Layer-resolved imaging of domain wall interactions in magnetic tunnel junction-like trilayers

We have performed a layer-resolved, microscopic study of interactions between domain walls in two magnetic layers separated by a non-magnetic one, using high-resolution x-ray photoemission electron microscopy. Domain walls in the hard magnetic Co layer of a Co/Al2O3/FeNi trilayer with in-plane uniaxial anisotropy strongly modify the local magnetization direction in the soft magnetic FeNi layer. The stray fields associated to the domain walls lead to an antiparallel coupling between the local Co and FeNi moments. For domain walls parallel to the easy magnetization axis this interaction is limited to the domain wall region itself. For strongly charged (head-on or tail-to-tail) walls, the antiparallel coupling dominates the interaction over radial distances up to several micrometers from the centre of the domain wall.Comment: Published version, J. Phys.: Condens. Matter 19, 476204 (2007

X-ray magnetic circular dichroism in (Ge,Mn) compounds: experiments and modeling

X-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra at the L$_{2,3}$ edges of Mn in (Ge,Mn) compounds have been measured and are compared to the results of first principles calculation. Early \textit{ab initio} studies show that the Density Functional Theory (DFT) can very well describe the valence band electronic properties but fails to reproduce a characteristic change of sign in the L$_{3}$ XMCD spectrum of Mn in Ge$_3$Mn$_5$, which is observed in experiments. In this work we demonstrate that this disagreement is partially related to an underestimation of the exchange splitting of Mn 2$p$ core states within the local density approximation. It is shown that the change in sign experimentally observed is reproduced if the exchange splitting is accurately calculated within the Hartree-Fock approximation, while the final states can be still described by the DFT. This approach is further used to calculate the XMCD in different (Ge,Mn) compounds. It demonstrates that the agreement between experimental and theoretical spectra can be improved by combining state of the art calculations for the core and valence states respectively.Comment: 8 page

From diluted magnetic semiconductors to self-organized nanocolumns of GeMn in Germanium

While achieving high Curie temperatures (above room temperature) in diluted magnetic semiconductors remains a challenge in the case of well controlled homogeneous alloys, several systems characterized by a strongly inhomogeneous incorporation of the magnetic component appear as promising. Incorporation of manganese into germanium drastically alters the growth conditions, and in certain conditions of low temperature Molecular Beam Epitaxy it leads to the formation of well organized nanocolumns of a Mn-rich material, with a crystalline structure in epitaxial relationship with the Mn-poor germanium matrix. A strong interaction between the Mn atoms in these nanocolums is demonstrated by x-ray absorption spectroscopy, giving rise to a ferromagnetic character as observed through magnetometry and x-ray magnetic circular dichroism. Most interesting, intense magneto-transport features are observed on the whole structure, which strongly depend on the magnetic configuration of the nanocolumns.Comment: SPIE Optics & Photonics Symposium, San Diego : \'Etats-Unis d'Am\'erique (2008

MORPHOLOGIE, STRUCTURE ET MAGNETISME DES COUCHES ULTRAMINCES D'ALLIAGES FE XNI 1 X DEPOSEES SUR DES SURFACES VICINALES DU CU(111)

NOUS AVONS ETUDIE LES PROPRIETES MAGNETIQUES ET STRUCTURALES DE FILMS ULTRAMINCES (2D) ET DE NANO-FILS (1D) D'ALLIAGES MAGNETIQUES DEPOSES SUR DES SURFACES A MARCHES PERIODIQUEMENT ESPACEES. NOTRE OBJECTIF ETAIT DE MONTRER L'EXISTENCE D'UNE ANISOTROPIE MAGNETIQUE INDUITE PAR LES MARCHES DU SUBSTRAT DANS LE PLAN DES COUCHES ET DE CORRELER CETTE ANISOTROPIE A LA MORPHOLOGIE (FORMES 1D, 2D, MEMOIRE DES MARCHES) AINSI QU'A LA STRUCTURE DE L'ALLIAGE (CONTRAINTES, RELAXATION) ; LE TOUT DANS UN SOUCI DE COMPREHENSION DES MECANISMES QUI GOUVERNENT L'ANISOTROPIE MAGNETIQUE. NOUS MONTRONS PAR STM QUE L'EPAISSEUR ET LA CONCENTRATION DE FE XNI 1 X/CU(111)-VIC SONT DES FACTEURS QUI INFLUENCENT FORTEMENT LA MORPHOLOGIE. NOTRE CHOIX C'EST NEANMOINS PORTE SUR L'ALLIAGE FE 0 . 6 5NI 0 . 3 5 QUI PRESENTE L'AVANTAGE D'ADOPTER UNE STRUCTURE CFC AINSI QU'UNE CROISSANCE EN DECORATION DE MARCHES. CETTE AUTO-ORGANISATION DES ATOMES LE LONG DU BORD SUPERIEUR DES MARCHES NOUS PERMET D'OBTENIR DES STRUCTURES A 1D (FILS) POUR DES FAIBLES TAUX DE RECOUVREMENT ALORS QU'AU DELA DE LA COALESCENCE 2D, DES RUBANS D'ALLIAGE SONT OBSERVES. L'APPLICATION DES REGLES DE SOMME AUX MESURES XMCD EFFECTUEES AUX SEUILS L 2 , 3 DU FE EN FONCTION DES ANGLES POLAIRE : ET AZIMUTAL : NOUS ONT PERMIS DE METTRE EN EVIDENCE L'EXISTENCE D'UNE FORTE ANISOTROPIE DU MOMENT ORBITAL DANS LE PLAN DES FILMS. L'EFFET LE PLUS IMPORTANT EST OBSERVE POUR LES FILS 1D AINSI QUE POUR LES RUBANS D'ALLIAGE A 3 MC. CETTE ANISOTROPIE MAGNETIQUE EST CORRELEE, DANS LE CAS DES RUBANS, A LA RELAXATION ANISOTROPE DES FILMS. NOUS AVONS MONTRE PAR S-EXAFS QU'UNE FAIBLE COMPRESSION DU PARAMETRE DE MAILLE PEUT INDUIRE UNE AUGMENTATION SIGNIFICATIVE DU MOMENT ORBITAL. LES DEPOTS ULTRAMINCES (0.3 ET 0.7 MC) MONTRENT QUANT A EUX UN ETAT DE SPIN ELEVE CORRELE AU VOLUME ATOMIQUE DU FER.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Interface Magnetoelectric Coupling in Co/Pb(Zr,Ti)O3

Vlašín, Ondřej et al.Magnetoelectric coupling at multiferroic interfaces is a promising route toward the nonvolatile electric-field control of magnetization. Here, we use optical measurements to study the static and dynamic variations of the interface magnetization induced by an electric field in Co/ PbZr0.2Ti0.8O3 (Co/PZT) bilayers at room temperature. The measurements allow us to identify different coupling mechanisms. We further investigate the local electronic and magnetic structure of the interface by means of transmission electron microscopy, soft X-ray magnetic circular dichroism, and density functional theory to corroborate the coupling mechanism. The measurements demonstrate a mixed linear and quadratic optical response to the electric field, which results from a magneto-electto-optical effect. We propose a decomposition method of the optical signal to discriminate between different components involved in the electric field-induced polarization rotation of the reflected light. This allows us to extract a signal that we can ascribe to interface magnetoelectric coupling. The associated surface magnetization exhibits a clear hysteretic variation of odd symmetry with respect to the electric field and nonzero remanence. The interface coupling is remarkably stable over a wide frequency range (1-50 kHz), and the application of a bias magnetic field is not necessary for the coupling to occur. These results show the potential of exploiting interface coupling with the prospect of optimizing the performance of magnetoelectric memory devices in terms of stability, as well as fast and dissipationless operation.This work is supported by the French National Research Agency (ANR) through JCJC program "DYNAMECS” ANR-11-JS10-009-01 and the TEM study was conducted in the framework of project "EMMA” ANR-12-BS10-013. Financial support by the Spanish Government for the CSIC JAE-predoc grant of O.V. is acknowledged. S. C.-H. acknowledges the technical assistance of J.-S. Pelle (IPHC, Strasbourg) and the team of the STnano cleanroom facility in Strasbourg for the optimization of the magnetoelectric micro-devices. R. Cours (CEMES, Toulouse) is acknowledged for his help with the preparation of the sample lamellas for TEM measurements.Peer reviewe

Focusing characteristics of polarized second-harmonic emission at non-Ising polar domain walls

International audienceNon-Ising polar domain walls have recently emerged as individual two-dimensional materials exhibiting localized nonlinear optical emission. The analysis of this emitted light often requires focusing with high apertures. As a result, the vectorial properties of light come into play. This study provides an analytic treatment of the vector light fields' effect on the polarized second-harmonic emission (SHG) arising at polar domain walls. While confined optical fields are expected to alter the SHG polarization response, we identify extrinsic and intrinsic properties capable of canceling focusing effects. We determine a precise combination of the fundamental wave polarization and orientation of the domain walls at which focusing effects are negligible. Furthermore, the perimeter defined by the domain walls intrinsic optical parameters below which focusing effects are negligible is extracted from a systematic focus-dependent analysis. Our study provides the necessary methodology and precautions to probe the internal structure of non-Ising domain walls with confined optical fields, and it can be extended to explore newly discovered ferroelectric topologic nanostructures

Polarization Control of the Interface Ferromagnetic to Antiferromagnetic Phase Transition in Co/Pb(Zr,Ti)O 3

International audienceBased on first-principles calculations, we predict the polarization control of the interfacial magnetic phase and a giant electronically driven magnetoelectric coupling (MEC) in Co/PbZr0.25Ti0.75O3 (PZT)(001). The effect of Co oxidation at the interface shared with (Zr,Ti)O2-terminated PZT is evidenced. The magnetic phase of the oxidized Co interface layer is electrically switched from the ferromagnetic to the antiferromagnetic state by reversing the PZT polarization from upward to downward, respectively. A comparative study between oxidized and unoxidized Co/PZT interfaces shows that in oxidized Co/PZT bilayers, the variation of the interface spin moment upon polarization reversal exceeds that of unoxidized Co/PZT bilayers by about 1 order of magnitude. We define a surface MEC constant αS taking into account the polarization dependence of both the spin and orbital moments. In unoxidized Co/PZT bilayers, we obtain αS ≈ 2 × 10–10 G cm2 V–1, while a giant surface coupling αS ≈ 12 × 10–10 G cm2 V–1 is found in the case of oxidized Co/PZT. We demonstrate that the polarization control of the magnetocrystalline anisotropy via spin–orbit coupling is not only effective at the interface but it extends to the Co film despite the interface origin of the MEC. This study shows that tailoring the nature of atomic bonding and electron occupancies allows for improving the performance of functional interfaces, enabling an efficient electric field control of spin–orbit interactions. Moreover, the nonlocal character of this effect holds promising perspectives for the application of electronically driven interface MEC in spin–orbitronic devices

Three-Dimensional Optical Analysis of Ferroelectric Domain Walls

International audienc

Composition of Ge(Si) Islands in the Growth of Ge on Si(111) by X-Ray Spectromicroscopy.

The stoichiometry of Ge/Si islands grown on Sis111d substrates at temperatures ranging from 460 to 560 degrees Celsius was investigated by x-ray photoemission electron microscopy sXPEEMd. By developing a specific analytical framework, quantitative information on the surface Ge/Si stoichiometry was extracted from laterally resolved XPEEM Si 2p and Ge 3d spectra, exploiting the chemical sensitivity of the technique. Our data show the existence of a correlation between the base area of the self-assembled islands and their average surface Si content: the larger the lateral dimensions of the 3D structures, the higher their relative Si concentration. The deposition temperature determines the characteristics of this relation, pointing to the thermal activation of kinetic diffusion processes