, are these households more or less vulnerable than, those headed by men? The indicator is the final household consumption 4 .

Demographic and economic characteristics of women heads of household

Unsurprisingly, 7 to 9 out of 10 widows and divorcee women are heads of household contrary to married ones (graph 1). In the rather brief span of time of ten year however, the proportions of female divorcees or currently married in a polygamous union registered as heads of households have increased significantly, whereas the categories of widowhood and monogamous marriage have almost not changed. A twofold societal change is undergoing. When families are broken by divorce, it is more and more common for women to become head of the household. Similarly, with the growing urbanisation, the lack of housing space may oblige polygamous households to split between two lodgings and the second wife may have declared herself as head of household.

Graph 1. Matrimonial status of women heads of household (Surveys of 2000/2001 and 2011)

The size of household is unfavourable to women. On average, male households have 2 more members.

The smaller size of female-headed households may represent a factor of vulnerability, since the fewer the members of the household, the more living conditions will depend on the income of the head of the household. In male-headed households, which are larger, other members can contribute or substitute the head of the household in case of the latter's incapacity. They can even be the mainstay of the family during certain periods, such as the lean period between two harvests.

Other individual characteristics of women heads of household contribute to their vulnerability: they have less human capital and are more often poorly integrated in the job market (table 1). Their poor education, combined with cultural factors that confine them to their role at home, hinder their participation in the labour market. Not only are a large minority excluded from it, but those who work are restricted, for most part, to activities of little productive value. In particular, the rate of activity of women heads of household is substantially lower than that of men: 53.5% versus 76.5%. They are therefore characterised by a high rate of inactivity. Finally, they have the least secure and least wellpaid jobs. 14.1% of women head of household are wage earners in the modern sector versus 21.4% of men and they earn three times less than men. In brief, women heads of household are more disadvantaged, whether in terms of education, jobs or income. [2000][2001]. 6,594 households were surveyed in 2000-2001 and 12,120 ten years later. was based on the analysis of the data of the first survey. 4 The methodology is described in Charbit and Kebe 2007.

More fragile female-headed households

Women's individual characteristics are exacerbated by the fact that their households are more fragile (table 2). Besides the absence of an active spouse, the secondary members of the household are to a great extent vulnerable person (young, old, etc.). Male and female members of male-headed households have almost the same age (23 years on average), whereas in female-headed households men are on average 8 years younger than women (18 years versus 26 years). Males heads of households thus benefit from a larger male population of active age. The economic participation of the members in women-headed households is not very high (table 2).

Table 2. Economic participation of members of the household by sex of head Among those old enough to work, only 39.5% are active. Likewise, the unemployment rate is very high, insofar as 13.4% of active persons are awaiting work, so only a small number of people work and they must ensure the existence of all the others. In male-headed households, the rate of activity of all the members is close to 60%, and their active members are less affected by unemployment (7.7%). Further, the average income of the male head is 10 times higher than that of the other members of the household whereas this difference is much lower (ratio of only 2 to 1) in female-headed households, a factor conducive to a lesser authority in decision-making and the functioning of the household in comparison to their male counterparts.

Compensating factors

However, female-headed households make use of social networks to mitigate the meagreness of their incomes: participation in mutual and other forms of credit, especially gifts and donations from persons outside the household, provide them with supplementary resources. Exchanging gifts and donations is a common practice in Senegal and the most disadvantaged can often count on the support of better-off households and persons. This solidarity is generally practiced at the time of festivities and ceremonies such as marriages, baptisms, deaths and so forth. In most cases the help provided is financial, though gifts in kind are far from negligible as about one in ten households receive only donations in kind.

Female households benefit most from this form of solidarity (85.9% versus 75.2%, see table 3,) and the amount of donations received is twice as high. The theory of reciprocal dependence between the migrant and their family explains the durability of the links despite distance and time [START_REF] Stark | The Migration of labour[END_REF]. Women heads receive far more often transfers on a regular basis: 57.7% versus 34.1%) (table 5).

Table 5. Characteristics of remittances received by sex of head of household Finally, does the balance eventually lean in favour of women? The distribution of households according to type, sex of head and prevalence of poverty shows (graph 2) that the prevalence of poverty is lower in households headed by women (34.7% versus 50.6%) except in the small category of single persons. In all other types, the vulnerability of women heads is certainly offset by the gifts contributed by other households and by remittances.

Graph 2. Incidence of poverty by sex of head and type of household

Are women being empowered?

To conclude, households headed by women are less exposed to monetary poverty than those headed by men because they benefit more from social networks (help provided by other households and transfers from migrants) that offset all the other factors of vulnerability. One should not however conclude that a process of empowerment is at work, understood as a greater autonomy vis-à-vis their partner, family of origin, the community to which they belong, society as a whole and also in terms of psychological, social, cultural, economic and political attainment. The stake is that of dependence. Gifts received by households are a source of income that undoubtedly permits reducing their difficulties, but these gifts cannot guarantee their regular functioning. And insofar as women are obliged to turn to networks of solidarity to satisfy the needs of their households, they risk finding themselves trapped in a relation of assistance. Although it is true that this is culturally and socially accepted, it does not contribute to increasing the autonomy of women heads of households. As for migratory transfers, they raise another problem regarding the possible change in the status of women linked to the reduction of poverty. If the money sent by the husband working abroad is controlled by third parties, their wives will not benefit from increased freedom in the decision-making needed for the functioning of their households. Thus everything depends on the cultural and social dimensions conditions associated with the micro economics of these households.

THE SOCIOLOGY OF ABSENCE: WOMEN AND CHILDREN IN THE COUNTRIES OF ORIGIN

As early as the 1950s and 1960s, millions of workers came from different family configurations in countries around the Mediterranean Sea (bachelors, husbands who had left wives and children, or who were accompanied or reunited later on with them) to settle in the industrialised countries of Western Europe. However the situations of the families that stayed in the countries of origin were given scant attention for a long time. Only in 1977 was an international seminar held in Ankara [START_REF] Yves | Children of Migrant Workers and their Home Countries[END_REF]. In 1980 the Human Rights Department of UNESCO stimulated exploratory studies of a sociological nature in selected countries (Egypt: [START_REF] Kafagy | The Impact of Male Migration on the Structure of Family and the Role of Women Left Behind in Rural and Urban Egypt[END_REF]. Greece: Tsilis 1986. Italy: Ginevra et al. 1986. Portugal : Wall 1981. Tunisia: Zamiti 1986. Turkey: [START_REF] Ulusan | Les femmes d'Altigrad[END_REF]. 5 The changes brought about by the migration of the head of the family varied according to the age of the women, their social background and place of residence, and they reacted differently to the departure of their husbands, showing the complexity of the interactions between migration and patriarchy. Almost at the same time, UNICEF entered the field and commissioned a synthesis [START_REF] Charbit | Enfants, familles, migrations dans le bassin méditerranéen[END_REF], who also provide a rich commented bibliography of 241 entries), addressing demographic data, legal protection, education, physical and mental health problems of left-behind children and mothers. I borrow here part of their analyses of intra-family dynamics. The oil crisis of 1974 put to an end the flows of active workers to Europe, while family reunification was tolerated, notably in France. The so-called "Thirty glorious decades" (1950's-1970's) therefore provide a unique case study to analyse the relationship between migration of the breadwinners and patriarchy in the countries of origin.

Children and the absent father

The same pattern was observed in various countries in spite of cultural differences (Spain: [START_REF] Mancho | Les enfants restés dans les pays d'origine alors que leur père et parfois leur mère ont émigré à l'étranger[END_REF]. Greece: Amira 1977. Italy: Di Nicola 1974 ;Secondolfo 1974. Morocco: Guignard et al. 1981. Portugal : Barreiros Santos and Serra 1977. Tunisia: Chater 1977 ;[START_REF] Zamiti | Influence de l'absence du père sur le développement psychoaffectif des enfants de migrants[END_REF]. Turkey: Dilzis 1977. Yougoslavia: Ilic et al. 1979;[START_REF] Zuzul | Neurotic disorders among children whose parents work abroad[END_REF]. The absence of the father profoundly disturbed the children, who were anxious, insecure, suffered from problems of identity and complained of lack of affection. They exhibited a greater frequency of psychiatric symptoms than children of non-migrants. The often very long separation, from five to ten years, created an irremediable void. The children, who could not mourn, recreated a partly mythical father, a situation that plunged them into a state of unreality unfavorable to the construction of a solid identity. When both parents were absent the same mechanism of idealization of the family took place to the detriment of grandparents in charge of the children. Children generally felt responsible for leaving their parents working abroad to improve their situation, hence an overt sense of guilt. But it was at least partly a transfer of guilt, when parents had stated that they had "abandoned" their children and invoked the argument that they sacrificed themselves. Such was the argument internalized by the children and re-expressed in the interviews.

"White widows"

In the absence of their husbands the women predictably became heads of the households and raised their children alone. Consequently, their responsibilities and workloads increased considerably: the education of children, managing savings, relations with different administrations, etc. But as the economic situation of the couple improved only after a period lasting from five to ten years or because the amounts of money transferred were too small, many of these women had to work. In Italy and Portugal, the families of migrants generally belonged to the lower social classes and most of the women worked in Portugal as farm labourers and in Italy as non-declared workers. In southern Italy, where hardly any modern industry existed and undeclared work was the main activity, women worked making clothes at home for paltry wages, sometimes helped by their children. In regions where social pressure was very strong, this form of work also allowed them to remain relatively close to the traditional image of the wife at home. In Greece, half the women of the sample worked and nearly all of them on their own farm. Many of them considered as a positive change to have been forced to take initiatives in areas traditionally reserved for men, but the traditional values of these patriarchal societies did not allow women to benefit from the social recognition befitting their new responsibilities. Still worse, in the absence of a man, they found themselves more isolated and marginalised than the wives of non-migrants. The very long absences of their husbands caused these women, to live long years of dissatisfaction and solitude as "white widows". Many of them feared abandonment. Solitude, marginalisation and overworking very often led to serious psychiatric and psychosomatic problems, with major impacts on the children.

The mother-child relationship

The marginalisation of mothers had major repercussions on their children's education, especially that of young boys. In Turkey, at an age when they are usually initiated by their father "outside", those whose father was absent and with whom they could not go out with, were refused activities in keeping with their age while others escaped supervision of any kind. These women who underwent moments of absence, anxiety and even depression, were unable to mitigate the absence of the father, a source of reassuring and protective authority, and were no longer able to control their families. An Italian study showed that the children in almost all the cases examined, declared that the maternal image was not enough to ensure family cohesion. This powerlessness even strengthened the negative image that some women had of themselves: "I'm incapable of being a mother … my children will turn out badly". In some cases, this lack of authority led the sons to fill the vacuum. Some Tunisian and Italian mothers complained about the aggressiveness of their children, which could go as far as physical violence [START_REF] Chater | Problèmes spécifiques aux enfants tunisiens restés dans leur pays[END_REF]. The absence of the father therefore resulted in the fragmentation of the family, unable to obtain economic or other forms of compensation. A Tunisian study of women consulting psychologists underlined the difficulties of mothers in coping with the imbalances caused by the departure of their husbands [START_REF] Zamiti | Influence de l'absence du père sur le développement psychoaffectif des enfants de migrants[END_REF]. Some of them wanted to substitute the father and wield strict authoritarian control over their children. These were generally women who had driven their husband to migrate. The father was rarely mentioned and only by way of emotional blackmail. This excess authority led to all sorts of inhibitions in the children, misguided aggression and functional disorders. These women, who fluctuated between constraint and resignation, found it hard to bear the absence of their husbands, sometimes projecting the responsibility of the situation on their children, and sometimes dowsing their aggression in an effusion of kindness. This climate of uncertainty led the children to become withdrawn and subject to psychomotor retardation.

Is the extended family a compensatory factor?

It had long been thought that the extended family, such as the zadrugas in Yugoslavia, could be a compensatory factor capable of mitigating the absence of the male heads of households. A woman could therefore count on her family or family-in-law for different forms of aid, including the children's education. For the latter this meant the presence of a man who, without being able to wholly take the father's place, nonetheless fulfilled the most important roles: image, authority, initiatory role. In Algeria where a strong patrilineal system prevailed at least until the 1990s, the mother and children lived under a "personal" roof, thus close to cousins, uncles and grandparents and often under the roof of the paternal grandfather, so that the child, whatever their age, was not deprived of the image of the adult man in the family, even if the father returned only once a year. The role of the paternal lineage in the child's education was very clear at the time of enrolment and throughout the school year. Enrolment was performed by the father on his return home in the country, and if not then most often by the mother, brothers and sisters, or even a cousin or uncle. The child's education was overseen by the mother, an uncle or cousin, or an elder brother. Thus, a male figure intervened alongside the mother in the education of children in 35% of cases, so paradoxically the father's absence resulted in strengthening the family's control over the child's education. However, tighter controls did not result in better school results when a cousin or uncle oversaw the child's education, certainly because it was a family obligation and a burden, to which they did not genuinely commit themselves.

Finally, these studies of psycho-sociological and sociological orientation show, within the specific context of economic immigration during the boom period and then recession , the need to clearly identify the real actors and to avoid essentializing patriarchy and migration. The behaviour of left-behind mothers and children was a blend of changes caused by migration with increased responsibilities, and submission to more severe social control due to the absence of the man. In parallel, men of the paternal lineage of all ages embodied the patriarchy of the country of origin and very concrete roles in defence of patriarchy conferred to them. Of course many individual factors are involved, warning against putting forward only one scenario of the evolution of behaviours.

Patriarchs abroad

But the story would not be complete if the situation in immigration countries was not taken into account. A weakening of patriarchy also occurred in European immigration countries. The historical pattern of immigration since the 1950s induced a threefold devaluation of the father. In the 1950's and 1960's, as a recently arrived migrant workers, men come to Europe alone discovered economic exploitation, different forms of discrimination in daily life and sometimes racism. They experienced "La plus haute des solitudes" (The deepest solitude) described by the novelist Tahar Benjelloun (1977). Then, as time passed, isolated workers married or brought their families over (for example, in the framework of family reunification), and a breach often widened between them and their children. If the latter were born or had been educated abroad, their knowhow and mastery of the language in daily life endowed them with an authority their fathers lacked. Lastly, when the Oil Crisis of 1974 occurred, the dismissal of large numbers of foreign workers aggravated the economic fragility of the family and fathers, now old and unemployed, found themselves in the domestic sphere usually reserved for women.

In most cases the mentality of migrating men changed, and they displayed weaker links with the values of patriarchy, especially when married with a woman of the country of settlement. One of the consequences was the importance given to the education of girls by fathers living abroad. They were faced with models different from the ideal type of the patriarch, especially in Sweden where equality between men and women is firmly entrenched. In France, these men massively employed in subaltern jobs in the metalworking industry or in construction, wanted their children to benefit from the French republican model which holds education to be the key to social mobility. On the contrary, the negative experience of life abroad mentioned above sometimes resulted in a quest for identity, an over-positive perception of the country of origin and the reassertion of patriarchal values, while children living abroad, often bi-national, have been faced with the sometimes painful contrast between their own identity and the devalued identity of their father and mother.

THE ANTHROPOLOGY OF DOGON WOMEN HEADS OF HOUSEHOLD

Up to now, I essentially focused on the relation between international migration and patriarchy, international migration creating the greatest distance possible with the values and functioning of patriarchy in the migrants' societies of origin. However, once again, a research bias is introduced by viewing migration solely from the standpoint of the industrialised countries. It is nevertheless vital to avoid narrowing the analysis. First, the migrants are only taken into account once they have arrived to seek work and a wide range of complex processes in the country of origin are ignored, notably the motivations of the rural exodus which often preceded international migration. This bias is increased by the separation between specialists in international migration and those of internal migration. Second, rather than taking a purely positivist view, imbued with western rationality, and sticking a stencil of interpretation on reality in view to strengthening his research problematic and hypotheses, the researcher should adopt a comprehensive Weberian view: how to explain the migrants' behaviour within the values and norms of their countries of origin? For instance, international migration is only one of the possible responses to poverty, in the same way as seeking employment and nondemographic responses such as pursuing multiple activities [START_REF] Charbit | Toward a Comprehensive Demography: Rethinking the Research Agenda on Change and Response[END_REF]. Obviously, the importance and nature of the resources used by households will vary as has been seen in the case of Senegal. But the very choice of response cannot be interpreted in terms of economic calculation and cultural and social dimensions cannot be ignored. In other words, anthropological analysis is the only means of showing the link between patriarchal structures and the rationale intrinsic to internal and international migrations. The Dogon of Mali are a cas d'école.

The Dogon represent a little less than 5% of the population of Mali. I studied the subgroup that lives in the arrondissement of Sangha on the cliffs of Bandiagara, located in the loop of the Niger northeast of Bamako. In 1991, during our fieldwork I numbered 21150 individuals in 1991 of which 19.7% were migrants, but women head of household were strikingly rare (Petit 1998): 13 out of the 1314 heads of household and about 8 women Dogon heads of household out of 10 were elderly or widows. It is clear that the absence of men in no way systematically led women to accede to the status of head of household. My hypothesis is that the distribution of gender roles and the strong hierarchy of age classes explained that despite a very low sex ratio, the function of family head remained reserved for men. In addition, since the families were not nuclear, as long as a man related to the man that had migrated remained, he could replace him or those that were absent. Furthermore, because the absent men were relatively young and did not necessarily have the status of head of household, they could still be associated with the household of their father or with that of a brother. Let us now examine the situation of the rare Dogon women heads of household. Two elements are decisive in understanding the low proportions of women heads of household: the status of women and the topographical environment. Other factors should be mentioned to account for other types of migrations, for example those of the young [START_REF] Véronique | Ana-yana : ceux qui partent loin du pays Dogon (Mali)[END_REF][START_REF] Véronique | Ana-yana : ceux qui partent loin du pays Dogon (Mali)[END_REF].

The place and status of women in society

Available anthropological work and by my own field observations showed that women could not become heads of family except in very precise and temporary situations. In a socially "normal" situation, the head of household was obligatorily the eldest male of the individuals present in the family. The gender relation prevailing over the age relation, a woman was statutorily a junior, outside any consideration of age. As Calame-Griaule observed because a woman never belonged to her husband's family, she remained a stranger, even after several years of marriage (Paulme 1940). One of the recognised freedoms of women was that they could break a union and start a new one or leave to re-join their paternal family. This freedom of movement in the social domain was incompatible with the status of head of family and she had little chance of becoming a head of household. Death also shows the statutory dependence of the Dogon woman. She could remain within the family as a widow or accept to marry, according to the custom of levirate marriage to the brother (as defined by the classificatory kinship) of the deceased husband. She could however decide to leave the family by returning to her father or remarry with a man outside the family of her deceased husband. In fact, the option chosen by the woman undoubtedly depended on her age and thus on her capacity to bear children. As long as she was fecund, she would seek to remarry quickly. If she was elderly or had reached the menopause, she was likely become a second or third wife by levirate marriage. Otherwise, she would remain a widow and her children would be bound to maintain her. Even if she had no children present or living in the family of her husband, she would not be driven away since that would amount to a lack of respect. If her son became the head of the household, she was sure that her place would not be contested.

Thus, for the Dogon woman, being the head of a household necessarily corresponded to a temporary situation. They occupied this position rather like a regent, in a situation of expectancy. The time needed to settle a succession could take longer than usual if the man called on to become the head of the family was a migrant, not to mention that he could refuse to return to the village. Women therefore only decided by proxy, as the real power lied elsewhere as normal attributes of the head of the household, especially economic and political power and women fell under the control of other men in the family under the gaze of the village elders. The brother of the wife's husband controlled the household's resources. The grain that feeds the family daily was stored in a male granary, traditionally that of the head of the family, and the woman only supplemented this food in case of need from her own reserves (Bouju 1984: 133-134). The "male granary", gê ana, was the largest of all the granaries. It could hold from 80 to 120 baskets of ears of corn, i.e. from 1600 to 2400 kg of grain. Having ownership over it is an assertion of power, that to feed people, make them one's dependents and control their fates. Women never owned these reserves.

The effect of the topographical environment

Another variable, the topographical position of the village of residence, was crucial in access of women to the status of head of household. In Dogon country, there is a contrast between the plateau and the escarpments. The former benefits from natural conditions that permit market gardening, notably onions, and from the impacts of tourism. On the other hand, the latter area suffers from isolation, topography and water resources hardly favourable for the development of market gardening which, with tourism, is the only source capable of generating monetary income. These economic differences have strong impacts on family structures. Firstly, 68.1% of the migrants came in 1991 from escarpment villages, which gathered only 55% of the total population. Secondly, the families were smaller on the escarpment, 13.4 people on average versus 32 people on the plateau, and they were above all composed of elderly persons and children. Families on the escarpment were often composed of a single household and the average age of this family is older than on the plateau. Despite the small sample available to us, the fact that all the women head of family lived on the escarpment meant that the sociodemographic situation (size and composition of family, origin of migrants) of the escarpment villages brought together the conditions that facilitated the accession of women to the status of head of household (tables 3 and 4).

Table 3. Distribution of heads of household by sex and topographical milieu Table 4. Characteristics of households by sex of head of household and topographical milieu These women, most often widowed and elderly, found themselves at the head of several people with lesser status, children and other old persons. Such a situation was socially tolerable as it did not completely call into question the functioning of a society based on the twofold hierarchy of age and gender. On the death of her husband, a wife could therefore more easily find herself at the head of a small family unit for want of men since they had often gone to settle definitively or temporarily in farming villages in the plain region. The seat of the large family, the gina, remained in the village of origin in the escarpment region, where the family fetishes were kept. The migrants on the plain always returned to the escarpment for the festival of boulo 6 , for the dama 7 . But the economic power increasingly lied in the villages of the plain, where the production of millet was most abundant. Those who had left for the plain frequently provided millet to the elderly remaining in the village and ensured their survival between harvests. The shift in authority was clear. A woman could "manage" children and other old people, but what was important for the future and the present of the family had moved elsewhere.

In brief, the possibility for a Dogon woman to be the head of a household depended only very marginally on the sex ratio. It was accepted only in exceptional circumstances, when family structures were the weakest or the economic situation dire and only as a temporary solution with limited attributes, because it obliged the group to go against one of the structural principles of its culture.

CONCLUSION: A FAR TOO SIMPLISTIC OPPOSITION

I started from the commonly accepted opposition between the patriarchal system, which "pub-counter ideas" assume to be conservative, and migration, also considered solely as a dynamic of modernity. To depart from such sketchy and simplistic views, demography, microeconomics of the household, sociology and anthropology have provided problematics, concepts and data. Finally, I would like to propose an epistemological discussion concerning each of the disciplines and their respective contributions to our knowledge of the structure and functioning of households.

Demographic and microeconomics quantification remains a unique contribution, notably for evaluating the feminisation of poverty. However, the limits of quantification become apparent immediately one sets aside measurement and tries to evaluate empowerment. But another criticism can be mentioned. The implicit assumption of demographic methodology is that everything occurs within the closed circle of the family living in a dwelling and that the male head of household is an allpowerful actor who exerts his domination over his wife and children. This is the western image of the patriarch holding absolute authority over the members of his family and of a largely autonomous individual who makes all the decisions "as a good father should", according to the Napoleonic Civil Code. In the liberal society of the 19 th century, the head of the family was assumed to be a totally autonomous and perfectly rational actor who pursued his economic interest according to utilitarian logic. In European history, and in that of non-European societies, this vision privileging individualism is sociologically and historically misleading. First sociologically, since patriarchy tends to perpetuate a social order that consecrates the predominance of the eldest men over their social juniors, the decisions are doubtless taken individually by each head of family but in conformity with what is common to the entire group of men in a dominant position: the system of values, norms, and sanctions deemed normal with respect to the members of the community under their domination. Secondly, historically, since the 5 th century BC in Athens, the separation between the public and private spheres has been one of the pillars of political order. The man was preeminent in the former while the woman dominated in what the Romans would later call the "domus". Under these conditions the authority of the patriarch is most often visible outside the domestic group, but the dynamic of the relations between husband and wife can give rise to mechanisms of influence of the wife over the husband, inspiring him discreetly but decisively regarding the decisions he takes. Likewise, in polygamous households in West Africa, the first wife plays a preeminent role in decisions concerning the domestic sphere and probably has an influence over the husband. What is more, the man is bound to respect precise rules regarding sexual relations with his spouses, with the practice of what is called "taking turns".

As for sociology, taking into account the complexity of the relations between migration and patriarchy implies considering migrant workers living abroad as representatives of patriarchal groups. In 1981, Boucebci, analysed the changes that had occurred in Algeria in extended families and in village communities, and observed that despite the strength of tradition, internal migrations and urbanisation had led to the reorganisation of family relations to the detriment of the group. The emigrant was no longer the delegate of the village group but an individual who attempted to survive and who merged in the world of negotiated labour. "The emigrant has become an almost anonymous member of a community whose administrative structure reflects an entity, the State, a body difficult to assimilate with the image of a father and relay an ancestral message. For the emigrant in a situation of cultural conflict, the calm reassurance retransmitted by the group remaining in the country no longer exists, and the latter ensures less and less its role as referent of the historic and cultural identity dreamt individually and collectively by the migrant group. This induces a distortion of the link between the emigrant and the family, both sources in the second degree of a multiplication of risk for the mother and child".

Boucebci's reflections of 1981 are premonitory since they foresee the obsolescence of the entire discussion of the supposed radical opposition between migration and patriarchy, brought about by globalisation and which affects both patriarchy and migration. Well before the advent of the Internet, the broadcasting of television in rural households from the 1960s onwards and even more so with the generalisation of parabolic antennas, considerably narrowed the gap between patriarchal and other societies. American telefilms, those produced by Bollywood, not to speak of the Brazilian telenovelas, all widely diffused around the world, have in some way placed a foot in the doorway of households and obliged all their members, both the men who embody authority and the others, to discover different modes of family functioning. In scenarios oozing fine sentiments, young adults fallen in love regularly come up against parental authority of a far less coercive nature and are aided by persons who play the role of mediators within the families. Regarding migration, the fall in the cost of transport has facilitated mobility, notwithstanding the barriers represented by restrictive immigrations policies implemented in Europe in particular. At the same time, new information technologies have contributed not only to the circulation of physical persons but also to that of ideas and models. The revolution of mobile phones, access to hitherto forbidden information, especially pornography, and more generally fascination for "Western" consumption, have had increasing stronger effects on the young. Thus, direct experience of migration, which called patriarchy into question, had been replaced by another mechanism based on the interiorization of models of behaviour deemed more prestigious than those experienced every day.

Last, the presumed opposition between patriarchy and migration is far too dichotomous from an anthropology point of view. Indeed, the patriarchal system can itself stimulate migration by designating who in the group of social juniors has the right to emigrate and by organising this emigration in the community, thus displaying its power and authority (examples: Petit on the Dogon, Sayad on Algeria; Timera on the West Africa). This social control of migration has been analysed as an initiatory rite: by travelling far from their society and country, the young must prove their courage, moral strength, endurance, and their capacity to assume and complete their project, the success of the enterprise guarantying them a number of rights on their return (access to the matrimonial system and to land). Initiatory rites aim at reproducing, under the control of the Elders (thus already initiated) who supervise the rituals, the social system through a series of physical and moral trials, and at transmitting an ensemble of knowhow and knowledge. The society of origin seeks to control the migrant from afar, by activating a value system strongly marked by notions of respect, honour and shame, that structure social and family links. This seems to echo various theoretical frameworks and concepts (migratory chain and networks, economy of debt, moral economy and notably the so-called migratory contract). But this appealing convergence between anthropology and socioeconomic theories of migration is vitiated, as migration corresponds more to a rite of separation. It allows individuals male and especially female migrants to acquire an economic capital that they can convert into a social and for men at least a political capital and migration sometimes jeopardises social order. Whereas the Elders hoped to control the energy, ambitions and claims of the young, migration can in time become an instrument of social subversion by generating (new) social disparities and by modifying the trajectories of success and access to power. Graph 1. Matrimonial status of women heads of household (Surveys of 2000(Surveys of /2001(Surveys of and 2011) ) Source: see note 2.

Table 1 .

 1 Characteristics of head of household by sex

3 I follow here Charbit and Kebe 2007. I am indebted to Kebe for updating data drawn from the 2011 Enquête de Suivi de la Pauvreté au Sénégal. I also quote data from the earlier one (

Table 3

 3

. Characteristics of gifts received by sex of head of household A still more significant source resides in transfers linked to internal and international emigration. Female-headed households are characterised by a larger proportion receiving money from abroad (40.8% as opposed to 27.1%), essentially from migrants living and working in Western countries (Europe, USA, Canada), from which are sent the largest and most regular monetary transfers: 32.9% (29.3 + 3.6) as opposed to 21.7 (19.0 + 2.7) in households headed by men (table 4).

Tableau 4. Place of residence of migrant sending remittances by sex of head of household

Table 1 .

 1 Characteristics of head of household by sex

	Characteristics of head of HH	Female	Male
	Percent with secondary education	13.6	19.2
	Rate of inactivity	53.5	76.5
	Percent wage earners	14.1	21.4
	Source: see note 2

Table 2 .

 2 Economic participation of members of the household by sex of head

	Female-headed HH	Male-headed HH

Table 3 .

 3 Characteristics of gifts received by sex of head of household Tableau 4. Place of residence of migrant sending remittances by sex of head of household

	Female-headed HH	Male-headed HH

Véronique Petit, Professeure de démographie, UMR CEPED Université Paris Descartes -IRD veronique.petit@ceped.org

Yves Charbit, Professeur émérite de démographie, UMR CEPED Université Paris Descartes -IRD yves.charbit@ceped.org

The method chosen was that of in-depth interviews of small samples of 30 to 40 women, with a common thematic guideline used in the different studies to maintain homogeneity of the entire survey: cause of departure of the head of the family, participation in decisions and reasons why the family stayed in the country, daily life (work, savings management, education of children, family relations, social life), difficulties due to the absence of the head of the family.

The boulo is a festival held every year in May, before sowing, and is aimed at preserving fertility of nature.

The dama is a funeral rite practiced a certain time after death whose purpose is to cause the soul of the deceased to leave the world of the living where it continues to stay.


Occurrences of energy-critical elements; Lithium Cobalt and Graphite in Europe

an overview.

Håvard Gautneb 1 , Janja Knežević Solberg 1 , Eric Gloaguen 2 , Jérémie Melleton 2 , Blandine Gourcerol 2 , Tuomo Törmänen 3 1) Geological Survey of Norway 2) BRGM France 3) Geological Survey of Finland

Methodology

We collected from European Geological Surveys basic geographical information, status (active/closed/not exploited), occurrence type (occurrence/prospect/deposit) and genetic type for each occurrence. We show the distribution of Li, Co and graphite occurrences in different EU countries (Fig. 1) and their locations and classification according to their genetic types, in Fig 2, 3 and 4..

Results: Lithium

Spatial distribution

The distribution of lithium in Europe shows a strong clustering (Fig. 2) highlighting the Li potential of the Variscan belt of south and central Europe.

Deposit types

High-grade Li deposits [START_REF] Gourcerol | Re-assessing the European lithium resource potential -A review of hard-rock resources and metallogeny[END_REF][START_REF] Gloaguen | Ressources métropolitaines en lithium et analyse du potentiel par méthodes de prédictivité[END_REF] are represented by:

• Li-rich LCT pegmatites • Rare metal granites [START_REF] Melleton | Rare-elements (Li-Be-Ta -Sn-Nb) magmatism in the European Variscan belt, a review[END_REF] • Atypical stratiform deposits such as Jadar, encountered in intramontane lacustrine evaporate basins

Medium-grade Li deposits [START_REF] Gourcerol | Re-assessing the European lithium resource potential -A review of hard-rock resources and metallogeny[END_REF] are represented by:

• hydrothermal deposits such as greisens • Li-bearing quartz veins associated with some peraluminous rare metal granites.

Other types [START_REF] Gloaguen | Ressources métropolitaines en lithium et analyse du potentiel par méthodes de prédictivité[END_REF] Li-rich (tosudite) hydrothermal alteration aureole around gold quartz veins; Cookeite/lithiophorite in black shales (Dauphiné: ave. 441 ppm Li 2 O, up to 1 847 ppm) or in bauxite deposits and Mn-(Fe) deposits. Li-rich clays (hectorite) are presently unknown in the EU.

Lithium carrier

Lithium is hosted by various minerals such as phosphates (amblygonite, montebrasite, triphylite), inosilicate (spodumene), phyllosilicates (lepidolite series, zinnwaldite, petalite) and scarce borosilicate (jadarite). Co-products are generally Ta, Nb, Sn, Be.

Examples, grades & tonnages

Jadar-type Li deposits occur in Serbia and Bosnia. In 2017, the total mineral resources report 135.7 Mt of ore at a grade of 1.

Results: Cobalt

Spatial distribution

Most of the known Co-bearing deposits and showings in Europe are clustered in the Nordic countries (Finland, Sweden and Norway). Deposits are more scattered throughout southerncentral Europe (Fig. 3). The deposits in the Nordic countries mostly represent magmatic Ni-Cu and Fe-Ti-V deposits and VMS deposits, whereas elsewhere in Europe genetic types are more varied from sediment-hosted, to lateritic and 5-element vein types, among others a common minor constituent in a number of different ore types.

Main deposit types:

• stratiform sediment-hosted deposits (60%)

• magmatic Ni-Cu-Co-PGE deposits (23%)

• lateritic Ni-Co deposits (15%), cobalt is recovered as a by-product (10-90 % recovery, [START_REF] Mudd | Quantifying the recoverable resources of byproduct metals: The case of cobalt[END_REF].

Other types of deposits containing Co IOCG, VMS (Besshi-type), 5-element vein deposits, black-shale hosted Ni-Cu-Zn deposits, and Fe-Cu-skarn deposits [START_REF] Slack | Critical mineral resources of the United States-Economic and environmental geology and prospects for future supply[END_REF]. Co-grades are between 0.01-0.1%.

Fig. 1 The distribution of occurrences within the different European nations.

Results: Graphite

Spatial distribution

Graphite is a common mineral in metamorphic rocks throughout Europe, however it is rare to find economically interesting deposits. The bulk of the graphite occurrences occur in Archaean and Proterozoic rocks of Northern Europe and Ukraine. In addition, a number of amorphous graphite occurrences are found in Phanerozoic rocks in Austria.

Deposit types

The graphite bearing rocks are typically organic rich paragneiss often associated with carbonates and iron formations.

In well-investigated areas (for instance in Norway) the graphite deposits show polyphasal deformation and are most commonly associated with upper amphibolite or granulite facies metamorphism [START_REF] Gautneb | Geological and ore dressing investigations of graphite occurrences in Bø, Sortland, Hadsel and Øksnes,municipalities[END_REF]. Graphite deposits are very good electrical conductors and are relatively easy to locate using geophysical methods.

We have in this study, divided graphite occurrences into flake and amorphous types which make up the bulk of the deposits. There are also a large number of showings where the genetic type is unknown. Vein deposits are known in Europe, but are mostly geological curiosities [START_REF] Luque | Vein graphite deposits: geological setting s, origin and economical significance[END_REF].

Examples, grades & tonnages

Active mines are situated in Ukraine, Austria and Norway. The graphite contents vary from 2-3% up to over 40% [START_REF] Gautneb | The geology, exploration and characterisation of graphite deposits in the Jennestad area, north Norway[END_REF]. The Traelen deposit in Norway is the world's richest graphite deposit in current production with an average ore grade of 31%, and 1.8 Mt proven reserves.

Examples, grades & tonnages

The

Fig. 2

 2 Fig. 2 The genetic types of Li-occurrences in Europe.

Fig. 4

 4 Fig. 4 The genetic types of graphite occurrences in Europe.

  only active mines producing cobalt are located in Finland. Kevitsa mine in northern Finland is a large low-grade Ni-Cu-PGE deposit, which produced 591 t of Co in 2018. Kylylahti mine is a small-sized Outokumpu-type Cu-Zn-Ni-Co deposit, which produced 278 t of Co in 2018 (New Boliden, 2019). Terrafame is a large, low-grade black-shale hosted Zn-Ni-Cu-Co mine which produces Co as by-product to Ni and Zn, but production figures for Co are not published (2013: 286 t). The highest grades occur in sediment-hosted deposits of the Central African Copper Belt (0.1-1.1% Co), lateritic Ni deposits (0.1-0.22% Co), some VMS deposit-types (Windy Craggy 0.66% Co, Outokumpu-type deposits 0.1-0.25% Co), and 5-element vein deposits and ultramafic-hosted hydrothermal deposits (up to %-level, e.g. Dobsina, Bou Azzer; Hitzman et al. 2017, Slack et al. 2017).


A Imam

T Gries

H Sezen

M Amati

D Mangin

T Belmonte
email: thierry.belmonte@univ-lorraine.fr

Plasma afterglow-assisted oxidation of iron-copper bilayers

Keywords: number: 81.07. ± b Nanoscale materials and structures: fabrication and characterization CuO, Fe2O3, Cu/Fe stacks, Nanostructuration, Afterglow, Oxidation 2

Iron layers with variable thicknesses, deposited onto copper thin films, are oxidized by a plasma afterglow at atmospheric pressure. Such a bilayer arrangement enables the growth of caterpillar-like patterns covered by CuO and Fe2O3 nanostructures. Two main mechanisms are at stake: either copper comes up to the surface through cracks or boundaries between the columns of the coating, or it diffuses through parts of the Fe2O3 layer made permeable by tensile stress. Structures grown by the former mechanisms are characterized by a central channel, whereas those grown by the latter exhibit a plane interface above which stands an equiaxed grain heap. This result was used to localize the growth of nanowires in cracks formed priory to the afterglow-assisted treatment. By resorting to XPS microscopy experiments carried out with the scanning photoelectron microscope at the ESCAMicroscopy beamline of the Elettra synchrotron facility in Trieste, we could gain access to the surface composition of a single isolated pattern. HIGHLIGHTS • Iron-copper bilayers are deposited by magnetron sputtering • Bilayers are oxidized by plasma afterglow at atmospheric pressure • Caterpillar-like patterns covered by CuO and Fe2O3 nanostructures are formed • Copper comes up through cracks or Fe2O3 layer made permeable by tensile stress • Localizing nanowires is achievable by creating crack patterns before oxidation.

INTRODUCTION

Growing tailored oxide nanostructures is of major importance in some applications like water splitting [START_REF] Su | Nanostructured WO3/BiVO4 heterojunction films for efficient photoelectrochemical water splitting[END_REF][START_REF] Zhang | Metal oxide nanomaterials for solar hydrogen generation from photoelectrochemical water splitting[END_REF][START_REF] Lin | Semiconductor nanostructure-based photoelectrochemical water splitting: A brief review[END_REF] or catalysis [START_REF] Zhong | Self-Assembled 3D flowerlike iron oxide nanostructures and their application in water treatment[END_REF][START_REF] Kniep | Rational design of nanostructured copper-zinc oxide catalysts for the steam reforming of methanol[END_REF][START_REF] Polshettiwar | Self-assembly of metal oxides into threedimensional nanostructures: synthesis and application in catalysis[END_REF]. It might be useful to control the way a surface could be patterned by nanostructures with well-defined properties. Resorting to non-equilibrium media, like direct plasmas or afterglows, opens up new opportunities to control the synthesis of nanostructures [START_REF] Ostrikov | Plasma nanoscience: from nano-solids in plasmas to nano-plasmas in solids[END_REF]. In direct low-pressure plasma processes, sputtering mechanisms can be partly responsible for the growth of nanostructures [START_REF] Cvelbar | Towards large-scale plasma-assisted synthesis of nanowires[END_REF][START_REF] Cvelbar | Control of morphology and nucleation density of iron oxide nanostructures by electric conditions on iron surfaces exposed to reactive oxygen plasmas[END_REF][START_REF] Ostrikov | From nucleation to nanowires: a single-step process in reactive plasmas[END_REF]. In plasma afterglow processes, oxygen atoms or metastable states of oxygen -mainly the singlet state O2(a 1 ∆g) -are available. They are more active than ground state oxygen at low temperature because of the extra amount of energy coming from dissociation or excitation. As low-temperature oxidation favours anisotropic crystal growth, and thus, the synthesis of nanostructures, non-equilibrium media promotes the growth of oxide nanostructures [START_REF] Altaweel | Controlled growth of copper oxide nanostructures by atmospheric pressure micro-afterglow[END_REF][START_REF] Arnoult | Flow transition in a small scale microwave plasma jet at atmospheric pressure[END_REF][START_REF] Arnoult | On the origin of selforganization of SiO2 nanodots deposited by CVD enhanced by atmospheric pressure remote microplasma[END_REF][START_REF] Arnoult | Self-organization of SiO2 nanodots deposited by chemical vapor deposition using an atmospheric pressure remote microplasma[END_REF][START_REF] Saa | Growth of ruthenium dioxide nanostructures by micro-afterglow oxidation at atmospheric pressure[END_REF][START_REF] Arnoult | Localized growth of silicon oxide nanowires by micro-afterglow oxidation[END_REF].

In a recent work [START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF], we showed that mixing iron and copper in alloy thin films strongly affects the growth processes of nanostructures, leading to original shapes. Oxidation of copper, because it is slower than iron, is strongly anisotropic, leading easily to nanostructures.

Increasing the amount of iron decreases the level of stress in the film and hinders but never stops the access of copper to the topmost surface.

Thermal oxidation of brass alloys in air was investigated by Zhu et al. [START_REF] Zhu | Co-synthesis of ZnO-CuO nanostructures by directly heating brass in air[END_REF]. They could simultaneously synthesize ZnO-CuO nanostructures between 623 and 813 K. By varying the brass composition, 1D nanowires/nanoflakes, 2D nanosheets, and 3D networks could be grown. Another group studied the influence of the oxygen partial pressure [START_REF] Xu | The effect of oxygen partial pressure on the growth of ZnO nanostructure on Cu0.62Zn0.38 brass during thermal oxidation[END_REF] and found that the compressive growth stress causes the plastic deformation of oxide scale and increases dislocation defects in scale, which is of benefit to the growth of nanowires and nanowalls. In a previous paper [START_REF] Xu | Growth of ZnO nanostructure on Cu0.62Zn0.38 brass foils by thermal oxidation[END_REF], the same authors described that the increase in oxidation temperature from 773 to 873 K typically increases the compressive growth stress. As plastic deformation is easier in the substrate than that in the oxide scale, buckling-induced patterns decrease in size, whereas micro-cracks form in the oxide scale. Micro-cracks act as short-circuits, resulting in the formation of flower-like nanostructures.

Finally, let's mention that radiofrequency plasma-assisted oxidation of brass was reported [START_REF] Bellakhal | Plasma and wet oxidation of (63Cu37Zn) brass[END_REF] but no nanostructures were formed in the studied conditions.

In the present work, we investigate the role of an iron thin film with variable thickness on the access of copper to the topmost surface. Fe/Cu bilayers are oxidizing with an Ar-O2 plasma afterglow operating at atmospheric pressure. Cu/Fe bilayers were also investigated but the corresponding results are not reported hereinafter, in so far as thus-grown nanostructures are similar to those formed with Cu-rich alloys and reported in reference [START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF].

EXPERIMENTAL SET-UP

The experimental set-up was described in details elsewhere [START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF]. Briefly, an atmospheric Ar-O2 microwave plasma was ignited in a fused silica tube (27 mm inner diameter) placed in a 2.45 GHz resonant cavity. The power absorbed by the plasma was 100 W. Flow rates of gases were controlled by mass flow controllers and the total flow rate was 275 sccm (standard cubic centimeter per minute). The partial pressure of oxygen in the gas mixture was set at 9.1 vol.%.

The treatment time was 2 hours to get sufficiently long structures to identify and characterize them as clearly as possible. The micro-afterglow was visible as a light beam escaping a hole (500 µm in diameter) drilled in a brass plate screwed on one wall of the cavity [START_REF] Altaweel | Controlled growth of copper oxide nanostructures by atmospheric pressure micro-afterglow[END_REF]. Oxidizing active species in the afterglow are mainly O, O2(a) and O2(X) because the afterglow is too hot to allow the formation of O3 [START_REF] Arnoult | Flow transition in a small scale microwave plasma jet at atmospheric pressure[END_REF].

A detailed thermal analysis reported in reference [START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF] showed that the maximum treatment temperature is about 733 K. When the plasma afterglow hits the metallic surface, it oxidizes concentric areas that extend radially up to several millimetres. Because of the temperature radial gradient that prevails in such conditions, Fe/Cu thin films, when submitted to the afterglow treatment, produce different nanostructures from the centre of the treatment outward, for a given thickness ratio.

Samples were Fe/Cu films deposited by DC magnetron sputtering. Thicknesses of copper and iron layers were set by adjusting the deposition time. The thickness of the bilayer was 1.3 µm.

The iron layer, always deposited onto the copper layer, had thicknesses of 50, 155 and 900 nm. The base vacuum in the sputtering chamber is 5×10 -6 mbar. The distance between the sample, made of fused silica, and the targets (50 mm in diameter, 3 mm thick and > 99.99 % purity for Cu target and 0.25 mm and 99.5 % purity for Fe target) was 100 mm. The substrateholder rotated at 28 revolutions per minute during deposition to ensure homogeneity. Thin films were deposited at a pressure of 5.8×10 -3 mbar in a 10 vol% H2 -90 vol% Ar mixture.

Treated surfaces were characterized by several surface diagnostics. Scanning Electron Microscopy (SEM) was made with a Philips XL 30. X-ray diffraction using Cu-Kα radiation (λ = 0.179026 nm) was performed with a Bruker D8 Discover diffractometer.

A CAMECA IMS 7F instrument was used for Secondary Ion Mass Spectrometry (SIMS). A Cs + primary ion beam of 30 nA operating at 5 kV was used to sputter sample with an impact energy of 3 keV. Mass Resolving Power was set to 2000. Depth-profiles were acquired over an area of 100×100 µm 2 .

The XPS microscopy experiments were carried out with the scanning photoelectron microscope (SPEM) at the ESCAMicroscopy beamline at the Elettra synchrotron facility in Trieste, Italy. In SPEM, the incident x-ray beam, with 648.5 eV photon energy, is focused on the sample to a spot of 130-150 nm diameter using zone plate optics. The sample is then scanned under the focused beam. The highest cross-section peaks of Cu 2p and Fe 2p are not accessible with such a photon energy, but Cu 3p and Fe 3p could be acquired. The SPEM instrument has two operation modes, microprobe photoelectron spectroscopy and spectromicroscopy imaging. The microprobe spectroscopy mode is identical to conventional XPS spectroscopy, i.e. energy distribution curves are measured from the selected area illuminated by a 130 nm focused beam with a 0.2 eV spectral resolution. The imaging mode maps the surface distribution of elements by collecting photoelectrons with a selected energy window while scanning the specimen with respect to the focused beam. More details about the instrument can be found in reference [START_REF] Abyaneh | Scanning Photoelectron Microscopy: a Powerful Technique for Probing Micro and Nano-Structures, e[END_REF].

RESULTS AND DISCUSSION

Layer and patterns structures

In figure 1, a map of the different nanostructures obtained with the afterglow process for iron thicknesses ranging from 50 to 900 nm is depicted. Four main types of nano-objects are found: Fe2O3 nanoblades, CuO nanowalls, nanotowers and nanowires. These nano-objects are described in details in [START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF]. Briefly, CuO nanowalls are characterized by their thickness, which is much larger than Fe2O3 nanoblades (~50 nm vs ~10 nm), their height, which is always lower than nanoblades for given conditions, and by the existence of amorphous domains embedded in a crystalline matrix (which is not observed with Fe2O3 nanoblades).

Nanotowers are an assembly of two nanowires with an amorphous phase in between. When the width of the nanostructure shrinks, large steps appear, producing a serrated shape as in a tower. CuO nanowires with diameters as small as 5 nm are also obtained.

The presence of the iron layer on top of the copper layer leads to the formation of caterpillarlike patterns (figures 2 and 3), especially in the external part of the radial distribution (figure 1). In figure 2, we show that each caterpillar-like pattern can be covered by different types of nanostructures, as indicated in figure 1, i.e. Fe2O3 nanoblades, CuO nanowalls, nanotowers and nanowires, depending on the temperature. Fe2O3 nanoblades are very easy to distinguish from other nanostructures, which makes finally all nanostructures easily recognizable.

Patterns get sparser and shorter when thickness of the iron layer is increased. It is important to consider that patterns only appear if the temperature is low enough, which seems to depend also on the initial iron thickness. Indeed, in figure 1, the minimum radial distance beyond which patterns appear, changes with the initial iron thickness. The relationship between patterns and cracks is tricky. First, patterns without cracks can be found (figure 3a). Next, if the iron layer is thin, large areas can be covered by uncorrelated patterns and cracks (figure 3b). However, at some places, patterns can also be split into 2 parts (figure 3c) or crossed by a crack that propagates in the film. Then, some cracks are formed after the synthesis of patterns. Finally, if the iron layer is thick, large areas are mainly (but not only) covered by strongly-correlated patterns and cracks (figure 3d). From figure 4, we readily understand that for a very thin iron layer (50 nm) patterns grow along the boundaries that separate the columns of the sputtered Cu layer. Then, copper has an easier access to the surface through these boundaries that go through the thin iron layer as well. So, if the iron layer is thin (50 nm), it does not stop the outward diffusion of copper, which occurs at column boundaries. If it is very thick (900 nm), copper can more easily reach the surface through cracks. If it is in between these two situations (155 nm), diffusion at column boundaries is limited. Then, the transport of copper to the surface can still occur by diffusion through cracks but, because of the possibility to find patterns without cracks, another mechanism must hold. This mechanism, which is limited but still exists if the layer is very thick, is discussed hereinafter.

The presence of Fe2O3 nanoblades when the iron layer is thick enough is simply due to its oxidation, iron being then readily available. SEM cross-section views of caterpillar-like patterns, obtained after a two-hour oxidation performed on a 155 nm Fe/Cu coating, show common features and specificities (figure 5). To describe them, we need to refer to figure 6 where the different possible layers are designed by segments. From the interface with the silica wafer to the top of the oxide layer, we observe: which is strongly marked in figure 5a, gives way to an almost equiaxed growth.

We could also observe the transverse structure (i.e. perpendicular to the surface and parallel to the pattern axis) of one pattern exhibiting a central channel (figure 7a). Regularly-spaced channels are also observed crosswise, as if the central feeding channel was cut into pieces, each of them feeding the pattern over a certain distance (~1 µm in figure 7b).

Diagnostics results

In figure 8, SIMS results recorded every millimetre from the centre of a treated sample outward are provided. They give very accurate results in perfect agreement with SEM images. Normalized intensities of Cu, Fe and O SIMS signals are depicted as a function of depth defined with respect to the interface between layer and silica. The sputtering time of the oxidized layer by the argon beam cannot be simply converted into a thickness scale because of the strong composition, density and structure changes through the layer. We also chose to present raw intensities, and not composition, the oxygen signal being difficult to interpret in terms of oxidation levels of each metallic element. This is due to the specificity of the SIMS technique where the emission yield of one element is strongly affected by its environment.

Besides, the porosity likely affects raw intensities all over the sample depth. We could perform spatially resolved XPS measurements on isolated patterns which is 3 mm away from centre of the treatment. The spectrum acquired on the film, beside the pattern (point O in figure 10) shows comparable amounts of iron and copper. The presence of iron on the topmost surface is consistent with SIMS profile given at this distance (figure 8). Traces of zinc (due to contamination during the sputtering process) and silicon (from the substrate) are also observed. Zinc was not sought in SIMS measurements, but silicon was. Results (available on demand) show that, at 3 mm from the centre, the 133 Cs 28 Si signal is null at the topmost surface, reaches ~30 counts after 750 s of Ar + sputtering, close to the initial interface between Fe and Cu, decays to zero after 1500 s, and increases again after 2000 s to reach its maximal value in the silica substrate with ~2800 counts. So, we infer that the relatively high intensity of the silicon contribution found in the XPS spectra is due to local cracks that open access to the substrate. Indeed, it would be extremely surprising to observe an outward transport of silicon, trapped in the form of silica, to the top of layer.

Both patterns exhibit almost identical spectra, which show a different stoichiometry in respect to the substrate, this latter being richer in iron content. Zinc and oxygen are also different on the patterns and on the film. More oxygen and more zinc are found on the substrate than on the pattern. Main contribution for a higher amount of O 1s intensity on the film is the presence of relatively higher amount of silica. In addition, the larger amount of iron oxide on the surface of the substrate, Fe2O3 containing 60 at.% of oxygen versus 50 at.% only for CuO.

We find, as predicted by the Ellingham diagram, that zinc is easier to oxidize than iron and copper. The two former elements oxidize first and, as they are trapped, are found at negligible concentration in CuO patterns that are formed in a second step. So, the role of zinc as trace contaminant is likely to be negligible.

Growth mechanism of patterns

By gathering the previous results, we obtain a relatively clear view of the way patterns grow in our conditions. Patterns are mainly made of CuO and may contain an amount iron that is all the larger as the distance from the centre is long. Copper has to find way to the topmost surface through the iron oxide layer that forms first. This occurs by outward diffusion of copper ions [START_REF] Altaweel | Controlled growth of copper oxide nanostructures by atmospheric pressure micro-afterglow[END_REF], either via a central channel if the Fe2O3 layer is blocking, which requires that the temperature be below a certain threshold or that the layer be thick enough, or via a locally-permeable Fe2O3 layer. Indeed, the growth rate of nanostructures scales as the square root of time because ion transportation is controlled by outward diffusion. The development of an inner porosity is due to this phenomenon, diffusion ions leaving vacancies behind them that coarsen to produce voids at the interface with the substrate. What is meant here by permeable is the loss of the diffusion barrier property of the layer, likely because of the increase in the number of voids per unit volume [START_REF] Zhou | Finite element analysis of oxidation induced metal depletion at oxide-metal interface[END_REF][START_REF] Kadiri | A theory for stress-driven interfacial damage upon cationic-selective oxidation of alloys[END_REF]. In the former mechanism, the central channel is either a boundary between two columns (figure 4), which requires a thin Fe2O3 layer (50 nm), or a crack (figure 3d) if the Fe2O3 layer is thick (900 nm). In the latter mechanism, the growth of the pattern is characterized by a plane interface through which copper ions diffuse (figures 5c and 6). In this case, a specific structure, characterized by equiaxed grain heap, is found. The cell-like layer adopts a special structure with aligned large grains, whereas layer [BC] is no longer a fine grain columnar layer, but an ultrafine grain layer (figures 5c and6). This transformation of layer [BC] is likely responsible for the diffusion, thus made possible, of copper ions. Over a short distance of 1-1.5 µm, this barrier layer is made permeable. We propose that the mechanism leading to this change is due to the local development of a high tensile stress in this part of the layer. As shown in reference [START_REF] Zhou | Finite element analysis of oxidation induced metal depletion at oxide-metal interface[END_REF], normal tensile stresses are generated at both the metal-oxide interface (here the Cu/Fe2O3 interface) and metal grain boundaries (here in copper). Those stresses are believed to be one of the important driven forces for oxide spallation. Besides, it has been pointed out that tensile forces are the cause of void nucleation by multi-axial stress field [START_REF] Kadiri | A theory for stress-driven interfacial damage upon cationic-selective oxidation of alloys[END_REF]. This would explain why a line made of separated spherical holes is observed within layer [BC]. The post-mortem formation of cracks is an indirect proof of the tensile stress that develops in the coating. The fact that they are most often aligned with patterns suggests that switching from one mechanism to the other is ruled by a threshold in the stress level beyond which cracks open, making the diffusion process change radically. This leads to a change in the structure of the cell-like layer, as mentioned previously. To our opinion, the cell-like structure would be made of an iron oxide skeleton, the cells, once copper starts leaving the layer. Iron would stay in this layer as an oxide under the shape of cell walls. However, this assertion still awaits proof.

If the transport of copper is not limited, either because the temperature is high enough, or because the thickness of the iron layer is sufficiently weak to let copper reach the surface within a period of time inferior to the treatment duration, copper oxide nanostructures (nanowalls and nanotowers, mainly) develop spontaneously. The growth mechanisms of these peculiar objects are the same as those already observed for Cu-Fe alloys described in a former paper of ours [START_REF] Imam | Nanostructures design by plasma afterglow-assisted oxidation of ironcopper thin films[END_REF]. The reason for the similarity comes from the way copper and iron segregate in these alloys and form stacks of oxide layers that are very similar to those depicted here (e.g. in figure 8). So, the reader is referred to this work for a detailed analysis of the growth mechanisms of these nano-objects.

Localized growth of nanostructures

When a substrate, coated by another material, is sufficiently bent, regularly-spaced cracks are formed in the coating [START_REF] Xia | Crack patterns in thin films[END_REF]. The oxidation process was applied to 300 nm Fe/Cu stack deposited on a 500 µm-thick silicon rectangle priory bent to form crack patterns (figure 11a).

Cracks could have been opened by any other method (like indentation for instance).

Nanowires grow almost exclusively in cracks (figure 11b). This feature does not depend on the orientation of the cracks (figures 11c and 11d). So, we can claim that we have found a way to localize nanowires on a surface.

CONCLUSION

We showed that the growth of oxides from Fe/Cu bilayer coatings makes possible the synthesis of a large number of nanostructures. Besides nanowalls, nanotowers and nanowires, the formation mechanisms of caterpillar-like patterns covered by CuO and/or Fe2O3 nanostructures have been clarified. Two main mechanisms are at stake: either copper comes up to the surface through cracks or boundaries between the columns of the coating, or it diffuses through parts of the Fe2O3 layer made permeable by tensile stress. The possibility to grow locally patterns covered by nanostructures opens up new opportunities to localize nanostructures on a substrate. For instance, nanoindentation might be advantageously employed to control "locally" the generation of stress in the metallic films. These structures might be also useful to develop sensors integrating several possible responses to given molecules that could react either with Fe2O3 or CuO nanostructures. Ongoing attempts to increase the temperature are achieved to check the possibility of growing spinel phases in the Fe-Cu-O system.

CAPTIONS

•

  a cell-like layer (denoted [AB] in figure 6) • a fine grain columnar layer (segment [BC]), often divided into two parts by a line made of separated spherical holes (segments [BB'] and [B'C]) • and protruding structure (segment [CD]) When a caterpillar-like pattern is found, the protruding structure [CD] can be made of: • a large grain columnar heap (segment [CC"]), which is not always present • a fine grain layer (segment [CC']) • a topmost thin layer where nanowires are embedded [C'D'] (figure 6) • sometimes topped by nanowires [D'D] (figures 5a to 5c and figure 6)Among the features common to all patterns, the presence of a cell-like layer [AB] is observed everywhere, but in figure5cand in figure 6, this layer, which is visible on one side of the image, adopt a special structure under the pattern. Then, aligned large grains are observed.Layer[BC] is no longer a fine grain columnar layer, but an ultrafine grain layer (figures 5c and 6). In these two cases, an equiaxed grain heap [CC"] is also visible. The presence of a central channel -also visible in layer [AB] -as in figures 5a and 5b is replaced by a plane interface extending all along the heap. The radial distribution of the grains in layer [CC'],

  Cu and Fe signals evolve strongly according to the radial position. As the temperature decreases from the centre of the treatment outward, the farthest spots are less oxidized. At +4 mm and +3 mm, an outer iron oxide layer covers a hardly-oxidized Cu layer. Nanostructures are too small to affect significantly the SIMS profile at the chosen sputtering rate. At +2 mm, the fine grain columnar layer [BC] is characterized by a Gaussian distribution of iron associated with depletion in copper that diffuses outward. At +1 mm, fine grain layer segment [CC'] is characterized by a weak SIMS intensity of iron in copper. The topmost thin layer [C'D'] is made of pure copper oxide and the nanowires start being long enough to contribute significantly to the SIMS signal, leading to the [D'D] segment. At the centre, the same layers are observed. A change in the proportion of Cu and Fe in the [BC] segment shows an inflection point on the oxygen profile, which might be attributed to the presence of spherical holes that form a well-visible separation line between [BB'] and [B'C] layers. Because of the size of the SIMS spot used to diagnose the surface composition (100×100 µm 2 ), patterns would only weakly contribute to the whole signal. Then, the equiaxed grain heap [C'C"] does not appear in the SIMS profiles. Space-resolved XRD patterns are depicted in figure 9. No spinel phases (CuFeO2, CuFe2O4 or Cu6Fe3O7) are observed in our conditions. Only Fe2O3 and CuO are found. The lack of FeO is likely due to the disproportionation of this phase into iron and Fe3O4, which occurs below 850 K.

Figure 1 :

 1 Figure 1: Map of as-grown nanostructures as a function of the radial distance for different

Figure 2 :

 2 Figure 2: Evolution of the caterpillar-like patterns observed after treatment of a 900 nm

Figure 3 :

 3 Figure 3: Examples showing caterpillar-like patterns observed after treatment and the way

Figure 4 :

 4 Figure 4: a) SEM image of the 50 nm Fe/ Cu coating before treatment. b) SEM image of the

Figure 5 :

 5 Figure 5: a) SEM cross-section view of a single caterpillar-like pattern observed 1380 µm

Figure 6 :

 6 Figure 6: a) SEM cross-section view of a single caterpillar-like pattern observed 1750 µm

Figure 7 :

 7 Figure 7: a) SEM cross-section view of a single caterpillar-like pattern observed 1750 µm

Figure 8 :

 8 Figure 8: Normalized intensities of Cu (red line), Fe (black line) and O (blue line) SIMS

Figure 9 :

 9 Figure 9: X-ray diffraction patterns recorded every millimetres from the centre outward. No

Figure 10 :

 10 Figure 10: a) A 100x100 µm 2 map acquired at Cu 3p signal with a 648.5 eV x-ray photon

Figure 11 :

 11 Figure 11: a) 300 nm Fe/Cu coating deposited on silicon and covered by regularly-spaced

Figure 1 :

 1 Figure 1: Map of as-grown nanostructures as a function of the radial distance for different

Figure 2 :

 2 Figure 2: Evolution of the caterpillar-like patterns observed after treatment of a 900 nm

Figure 3 :

 3 Figure 3: Examples showing caterpillar-like patterns observed after treatment and the way cracks can be distributed. a) Patterns without cracks.

Figure 4 :

 4 Figure 4: a) SEM image of the 50 nm Fe/Cu coating before treatment. b) SEM image of the

Figure 5 :

 5 Figure 5: a) SEM cross-section view of a single caterpillar-like pattern observed 1380 µm

Figure 6 :

 6 Figure 6: a) SEM cross-section view of a single caterpillar-like pattern observed 1750 µm

Figure 7 :

 7 Figure 7: a) SEM cross-section view of a single caterpillar-like pattern observed 1750 µm

Figure 8 :

 8 Figure 8: Normalized intensities of Cu (red line), Fe (black line) and O (blue line) SIMS signals as a function of the sputtering time for various

Figure 9 :

 9 Figure 9: X-ray diffraction patterns recorded every millimetres from the centre outward. No

Figure 10 :

 10 Figure 10: a) A 100x100 µm 2 map acquired at Cu 3p signal with a 648.5 eV x-ray photon energy: and a 130-150 nm spot diameter. XPS spectra

Figure 11 :

 11 Figure 11: a) 300 nm Fe/Cu coating deposited on silicon and covered by regularly-spaced