Pour obtenir le grade de. Spécialité : PHYSIQUE. Arrêté ministériel : 7 août Raffaella TORCHIO - PDF

THÈSE Pour obtenir le grade de DOCTEUR DE L UNIVERSITÉ DE GRENOBLE Spécialité : PHYSIQUE Arrêté ministériel : 7 août 2006 Présentée par Raffaella TORCHIO Thèse dirigée par Dr. Sakura Pascarelli et codirigée

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THÈSE Pour obtenir le grade de DOCTEUR DE L UNIVERSITÉ DE GRENOBLE Spécialité : PHYSIQUE Arrêté ministériel : 7 août 2006 Présentée par Raffaella TORCHIO Thèse dirigée par Dr. Sakura Pascarelli et codirigée par Prof. S. Mobilio et Prof. C. Meneghini préparée au sein du European Synchrotron Radiation Facility dans l'école Doctorale de Physique Magnétisme, structure et ordre chimique dans les métaux 3d et leur alliages à très hautes pressions Thèse soutenue publiquement le 23 janvier 2012, devant le jury composé de : Prof. Settimio MOBILIO Professeur à l' Università degli Studi di ROMA TRE, Italie; Membre Dr. Sakura PASCARELLI Responsable des lignes de lumière à l' ESRF, Grenoble, France; Membre Prof. Francesco SACCHETTI Professeur à l' Università degli Studi di Perugia, Italie; Président Prof. M.Luisa F-dez GUBIEDA Professeur à l' Universidad del Pais Vasco, Bilbao, Espagne; Rapporteur Prof. Stefan KLOTZ Directeur de recherche CNRS et Professeur à l' Université P&M Curie, Paris, France; Rapporteur 2 DOTTORATO DI RICERCA IN FISICA XXIV ciclo MAGNETISM STRUCTURE AND CHEMICAL ORDER IN THE 3D METALS AND THEIR ALLOYS AT EXTREME PRESSURES Raffaella Torchio Tutor: Prof. S. Mobilio, Dr. S. Pascarelli, Prof. C. Meneghini Coordinatore: Prof. O. Ragnisco Tesi realizzata con il supporto dell Università Italo Francese 4 Abstract This thesis concerns the study of the magnetic and structural transformations that occur in the 3d metals when they are compressed up to extreme pressures. The investigation has been carried out using polarized X-ray absorption (X-ray magnetic circular dichroism or XMCD) coupled to X-ray diffraction and DTF calculations and applied to the cases of cobalt, nickel and iron-cobalt (FeCo) alloys. In particular, in cobalt we present the first experimental evidence of pressure-induced suppression of ferromagnetism and we explore the interplay between structural and magnetic changes. The case of nickel, that is structurally stable over a wide range of pressures, allows to go deeper into the interpretation of the K-edge XMCD signal, so far still unsettled. Finally the investigation of the FeCo alloys is aimed at understanding the role played by the chemical order in tuning the high pressure structural and magnetic properties. 6 Resumé Cette thèse traite des transformations structurelles et magnétiques qui se produisent dans les métaux 3d quand ils sont comprimés jusqu à des pressions extrèmes. L étude a été réalisée en utilisant les techniques d absorption polarisée des rayons X (Dichroïsme circulaire magnetique de rayons X ou XMCD) et de diffraction des rayons X, couplées à des calculs DFT; elle a été appliquée aux cas du cobalt, du nickel et des alliages de fer et cobalt (FeCo). En particulier, cette thèse présente la première preuve expérimentale de la suppression du ferromagnétisme du cobalt, induite par la pression, et explore la relation entre les changements structurels et magnétiques. Le cas du nickel, structurellement stable sur une large échelle de pressions, permet d aller plus loin dans l interprétation du signal XMCD au seuil K, encore débattue aujourd hui. Enfin, l enquéte sur les alliages FeCo vise à comprendre le rôle joué par l ordre chimique dans la détermination des propriétés structurelles et magnétiques en conditions de haute pression. 8 Riassunto Questa tesi riguarda lo studio delle trasformazioni magnetiche e strutturali che avvengono nei metalli 3d quando sono sottoposti a pressioni estreme. L indagine è stata svolta usando le tecniche di assorbimento di raggi X polarizzato (X-ray magnetic circular dichroism o XMCD) e diffrazione da raggi X, accoppiate a calcoli teorici DFT. Tale studio è stato applicato ai casi di cobalto, nichel e leghe di ferro-cobalto (FeCo). In particolare, per il cobalto, si presenta la prima evidenza sperimentale della soppressione del ferromagnetismo indotta dall applicazione della pressione e si indaga la relazione tra le trasformazioni magnetiche e quelle strutturali. Il caso del nichel, strutturalmente stabile in un ampio intervallo di pressioni, permette di approfondire l interpretazione del segnale XMCD alla soglia K, a tutt oggi ancora dibattuta. Lo studio delle leghe di FeCo è invece volto alla comprensione di quale sia il ruolo giocato dall ordine chimico nel determinare le proprietà strutturali e magnetiche in condizioni di alta pressione. 10 Contents Abstract 5 Resumé 7 Riassunto 9 Introduction iii 1 Magnetism and structure in the 3d metals Itinerant ferromagnetism Basic bands theoretical results for the transition metals The magnetism of alloys: covalent magnetism Structure and magnetism in the 3d metals High pressure studies Pressure effects on magnetism and structure in the 3d metals The iron case Experimental methods High-pressure experimental methods Amagnetic diamond anvil cells Pressure measuring methods X-ray Absorption Spectroscopy X-ray Magnetic Circular Dichroism XMCD on the 3d metals under high pressure Interpretation of the K-edge XMCD signal Polarized XAS under high pressure: experimental details The dispersive beamline ID The HP XMCD setup ii CONTENTS 3 Cobalt Introduction Extreme conditions structure and magnetism in cobalt The HP/HT phase diagram High pressure structure Elastic anomalies High pressure magnetism Experiment Results and discussion Conclusions Nickel Introduction Extreme conditions structure and magnetism in nickel The HP/HT phase diagram High pressure magnetism Experiment Results and discussion Conclusions Iron-Cobalt Alloys Introduction Overview of structural, magnetic and chemical properties in FeCo The Fe 1 x Co x (x,t) phase diagram Ambient magnetic properties High pressure studies Experiment Results and discussion Conclusions Conclusions 117 Bibliography 123 Introduction The appearance of ferromagnetism in Fe(bcc), Co(hcp), and Ni(fcc) metals is one of the most fundamental questions in solid-state physics. In these transition metals, magnetism plays a fundamental role in determining the physical properties; for example, it greatly influences the crystalline structure, stabilizing bcc Fe and hcp Co instead of hcp Fe and fcc Co predicted in absence of magnetic interactions. Our modern understanding of the magnetic structure of metals relies on the Stoner-Wolfahrt model, which points out that the density of state at the Fermi level and the electronic correlations are the two most crucial factors at absolute zero temperature. In Fe, Co and Ni magnetism arises from the partially filled spin-polarized 3d band whose properties are strongly determined by the crystal structure and by external factors such as temperature, magnetic field, and pressure. Therefore, exploring the stability limits of ferromagnetism as a function of these thermodynamical variables is an essential issue to get a deeper insight on its appearance. Application of pressure is an effective way to address the complex interplay between magnetic, structural and electronic degrees of freedom. Directly acting on the interatomic distances it allows to modify the band hybridization strength thus inducing structural and magnetic instabilities. The knowledge and understanding of correlations between structure and magnetism is of paramount importance both from a fundamental and a technological point of view. First for the implications in geophysics for planetary interior s studies, being the Earth core mainly composed of iron dominated alloys; secondly for the tremendous number of technological applications in which these metals are involved such as high-density magneto-optical storage media. The compression of interatomic distances leads to the broadening of the electronic bands, which reflects in a decreased density of state at the Fermi level. In the framework of the Stoner-Wolfahrt model this implies that at sufficiently high compression the condition for ferromagnetism stability is no longer satisfied, and ferromagnetism is lost. In iron the collapse of ferromagnetism with pressure has been experimentally observed iv introduction in correspondence to the bcc-hcp structural phase transition around 13 GPa. An intense debate is still open to understand which, between structural and magnetic degrees of freedom, plays a driving role in the bcc ferromagnetic to hcp non-ferromagnetic transition. In cobalt, the suppression of ferromagnetism is predicted to occur with the hcp-fcc transition in the GPa range and the pressure limits for ferromagnetism stability have been established only recently with the results presented in this Thesis and, simultaneously, by Ishimatsu and co-workers. Ni is predicted to maintain a stable fcc ferromagnetic phase up to the 10 2 GPa range, however the pressure limit of ferromagnetism in fcc Ni is still not established. The scientific goal of this project has been to probe magnetism, structure and chemical order in 3d metals and their alloys at extreme pressures by means of K-edge polarized absorption. K-edge X-ray Absorption Spectroscopy (XAS) and associated techniques exploiting the polarization properties of synchrotron radiation, have shown to be very useful in probing simultaneously the structural, electronic and magnetic properties of the transition metals at high pressure, for example in pure iron where they allowed to simultaneously follow the structural-magnetic phase transition from bcc-ferromagnetic to hcp- non ferromagnetic around 13 GPa. In fact, polarized X-ray absorption spectroscopy contains intrinsic structural, electronic, and magnetic probes through Extended X-ray Absorption Fine Structure (EXAFS), X-ray Absorption Near Edge Structure (XANES) and X-ray Magnetic Circular Dichroism (XMCD). The first two clearly determine and quantify the local symmetry, while the third is very sensitive to polarized magnetic moments variation. Thus, at each pressure point, simultaneous information on both the magnetic and the structural properties of the system are provided without any relative pressure incertitude, which is very important in the high pressure domain where the reproducibility of high-pressure hydrostatic conditions is difficult to obtain. The study of electronic structure properties of materials at extreme conditions is a challenging problem both from theoretical and experimental viewpoints. On one hand, high pressure and temperature conditions push experimental data towards limits where conventional data analysis and interpretation approaches start to present their weaknesses. On the other hand, a complete theoretical understanding of the deep interplay between the electronic and atomic structure of systems in such conditions is still far from being reached. A tight connection between theoretical and experimental results is an asset in view of making significant advances in the field. At this purpose a close collaboration was established with the ESRF (European Synchrotron Radiation Facility) Theory group (Y.O. Kvashnin, L.Genovese, P. Bruno). In a parallel theoretical PhD project entitled Ab initio study of transition metals under high pressure theoretical methods are currently developed to address electronic and structural properties of 3d transition metals under extreme conditions, in view of a better undertel , version 1-16 May 2012 introduction v standing of the mechanisms at the atomistic scale determining their behavior. In particular the experimental-theoretical collaboration has been devoted to the interpretation of the K-edge XMCD signal behavior under pressure, still controversial in the literature. In this Thesis, combined XANES-XMCD and X-ray Diffraction (XRD) measurements are used to explore the high pressure limits for ferromagnetism stability and associated structural transitions in the case of cobalt, nickel and iron-cobalt (FeCo) alloys. The case of the FeCo alloy was investigated in order to study the role played by the chemical order in tuning the structural and magnetic properties under compression. The following main results were obtained. In cobalt, our results provide the first experimental evidence of the pressure induced collapse of ferromagnetism occurring around 120 GPa where the fcc/hcp phase fraction is at least 40%. The XMCD signal decreases linearly and continuously up to total extinction whereas a progressive change in the arrangement of the first atomic shells from an hcp- to an fcc-like local structure is observed starting from around 80 GPa. Thus the magnetic response in Co seems to be quite independent from the structural modifications. A coherent scenario correlating magnetism, elastic-structural anomalies and theoretical results is furnished, highlighting major differences with the iron case. In the case of nickel, the structural stability of the fcc phase over a wide pressure range offers a unique opportunity to experimentally investigate how magnetism is modified by the simple compression of interatomic distances. New high pressure K-edge XANES- XMCD measurements coupled to XRD measurements on pure Nickel are presented up to 200 GPa, a record pressure for absorption techniques. Our data show that fcc Ni is structurally stable and ferromagnetic up to 200 GPa contrary to the recent predictions of an abrupt transition to a paramagnetic state at 160 GPa. Moreover, novel DFT calculations describing the different behavior of orbital and spin moments in compressed Ni, point out that the pressure evolution of the Ni K-edge XMCD closely follows that of the p projected orbital moment rather than that of the total spin moment. The disappearance of magnetism in Ni is predicted to occur above 400 GPa. The structural and magnetic phase diagram of Fe 1 x Co x under high pressure is attractive because of the very different behavior of the pure components: Fe looses its ferromagnetism across the bcc-hcp transition around 13 GPa and Co retains an hcp ferromagnetic phase up to around 100 GPa. Our systematic study in an unexplored pressure (P 70 GPa) and composition range (0.5 x 0.9) provides a detailed description of the pressure-induced structural and magnetic transitions occurring in this range pointing out the close relationship between structure, magnetism and chemical order in this system. The comparison with DTF calculations shows that the chemical order plays a crucial role in determining the high pressure magnetic and structural properties. A first (x,p) magnetic and structural phase diagram for FeCo is drawn. The results of the present work have been the subject of the following publications: vi introduction 1. Pressure-induced collapse of ferromagnetism in cobalt up to 120 GPa as seen via x-ray magnetic circular dichroism, R. Torchio, A. Monza, F. Baudelet, S. Pascarelli, O. Mathon, E. Pugh, D. Antonangeli, J. Paul Itié, Phys. Rev. B Rapid communications, Phys. Rev. B 84, (R) (2011) 2. XMCD measurements in Ni up to 200 GPa: resistant ferromagnetism., R. Torchio, Y.O Kvashnin, S. Pascarelli, O. Mathon, C. Marini, L. Genovese, P. Bruno, G. Garbarino, A. Dewaele, F. Occelli, P. Loubeyre, Phys. Rev. Lett. 107, (2011) 3. Structure and magnetism in compressed Iron-Cobalt alloys, R.Torchio, S. Pascarelli, O. Mathon, C. Marini, S. Anzellini, P. Centomo, C. Meneghini, S. Mobilio, N.A. Morley and M.R.J. Gibbs, High Pressure Research 31 (2011) completing papers are under preparation. This Thesis is organized as follows: Physical background and experimental techniques In Chapter 1 an historical overview of the scientific understanding of magnetism in the transition metals is given, with particular attention devoted to the Stoner band model in its merits and limitations. A covalent magnetism model, addressing the description of magnetism in 3d metal alloys is presented and the close interrelationship between structure and magnetism in the transition metals is discussed. The effect of pressure on structure and magnetism is analyzed on the basis of the experimental and theoretical results present in the literature and the case of iron is reported as an example. Chapter 2 surveys some high-pressure apparata. In particular, the diamond anvil cells and related equipment. The polarized XAS technique is described and the scientific and technical implications given by the high pressure setup are discussed. Special focus is dedicated to the interpretation of the K-edge X-ray Magnetic Circular Dichroism signal. Finally the principle of the energy dispersive spectrometer and high pressure XMCD setup of beamline ID24 is briefly described, pinpointing its suitability to this kind of experiments. Experimental results and discussion In Chapter 3, 4 and 5 our new results on, respectively, cobalt, nickel and iron-cobalt alloys are presented. In each chapter the most relevant experimental and theoretical literature is overviewed and the experimental setup, used in each experiment, described. Then, the experimental results are discussed in comparison with previous studies and theoretical calculations. introduction vii Finally the overall conclusion of this work are presented. 1 Magnetism and structure in the 3d metals In this chapter an historical overview of the fundamental understanding of magnetism in the transition metals is given, with particular attention devoted to the Stoner band model. A covalent magnetism model, describing the magnetism of metallic alloys is presented. The close relationship between structure and magnetism in the 3d metal is discussed and the relevance of high pressure studies in addressing this complex interplay is underlined. The structural and magnetic modifications induced by compression in these metals are analyzed on the basis of the experimental and theoretical results present in the literature and the case of iron is reported as an example. 1.1 Itinerant ferromagnetism In the history of magnetism and still today the magnetic properties of the transition metals Fe, Co and Ni and their alloys have constituted the core of the whole field of magnetism. The reason is that for these metals the electronic structure gives rise to sizable magnetic moments at room temperature. While the rare earths (or 4f elements) have also been important in magnetism, they become attractive for potential applications only when alloyed with the transition metals, because this raises their Curie Temperature above room temperature. Therefore the scientific understanding and technological applications of the 3d transition metals have dominated the field. The difficulty and, at the same time, fascination with the transition metals is mainly in their duality between localized and delocalized, so called itinerant, behavior. This duality forms the core of the electronic many-body problem and is the essence of magnetism. The atomic moments in Fe, Co and Ni are not multiples of the Bohr magneton but rather odd fraction of it, therefore cannot be explained by a successive orbital occupation as described by the Hund s rule. The puzzle about the broken Bohr magneton numbers in the ferromagnetic metals was solved through the development of band theory, which 2 Magnetism and structure in the 3d metals was first applied to magnetic systems around 1935 by Mott [1], Slater [2, 3], and Stoner [4, 5]. The simplest band-like model of the ferromagnetic metals is often called the Stonermodel or Stoner-Wohlfarth-Slater-model and underlies our modern understanding of the magnetic structure of metals. The assumption behind the SWS-model is that the bonding interaction between the 3d electrons causes a smearing of their energy into a band. In the presence of a Weiss Field (the exchange field H ex ) the centers of gravity of the states characterized by opposite spins exhibit an exchange separation, the exchange splitting, determining an unbalance in the spin-up and spin-down electron (holes) population. The magnetic moment m is then given by the difference in the number of electrons in the majority and minority bands, as defined in Fig. 1.1 according to: m = µ B (N
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