Ferroelectric control of the spin texture in germanium telluride

The electrical manipulation of spins in semiconductors, without magnetic fields or auxiliary ferromagnetic materials, represents the holy grail for spintronics. The use of Rashba effect is very attractive because the k-dependent spin-splitting is originated by an electric field. So far only tiny effects in two-dimensional electron gases (2DEG) have been exploited. Recently, GeTe has been predicted to have bulk bands with giant Rashba-like splitting, originated by the inversion symmetry breaking due to ferroelectric polarization. In this work, we show that GeTe(111) surfaces with inwards or outwards ferroelectric polarizations display opposite sense of circulation of spin in bulk Rashba bands, as seen by spin and angular resolved photoemission experiments. Our results represent the first experimental demonstration of ferroelectric control of the spin texture in a semiconductor, a fundamental milestone towards the exploitation of the non-volatile electrically switchable spin texture of GeTe in spintronic devices.Comment: 18 pages, 4 figure

Electronically ordered ultrathin Cr2O3 on Pt(1 1 1) in presence of a multidomain graphene intralayer

In the last decade, reducing the dimensionality of materials to few atomic layers thickness has allowed exploring new physical properties and functionalities otherwise absent out of the two dimensional limit. In this regime, interfaces and interlayers play a crucial role. Here, we investigate their influence on the electronic properties and structural quality of ultrathin Cr2O3 on Pt(111), in presence of a multidomain graphene intralayer. Specifically, by combining Low-Energy Electron Diffraction, X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy, we confirm the growth of high-quality ultrathin Cr2O3 on bare Pt, with sharp surface reconstructions, proper stoichiometry and good electronic quality. Once a multidomain graphene intralayer is included at the metal/oxide interface, the Cr2O3 maintained its correct stoichiometry and a comparable electronic quality, even at the very first monolayers, despite the partially lost of the morphological long-range order. These results show how ultrathin Cr2O3 films are slightly affected by the interfacial epitaxial quality from the electronic point of view, making them potential candidates for graphene-integrated heterostructures

Influence of orbital character on the ground state electronic properties in the van Der Waals transition metal iodides VI3 and CrI3

This work was performed in the framework of the Nanoscience Foundry and Fine Analysis (NFFA-MUR Italy) facility and was supported by JST-CREST (No. JPMJCR18T1). A part of the computation in this work, using the VASP code (43) in the GGA approximation (44), was performed by using the facilities of the Supercomputer Center, the Institute for Solid State Physics, the University of Tokyo and MASAMUNE-IMR, Center for Computational Materials Science, Institute for Materials Research, Tohoku University (Project No. 20K0045).Two-dimensional van der Waals magnetic semiconductors display emergent chemical and physical properties and hold promise for novel optical, electronic and magnetic “few-layers” functionalities. Transition-metal iodides such as CrI3 and VI3 are relevant for future electronic and spintronic applications; however, detailed experimental information on their ground state electronic properties is lacking often due to their challenging chemical environment. By combining X-ray electron spectroscopies and first-principles calculations, we report a complete determination of CrI3 and VI3 electronic ground states. We show that the transition metal-induced orbital filling drives the stabilization of distinct electronic phases: a wide bandgap in CrI3 and a Mott insulating state in VI3. Comparison of surface-sensitive (angular-resolved photoemission spectroscopy) and bulk-sensitive (X-ray absorption spectroscopy) measurements in VI3 reveals a surface-only V2+ oxidation state, suggesting that ground state electronic properties are strongly influenced by dimensionality effects. Our results have direct implications in band engineering and layer-dependent properties of two-dimensional systems.Publisher PDFPeer reviewe

Observation of termination-dependent topological connectivity in a magnetic Weyl Kagome lattice

The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement 897276. The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. (https://www.gauss-centre.eu) for funding this project by providing computing time on the GCS Supercomputer SuperMUC-NG at Leibniz Supercomputing Centre (https://www.lrz.de). The authors are grateful for funding support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project 390858490), through FOR 5249-449872909 (Project P5), and through the Collaborative Research Center SFB 1170 ToCoTronics (Project 258499086). The authors greatly acknowledge the Diamond Light Source that supported the entire micro-ARPES experiment and corresponding costs. The Flatiron Institute is a division of the Simons Foundation. P.D.C.K. and C.B. gratefully acknowledge support from The Leverhulme Trust via Grant RL-2016-006.Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designers, with the opportunity to drive new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co3Sn2S2 and show how for the terminations of different samples the Weyl points connect differently, still preserving the bulk-boundary correspondence. Scanning tunneling microscopy has suggested such a scenario indirectly, and here, we probe the Fermiology of Co3Sn2S2 directly, by linking it to its real space surface distribution. By combining micro-ARPES and first-principles calculations, we measure the energy-momentum spectra and the Fermi surfaces of Co3Sn2S2 for different surface terminations and show the existence of topological features depending on the top-layer electronic environment. Our work helps to define a route for controlling bulk-derived topological properties by means of surface electrostatic potentials, offering a methodology for using Weyl kagome metals in responsive magnetic spintronics.Publisher PDFPeer reviewe

Spin injection in the doped bad metal SrTiO3

In this paper, we demonstrate the capability to establish spin-polarized currents in doped SrTiO3 (STO). The results are based on the study of charge and spin transport in STO layers doped by the reversible electromigration of oxygen atoms in resistive-switching La0.7Sr0.3MnO3/STO/Co vertical stacks. The formation of oxygen vacancies inside STO results in a metallic conductivity at temperatures <200–250 K, above which a transitionto an insulating like behavior is detected. A detailed theoretical analysis shows that the behavior of the metallic phase in our samples corresponds to the well-known state of the thermodynamically doped STO featuring the so-called bad metal behavior. Thus, our findings introduce this class of unconventional materials as valuable candidates for innovative spintronic devices

Covalency, correlations, and interlayer interactions governing the magnetic and electronic structure of Mn3Si2Te6

Mn3Si2Te6 is a rare example of a layered ferrimagnet. It has recently been shown to host a colossal angular magnetoresistance as the spin orientation is rotated from the in- to out-of-plane direction, proposed to be underpinned by a topological nodal-line degeneracy in its electronic structure. Nonetheless, the origins of its ferrimagnetic structure remain controversial, while its experimental electronic structure, and the role of correlations in shaping this, are little explored to date. Here, we combine x-ray and photoemission-based spectroscopies with first-principles calculations to probe the elemental-selective electronic structure and magnetic order in Mn3Si2Te6. Through these, we identify a marked Mn-Te hybridization, which weakens the electronic correlations and enhances the magnetic anisotropy.We demonstrate how this strengthens the magnetic frustration in Mn3Si2Te6, which is key to stabilizing its ferrimagnetic order, and find a crucial role of both exchange interactions extending beyond nearest-neighbors and antisymmetric exchange in dictating its ordering temperature. Together, our results demonstrate a powerful methodology of using experimental electronic structure probes to constrain the parameter space for first-principles calculations of magnetic materials, and through this approach, reveal a pivotal role played by covalency in stabilizing the ferrimagnetic order in Mn3Si2Te6

Scalabilité et amélioration des propriétés de couplage d'échange pour TA-MRAM

Exchange coupling between a ferromagnetic (F) and an antiferromagnetic (AF) layer is responsible of a higher coercivity and of a shift in the hysteresis loop. This phenomenon is widely used in magnetic systems like spin-valves and MRAM to set the reference layer, that remains fixed during the writing processes of the storage layer. It has been noticed that, for systems with reduced lateral size , the magnetization of the reference layer can (completely or partially) reverse because of spin switches in the AF layer. The aim of this thesis project is the understanding of these reversal phenomena, the quantification as a function of lateral dimension and the proposal of feasable solutions in order to increase the stability of the storage layer. The thesis will be maily experimental, including deposition, lithography and characterization processes. The main part of the thesis will be spent at SPINTEC (UMR8191) laboratories. The student will also collaborate with other research groups in Grenoble, in particular for magneto-optical measurements, crystallographic analysis, and atomic simulations. He will also manage the industrial integration of his studies, by sharing and discussing his results with Crocus Technology. The thesis, during a period of three years, will cover the following subjects: i) Study of the termal stability as a function of lateral size (0-15 months). The student will deposit F/AF bilayers being AF FeMn, PtMn or IrMn) by magnetron sputtening. These layers will be etched at Pthe TA cleanroom facility, in CEA-Grenoble. TB and exchange field distributions will be characterized by Kerr effect and magnetotrasport measurements as a function of lateral size. He will collaborate to the crystallographic analysis with the X-ray general service and the 'laboratoires d'Etude des Matériaux par Microscopie Avancée' (LEMMA) (CEA/Grenoble/INAC/SP2M). These analysis, that will give informations about grain size and distribution, will help the understanding of the magnetic measurements and will be a starting point for an optimization, through annealing steps or additional elements, of the stability of systems with reduced lateral size (typically below 100nm). ii) Study of AF inter-grain coupling (15-22 months) The student will perform magnetic training measurements. He will determine the nucleation volumes in the AF and compare them with the crystallographic results. He will manage to establish the importance of this coupling in the stability of magnetic memories and to vary its intensity (annealing, additional elements). This study will contribute to optimize the exchange anisotropy at reduced dimensions presented in the previous point. iii) Atomic simulations (22-30 months) The student will collaborate with LSIM laboratory, in particular with F.Lançon. He will simulate the impact of crystallography (grain size, inter-grain coupling, interfacial disorder, rugosity) on exchange anisotropy properties in F/AF systems with reduced lateral size. Simulations will be performed with a code developped in the lab. These simulations will help in understanding the experimental measures performed previously, and will give new suggestions in the optimization process of the exchange field for technological integration.Le couplage d’échange entre une couche ferromagnétique (F) et une couche antiferromagnétique (AF) permet de piéger l’aimantation de la couche ferromagnétique. Ce phénomène est largement utilisé dans des systèmes magnétiques complexes, telles que les vannes de spins, ou les mémoires MRAM, où il permet de constituer des couches de références, normalement insensibles aux cycles d’écriture des couches de stockage. On remarque aux petites dimensions, lorsque la taille des cellules diminue en dessous de la centaine de nm, des renversements partiels ou complets des électrodes de référence, dus à un basculement du réseau de spins dans l’AF. L’objectif de cette thèse est de comprendre ces phénomènes de renversement, de les quantifier en fonction de la dimension latérale des dispositifs, et de présenter des solutions viables afin d’accroître la stabilité des systèmes de stockage. Ce travail essentiellement expérimental, comprenant dépôts, lithogravure et caractérisations, se déroulera pour la majeure partie au sein du laboratoire SPINTEC (UMR8191). L’étudiant sera cependant amené à collaborer avec plusieurs entités du pôle grenoblois, notamment pour les mesures magnéto-optiques, les analyses cristallographiques, ainsi que pour une partie de simulation atomistique ; il devra aussi s’intéresser à l’intégration industrielle de ses études en rendant compte de ses résultats, en les discutant, afin que Crocus Technology en bénéficie directement. La thèse, se déroulant sur trois ans, explorera les points suivants : i) Etude de la stabilité thermique en fonction de la taille des motifs (0-15mois) L’étudiant déposera par pulvérisation cathodique des bicouches F/AF (AF=FeMn, PtMn ou IrMn) qui seront gravées sur la plate forme de technologie amont (PTA) localisée sur le site du CEA/Grenoble. Il caractérisera par des mesures d’effet kerr ou de magnétotransport les propriétés magnétiques des bicouches, notamment les distributions de TB, de champ d’échange, en fonction de la taille des motifs. Il participera aux analyses cristallographiques en collaboration avec le Service général des rayons X et le laboratoires d'Etude des Matériaux par Microscopie Avancée (LEMMA) (du CEA/Grenoble/INAC/SP2M). Ces analyses qui donneront des renseignements sur les tailles de grains et leur distribution seront utilisées pour comprendre les mesures magnétiques dans un premier temps, et seront un point de départ pour optimiser via des recuits, ou l’ajout d’éléments d’addition, la stabilité des systèmes aux dimensions réduites, typiquement <100nm. ii) Etude du couplage inter-grain dans l’AF (15-22mois) L’étudiant réalisera des mesures de trainage magnétique et déterminera les volumes de nucléation dans l’AF et les comparera aux données cristallographiques. Il essaiera de déterminer l’importance de ce couplage dans la stabilité des points mémoire en jouant sur son intensité (recuits, éléments d’addition…), ceci participant de l’optimisation de l’anisotropie d’échange aux petites dimensions présentée dans la partie précédente. iii) Volet de simulations atomistiques (22-30mois) L’étudiant collaborera avec le laboratoire LSIM et notamment F. Lançon afin de simuler, grâce à un code de calcul développé localement, l’impact de la cristallographie (taille de grains, couplage inter-grains, désordre interfacial et rugosité) sur les propriétés de l’anisotropie d’échange dans les systèmes F/AF de taille réduite. Ces simulations permettront de comprendre les mesures expérimentales réalisées en parallèle et d’ouvrir de nouvelles voies exploratoires pour optimiser les valeurs de champ d’échange en vue de leur intégration dans les dispositifs

Scalability and improvement of exchange bias properties for Thermally Assisted MRAM

Le couplage d’échange entre une couche ferromagnétique (F) et une couche antiferromagnétique (AF) permet de piéger l’aimantation de la couche ferromagnétique. Ce phénomène est largement utilisé dans des systèmes magnétiques complexes, telles que les vannes de spins, ou les mémoires MRAM, où il permet de constituer des couches de références, normalement insensibles aux cycles d’écriture des couches de stockage. On remarque aux petites dimensions, lorsque la taille des cellules diminue en dessous de la centaine de nm, des renversements partiels ou complets des électrodes de référence, dus à un basculement du réseau de spins dans l’AF. L’objectif de cette thèse est de comprendre ces phénomènes de renversement, de les quantifier en fonction de la dimension latérale des dispositifs, et de présenter des solutions viables afin d’accroître la stabilité des systèmes de stockage. Ce travail essentiellement expérimental, comprenant dépôts, lithogravure et caractérisations, se déroulera pour la majeure partie au sein du laboratoire SPINTEC (UMR8191). L’étudiant sera cependant amené à collaborer avec plusieurs entités du pôle grenoblois, notamment pour les mesures magnéto-optiques, les analyses cristallographiques, ainsi que pour une partie de simulation atomistique ; il devra aussi s’intéresser à l’intégration industrielle de ses études en rendant compte de ses résultats, en les discutant, afin que Crocus Technology en bénéficie directement. La thèse, se déroulant sur trois ans, explorera les points suivants : i) Etude de la stabilité thermique en fonction de la taille des motifs (0-15mois) L’étudiant déposera par pulvérisation cathodique des bicouches F/AF (AF=FeMn, PtMn ou IrMn) qui seront gravées sur la plate forme de technologie amont (PTA) localisée sur le site du CEA/Grenoble. Il caractérisera par des mesures d’effet kerr ou de magnétotransport les propriétés magnétiques des bicouches, notamment les distributions de TB, de champ d’échange, en fonction de la taille des motifs. Il participera aux analyses cristallographiques en collaboration avec le Service général des rayons X et le laboratoires d'Etude des Matériaux par Microscopie Avancée (LEMMA) (du CEA/Grenoble/INAC/SP2M). Ces analyses qui donneront des renseignements sur les tailles de grains et leur distribution seront utilisées pour comprendre les mesures magnétiques dans un premier temps, et seront un point de départ pour optimiser via des recuits, ou l’ajout d’éléments d’addition, la stabilité des systèmes aux dimensions réduites, typiquement <100nm. ii) Etude du couplage inter-grain dans l’AF (15-22mois) L’étudiant réalisera des mesures de trainage magnétique et déterminera les volumes de nucléation dans l’AF et les comparera aux données cristallographiques. Il essaiera de déterminer l’importance de ce couplage dans la stabilité des points mémoire en jouant sur son intensité (recuits, éléments d’addition…), ceci participant de l’optimisation de l’anisotropie d’échange aux petites dimensions présentée dans la partie précédente. iii) Volet de simulations atomistiques (22-30mois) L’étudiant collaborera avec le laboratoire LSIM et notamment F. Lançon afin de simuler, grâce à un code de calcul développé localement, l’impact de la cristallographie (taille de grains, couplage inter-grains, désordre interfacial et rugosité) sur les propriétés de l’anisotropie d’échange dans les systèmes F/AF de taille réduite. Ces simulations permettront de comprendre les mesures expérimentales réalisées en parallèle et d’ouvrir de nouvelles voies exploratoires pour optimiser les valeurs de champ d’échange en vue de leur intégration dans les dispositifs.Exchange coupling between a ferromagnetic (F) and an antiferromagnetic (AF) layer is responsible of a higher coercivity and of a shift in the hysteresis loop. This phenomenon is widely used in magnetic systems like spin-valves and MRAM to set the reference layer, that remains fixed during the writing processes of the storage layer. It has been noticed that, for systems with reduced lateral size , the magnetization of the reference layer can (completely or partially) reverse because of spin switches in the AF layer. The aim of this thesis project is the understanding of these reversal phenomena, the quantification as a function of lateral dimension and the proposal of feasable solutions in order to increase the stability of the storage layer. The thesis will be maily experimental, including deposition, lithography and characterization processes. The main part of the thesis will be spent at SPINTEC (UMR8191) laboratories. The student will also collaborate with other research groups in Grenoble, in particular for magneto-optical measurements, crystallographic analysis, and atomic simulations. He will also manage the industrial integration of his studies, by sharing and discussing his results with Crocus Technology. The thesis, during a period of three years, will cover the following subjects: i) Study of the termal stability as a function of lateral size (0-15 months). The student will deposit F/AF bilayers being AF FeMn, PtMn or IrMn) by magnetron sputtening. These layers will be etched at Pthe TA cleanroom facility, in CEA-Grenoble. TB and exchange field distributions will be characterized by Kerr effect and magnetotrasport measurements as a function of lateral size. He will collaborate to the crystallographic analysis with the X-ray general service and the 'laboratoires d'Etude des Matériaux par Microscopie Avancée' (LEMMA) (CEA/Grenoble/INAC/SP2M). These analysis, that will give informations about grain size and distribution, will help the understanding of the magnetic measurements and will be a starting point for an optimization, through annealing steps or additional elements, of the stability of systems with reduced lateral size (typically below 100nm). ii) Study of AF inter-grain coupling (15-22 months) The student will perform magnetic training measurements. He will determine the nucleation volumes in the AF and compare them with the crystallographic results. He will manage to establish the importance of this coupling in the stability of magnetic memories and to vary its intensity (annealing, additional elements). This study will contribute to optimize the exchange anisotropy at reduced dimensions presented in the previous point. iii) Atomic simulations (22-30 months) The student will collaborate with LSIM laboratory, in particular with F.Lançon. He will simulate the impact of crystallography (grain size, inter-grain coupling, interfacial disorder, rugosity) on exchange anisotropy properties in F/AF systems with reduced lateral size. Simulations will be performed with a code developped in the lab. These simulations will help in understanding the experimental measures performed previously, and will give new suggestions in the optimization process of the exchange field for technological integration