X-ray Magnetic Circular Dichroism Analysis Research Articles

Strong signature of uncompensated magnetization in frustrated cobalt manganites using x-ray magnetic circular dichroism study

The present study is focused on the investigation of the distorted tetragonal phase of mixed spinel oxides, due to their technological relevance in the field of electronics, spintronics, magnetism, catalysis, and electrochemical energy storage. Herein, we report on solgel synthesized multivalent cobalt manganites, CoMn2O4 (CMO), and subjected them to a comprehensive analysis to elucidate their physicochemical characteristics at room temperature. Analysis employing powder x-ray diffraction patterns and electron microscopy (including field-emission scanning electron microscopy and high-resolution transmission electron microscopy) results confirmed the formation of a pure and exceptionally crystalline, distorted tetragonal phase of mixed CMO. Synchrotron-based x-ray absorption spectroscopic (XAS) measurements in the total electron yield mode examined local electronic structures affirming the formation of CMO with uncompensated electronic states involving Co2+, Co3+, Mn2+, Mn3+, and Mn4+ cations. Concurrently, XAS and x-ray magnetic circular dichroism analyses revealed antiferromagnetic coupling within Co and Mn sublattices in CMO, indicating the presence of uncompensated electronic states. Vibrating sample magnetometry results demonstrated clear hysteresis behavior, explicitly indicating the coexistence of super-paramagnetic and canted antiferromagnetic characteristics in CMO, as validated through the Langevin function fitting and x-ray magnetic circular dichroism results. The noticeable absence of saturated magnetization confirmed the high degree of spin canting, primarily stemming from the presence of the Yafet–Kittel spin arrangement.

Magnetic and Impedance Analysis of Fe2O3 Nanoparticles for Chemical Warfare Agent Sensing Applications

A dire need for real-time detection of toxic chemical compounds exists in both civilian and military spheres. In this paper, we demonstrate that inexpensive, commercially available Fe2O3 nanoparticles are capable of selective sensing of chemical warfare agents (CWAs) using frequency-dependent impedance spectroscopy, with additional potential as an orthogonal magnetic sensor. X-ray magnetic circular dichroism analysis shows that Fe2O3 nanoparticles possess moderately lowered moment upon exposure to 2-chloroethyl ethyl sulfide (2-CEES) and diisopropyl methylphosphonate (DIMP) and significantly lowered moment upon exposure to dimethyl methylphosphonate (DMMP) and dimethyl chlorophosphate (DMCP). Associated X-ray absorption spectra confirm a redox reaction in the Fe2O3 nanoparticles due to CWA structural analog exposure, with differentiable energy-dependent features that suggest selective sensing is possible, given the correct method. Impedance spectroscopy performed on samples dosed with DMMP, DMCP, and tabun (GA, chemical warfare nerve agent) showed strong, differentiable, frequency-dependent responses. The frequency profiles provide unique “shift fingerprints” with which high specificity can be determined, even amongst similar analytes. The results suggest that frequency-dependent impedance fingerprinting using commercially available Fe2O3 nanoparticles as a sensor material is a feasible route to selective detection.

Effect of annealing temperature on the structural, electronic and magnetic properties of Co doped TiO2 nanoparticles: An investigation by synchrotron-based experimental techniques

The present study examines the effect of annealing temperature on the structural, electronic and magnetic properties of Co-doped TiO2 (TCO) nanoparticles (NPs) synthesized by the sol-gel method. X-ray powder diffractograms of TCO NPs annealed at 400, 600 and 800 °C exhibited the monophasic anatase, diphase (anatase and rutile) and monophasic rutile structures respectively. A morphological investigation using high-resolution scanning electron microscopy (HR-SEM) reveals the presence of nanoparticles with diameters ranging from ∼11 to ∼51 nm. The widening and shifting of the most intense Eg peak in Raman studies and SEM images of TCO NPs suggest Co incorporation in the TiO2 matrix and the formation of oxygen vacancies. Magnetic measurements of TCO at room temperature demonstrate ferromagnetic behaviour which is caused by the existence of oxygen vacancies and the doped Co content. Element specific technique, soft X-ray magnetic circular dichroism (XMCD) was used to study the magnetism in Co doped TiO2 samples. The estimated total magnetic moment by XMCD was in the range of ∼1.3 ± 0.1–2.3 ± 0.2 μB/Co. This huge magnetization was first time observed by the surface-sensitive Total Electron Yield (TEY) mode. The estimated saturation magnetic moment is in the range of 1.0–1.5 μB/Co, also consistent with XMCD analysis and cluster model calculation. The results obtained by XMCD recommend an intrinsic behaviour of ferromagnetism. The X-ray absorption spectra (XAS) of Co-doped TiO2 nanoparticles at Ti L2,3 and Co L2,3 -edge in TEY mode concludes that the valence states of Co and Ti ions are in 2 + and 4 + respectively. Cluster model calculation also confirmed that Co ions are in a 2 + valence state with D2 h high spin crystal-field symmetry at the surface while a random crystal field symmetry in bulk has been observed which is well consistent with experimental data.

Multimodal analysis of Zr substitution effects on magnetic and crystallographic properties in (Sm1−xZrx)(Fe0.8Co0.2)12 compounds with ThMn12 structure

SmFe12-based compounds containing Zr atoms have been recognized as candidates for novel high-performance permanent magnet materials with a high saturation magnetization. Partial substitution of Sm with Zr in (Sm1−xZrx)(Fe0.8Co0.2)12 has been known to enhance the magnetization of the compound. This study focused on (Sm1−xZrx)(Fe0.8Co0.2)12 single-crystalline films to evaluate the effects of Zr substitution for Sm on their structural and magnetic properties via systematic multimodal analysis. X-ray absorption fine structure (XAFS) and scanning transmission electron microscopy indicated a selective occupation of the Sm site by Zr atoms without disturbing the Fe sublattices. The chemical shift in the Zr-K edge XAFS spectra suggested electron transfer from Zr to the Fe band. Soft X-ray magnetic circular dichroism analysis proved that the magnetic moment of Sm was two orders of magnitude smaller than the magnetic moments of Fe and Co, and its contribution to the total magnetization was negligible. The partial substitution of Sm with Zr resulted in a volumetric expansion and c-axis length elongation despite Zr having a smaller atomic radius, which is unique to (Sm1−xZrx)(Fe0.8Co0.2)12 without structure-stabilizing elements such as Ti, V, and Si. This anisotropic expansion is explained by an increase in the magnetic moment and magnetic interactions in the Fe8f site, which is induced by the electron transfer from Zr to Fe8f atoms. Thus, we conclude that the electron transfer from Zr to the Fe8f atoms principally drives magnetization enhancement in (Sm1−xZrx)(Fe0.8Co0.2)12 single-crystalline films.

Strain assisted magnetoelectric coupling in ordered nanomagnets of CoFe2O4/SrRuO3/(Pb(Mg1/3Nb2/3)O3–PbTiO3) heterostructures

We have explored the electric field controlled magnetization in the nanodot CoFe2O4/SrRuO3/PMN-PT (CFO/SRO/PMN-PT) heterostructures. Ordered ferromagnetic CFO nanodots (∼300 nm lateral dimension) are developed on the PMN-PT substrate (ferroelectric as well as piezoelectric) using a nanostencil-mask pattering method during pulsed laser deposition. The nanostructures reveal electric field induced magnetization reversal in the single domain CFO nanodots through transfer of piezostrains from the piezoelectric PMN-PT substrate to the CFO. Further, electric field modulated spin structure of CFO nanomagnets is analyzed by using x-ray magnetic circular dichroism (XMCD). The XMCD analysis reveals cations (Fe3+/Co2+) redistribution on the octahedral and tetrahedral site in the electric field poled CFO nanodots, establishing the strain induced magneto-electric coupling effects. The CFO/SRO/PMN-PT nanodots structure demonstrate multilevel switching of ME coupling coefficient (α) by applying selective positive and negative electric fields in a non-volatile manner. The retention of two stable states of α is illustrated for ∼106 seconds, which can be employed to store the digital data in non-volatile memory devices. Thus the voltage controlled magnetization in the nanodot structures leads a path towards the invention of energy efficient high-density memory devices.

Unveiling a Chemisorbed Crystallographically Heterogeneous Graphene/L10-FePd Interface with a Robust and Perpendicular Orbital Moment.

A crystallographically heterogeneous interface was fabricated by growing hexagonal graphene (Gr) using chemical vapor deposition (CVD) on a tetragonal FePd epitaxial film grown by magnetron sputtering. FePd was alternately arranged with Fe and Pd in the vertical direction, and the outermost surface atom was identified primarily as Fe rather than Pd. This means that FePd has a high degree of L10-ordering, and the outermost Fe bonds to the carbon of Gr at the interface. When Gr is grown by CVD, the crystal orientation of hexagonal Gr toward tetragonal L10-FePd selects an energetically stable structure based on the van der Waals (vdW) force. The atomic relationship of Gr/L10-FePd, which is an energetically stable interface, was unveiled theoretically and experimentally. The Gr armchair axis was parallel to FePd [100]L10, where Gr was under a small strain by chemical bonding. Focusing on the interatomic distance between the Gr and FePd layers, the distance was theoretically and experimentally determined to be approximately 0.2 nm. This shorter distance (≈0.2 nm) can be explained by the chemisorption-type vdW force of strong orbital hybridization, rather than the longer distance (≈0.38 nm) of the physisorption-type vdW force. Notably, depth-resolved X-ray magnetic circular dichroism analyses revealed that the orbital magnetic moment (Ml) of Fe in FePd emerged at the Gr/FePd interface (@inner FePd: Ml = 0.16 μB → @Gr/FePd interface: Ml = 0.32 μB). This interfacially enhanced Ml showed obvious anisotropy in the perpendicular direction, which contributed to interfacial perpendicular magnetic anisotropy (IPMA). Moreover, the interfacially enhanced Ml and interfacially enhanced electron density exhibited robustness. It is considered that the shortening of the interatomic distance produces a robust high electron density at the interface, resulting in a chemisorption-type vdW force and orbital hybridization. Eventually, the robust interfacial anisotropic Ml emerged at the crystallographically heterogeneous Gr/L10-FePd interface. From a practical viewpoint, IPMA is useful because it can be incorporated into the large bulk perpendicular magnetic anisotropy (PMA) of L10-FePd. A micromagnetic simulation assuming both PMA and IPMA predicted that perpendicularly magnetized magnetic tunnel junctions (p-MTJs) using Gr/L10-FePd could realize 10-year data retention in a small recording layer with a circular diameter and thickness of 10 and 2 nm, respectively. We unveiled the energetically stable atomic structure in the crystallographically heterogeneous interface, discovered the emergence of the robust IPMA, and predicted that the Gr/L10-FePd p-MTJ is significant for high-density X nm generation magnetic random-access memory (MRAM) applications.

Magnetic anisotropic of thermally evaporated FeNi thin film: A soft X‐ray magnetic circular dichroism study

FeNi film was fabricated on Cu/MgO (001) substrates using thermal evaporation technique of a FeNi target to realize the formation of an L10‐FeNi phase. The magnetic anisotropy of FeNi film was systematically investigated by means of X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD) techniques. Our L‐edge XAS and XMCD analysis is performed for 30‐nm FeNi film grown on Cu/MgO (001) substrate. The present experimental results of XAS and XMCD indicate that ~5% Fe atoms are oxidized. Our findings also showed that the orbital magnetic moment of Fe has a significant incident X‐ray angle dependence that exhibits a maximum value at the out‐of‐plane direction corresponding perpendicular to the plane. The origin of anisotropy is successfully explained by element‐specific XMCD technique. The enhancement in magnetic anisotropy energy (MAE) is due to orbital magnetic moments of Fe through spin–orbit interaction. The uniaxial anisotropy energy showed quantitatively good agreement with the volume MAE.

Effects of swift heavy ion-irradiation on magnetic properties of Co-doped TiO2

Ferromagnetic Co-doped TiO2 thin films at room temperature have been synthesized by RF sputtering and ion-irradiated with 100 MeV Au ions. The surface roughness of the film increases with increasing Au-ion fluence. Kerr microscopy measurements at room temperature give isotropic hysteresis loop with negligible coercivity, which gets modified into in-plane uniaxial magnetic anisotropy after anion fluence of 5×1011 ions/cm2. Further ion-irradiation reduced the coercivity of the film. X-ray magnetic circular dichroism (XMCD) measurements have been performed in total electron yield mode, to investigate the effects of ion-irradiation on magnetic properties of the Co-doped TiO2 thin films. From the XMCD analysis, we observed that the total magnetic moment of Co atoms in case of as-deposited film is 1.771±0.016μB. After the ion-irradiation it became 0.3818±0.014μB at room temperature.

Bacterial production of vanadium ferrite spinel (Fe,V)3O4 nanoparticles

AbstractBiogenic nanoscale vanadium magnetite is produced by converting V(V)-bearing ferrihydrites through reductive transformation using the metal-reducing bacterium Geobacter sulfurreducens. With increasing vanadium in the ferrihydrite, the amount of V-doped magnetite produced decreased due to V-toxicity which interrupted the reduction pathway ferrihydrite–magnetite, resulting in siderite or goethite formation. Fe L2,3 and V L2,3 X-ray absorption spectra and data from X-ray magnetic circular dichroism analysis revealed the magnetite to contain the V in the Fe(III) Oh site, predominately as V(III) but always with a component of V(VI), present a consistent V(IV)/V(III) ratio in the range 0.28 to 0.33. The bacteriogenic production of V-doped magnetite nanoparticles from V-doped ferrihydrite is confirmed and the work reveals that microbial reduction of contaminant V(V) to V(III)/V(IV) in the environment will occur below the Fe-redox boundary where it will be immobilised in biomagnetite nanoparticles.

Layer-dependence of macroscopic and atomic magnetic correlations in Co/Pd multilayers

The development of multilayered materials with engineered magnetic properties compels a deep knowledge of physical properties at the atomic scale. The magnetic anisotropy is a key property in these materials. This work accounts for the magnetic anisotropy energy and its correlation with atomic properties of Co/Pd multilayers with the number of Co/Pd repetitions. Magnetometry measurements confirm stronger perpendicular magnetic anisotropy energies as the number of repetitions increases up to 40. However, the intrinsic anisotropy, related to the Co–Pd orbital hybridization and spin–orbit coupling, saturates at 15 repetitions. This finding is supported by x-ray magnetic circular dichroism analysis that reveals a direct correlation of the atomic Co and Pd orbital magnetic moments and the effective anisotropy of the system. The proximity effect that accounts for the Pd induced magnetization, along with the increasing Co moment, provides a suitable mechanism for the observed anisotropy energy layer dependence.

Ferromagnet/Two-Dimensional Semiconducting Transition-Metal Dichalcogenide Interface with Perpendicular Magnetic Anisotropy.

Ferromagnet/two-dimensional transition-metal dichalcogenide (FM/2D TMD) interfaces provide attractive opportunities to push magnetic information storage to the atomically thin limit. Existing work has focused on FMs contacted with mechanically exfoliated or chemically vapor-deposition-grown TMDs, where clean interfaces cannot be guaranteed. Here, we report a reliable way to achieve contamination-free interfaces between ferromagnetic CoFeB and molecular-beam epitaxial MoSe2. We show a spin reorientation arising from the interface, leading to a perpendicular magnetic anisotropy (PMA), and reveal the CoFeB/2D MoSe2 interface allowing for the PMA development in a broader CoFeB thickness-range than common systems such as CoFeB/MgO. Using X-ray magnetic circular dichroism analysis, we attribute generation of this PMA to interfacial d-d hybridization and deduce a general rule to enhance its magnitude. We also demonstrate favorable magnetic softness and considerable magnetic moment preserved at the interface and theoretically predict the interfacial band matching for spin filtering. Our work highlights the CoFeB/2D MoSe2 interface as a promising platform for examination of TMD-based spintronic applications and might stimulate further development with other combinations of FM/2D TMD interfaces.

Observation of an electric field-induced interface redox reaction and magnetic modification in GdOx/Co thin film by means of depth-resolved X-ray absorption spectroscopy.

We study an electric field-induced redox reaction at the interface of GdOx/Co thin film, by means of soft X-ray absorption spectroscopy (XAS). The fluorescence-yield depth-resolved XAS analysis reveals that the interfacial Co layer at GdOx/Co is composed of ∼50% Co oxide when the negative field is applied, while metallic Co is dominant in the case of the positive field. We suppose that the interfacial layer is oxidized by oxygen migration from the GdOx layer with the negative field while the interfacial layer shows the metallic state with the positive field, which means that the redox reaction is induced by the electric field. In addition, it is found from the X-ray magnetic circular dichroism (XMCD) measurement that the orbital magnetic moment of Co is larger when the negative electric field is applied to the film, as compared to the positive field. Moreover, the depth-resolved XMCD analysis reveals that the interfacial Co layer shows no or little magnetization regardless of the electric field, while for the inner layer, an increase of the orbital magnetic moment is suggested with the negative field. The field-induced magnetic modification could be attributed to the change of the orbital moment in the inner Co layer due to interfacial modifications. We thus succeed in the direct observation of the redox reaction-induced change in the interface magnetic state.

Analyzing the magnetic profile in NiFe/NiO bilayers

The magnetic profile of the Si(100)/NiO(35nm)/NiFe(10nm)/Ta(1nm) sample has been obtained by X-ray absorption spectroscopy (XAS) and the X-ray magnetic circular dichroism (XMCD). Two experimental procedures were used. In the procedure 1, the magnetic depth profile has been determines using samples deposited with different NiFe thicknesses, Si(100)/NiO(35nm)/NiFe(t)/Ta(1nm), t=1,3,5,7,10nm. In procedure 2, the sample (NiFe=10nm), was thinned by several in situ sputtering cycles with Ar+ ions, followed by XAS and XMCD analysis. In both procedures, the calculated magnetic moments values tend to decrease close to interface with the NiO antiferromagnetic (AF) layer, however, this decreasement is more evidenced in the sputtered sample. There is no charge transfer between Ni and Fe in the inner part of the NiFe layer, a reduction of the morb/mspineff has been found at the NiFe/NiO interface. Procedure 1 emerged as the most indicated to analyse the interface region.

First-principles analysis of X-ray magnetic circular dichroism for transition metal complex oxides

X-ray magnetic circular dichroism (XMCD) is widely used for the characterization of magnetism of materials. However, information from XMCD related to the atomic, electronic, and magnetic structures is not fully utilized due to the lack of reliable theoretical tools for spectral analysis. In this work, the first-principles configuration interaction (CI) calculations for X-ray absorption spectra developed by the author were extended for the calculation of XMCD, where the Zeeman energy was taken into the Hamiltonian of the CI to mimic magnetic polarization in the solid state. This technique was applied to interpret the L2,3 XMCD from 3d transition metal complex oxides, such as NiFe2O4 and FeTiO3. The experimental XMCD spectra were quantitatively reproduced using this method. The oxidation states as well as the magnetic ordering between transition metal ions on crystallographically different sites in NiFe2O4 can be unambiguously determined. A first-principles analysis of XMCD in FeTiO3 revealed the presence of Fe3+ and Ti3+ ions, which indicates that the charge transfer from Fe to Ti ions occurs. The origin of magnetic polarization of Ti ions in FeTiO3 was also discussed.

Controlled cobalt doping in the spinel structure of magnetosome magnetite: new evidences from element- and site-specific X-ray magnetic circular dichroism analyses.

The biomineralization of magnetite nanocrystals (called magnetosomes) by magnetotactic bacteria (MTB) has attracted intense interest in biology, geology and materials science due to the precise morphology of the particles, the chain-like assembly and their unique magnetic properties. Great efforts have been recently made in producing transition metal-doped magnetosomes with modified magnetic properties for a range of applications. Despite some successful outcomes, the coordination chemistry and magnetism of such metal-doped magnetosomes still remain largely unknown. Here, we present new evidences from X-ray magnetic circular dichroism (XMCD) for element- and site-specific magnetic analyses that cobalt is incorporated in the spinel structure of the magnetosomes within Magnetospirillum magneticum AMB-1 through the replacement of Fe(2+) ions by Co(2+) ions in octahedral (Oh) sites of magnetite. Both XMCD at Fe and Co L2,3 edges, and energy-dispersive X-ray spectroscopy on transmission electron microscopy analyses reveal a heterogeneous distribution of cobalt occurring either in different particles or inside individual particles. Compared with non-doped one, cobalt-doped magnetosome sample has lower Verwey transition temperature and larger magnetic coercivity, related to the amount of doped cobalt. This study also demonstrates that the addition of trace cobalt in the growth medium can significantly improve both the cell growth and the magnetosome formation within M. magneticum AMB-1. Together with the cobalt occupancy within the spinel structure of magnetosomes, this study indicates that MTB may provide a promising biomimetic system for producing chains of metal-doped single-domain magnetite with an appropriate tuning of the magnetic properties for technological and biomedical applications.

On the magnetic properties of iron nanostructures fabricated via focused electron beam induced deposition and autocatalytic growth processes

We employ Electron beam induced deposition (EBID) in combination with autocatalytic growth (AG) processes to fabricate magnetic nanostructures with controllable shapes and thicknesses. Following this route, different Fe deposits were prepared on silicon nitride membranes under ultra-high vacuum conditions and studied by scanning electron microscopy (SEM) and scanning transmission x-ray microspectroscopy (STXM). The originally deposited Fe nanostructures are composed of pure iron, especially when fabricated via autocatalytic growth processes. Quantitative near-edge x-ray absorption fine structure (NEXAFS) spectroscopy was employed to derive information on the thickness dependent composition. X-ray magnetic circular dichroism (XMCD) in STXM was used to derive the magnetic properties of the EBID prepared structures. STXM and XMCD analysis evinces the existence of a thin iron oxide layer at the deposit–vacuum interface, which is formed during exposure to ambient conditions. We were able to extract magnetic hysteresis loops for individual deposits from XMCD micrographs with varying external magnetic field. Within the investigated thickness range (2–16 nm), the magnetic coercivity, as evaluated from the width of the hysteresis loops, increases with deposit thickness and reaches a maximum value of ∼160 Oe at around 10 nm. In summary, we present a viable technique to fabricate ferromagnetic nanostructures in a controllable way and gain detailed insight into their chemical and magnetic properties.

Depth-resolved X-ray magnetic circular dichroism measurement by a multi-anode microchannel plate detector combined with polarization switching

A depth-resolved X-ray magnetic circular dichroism (XMCD) technique has been developed in the soft X-ray region by combining a multi-anode microchannel plate detector with polarization switching between right and left circular polarizations. A simultaneous measurement for 30-channel anodes has been achieved with a continuous data acquisition rate of 1 kHz, which is fast enough for 10 Hz polarization switching. This enables us to realize the layer-resolved XMCD analysis with a high signal-to-noise ratio.

Striped domains of coarse-grained magnetite observed by X-ray photoemission electron microscopy as a source of the high remanence of granites in the Vredefort dome

The characteristics of a coarse-grained high-remanence magnetite obtained from shocked Vredefort granite were investigated by X-ray magnetic circular dichroism (XMCD) analysis and X-ray absorption spectroscopy (XAS). The study utilized a spectroscopic photoelectron low-energy electron emission microscope (SPELEEM) and was conducted in the SPring-8 large-synchrotron radiation facility. It is generally believed that the strong and stable bulk remanence of Vredefort granites is due to the presence of minerals that have been strongly magnetized by either an impact-generated magnetic field or terrestrial lightning strikes. Although coarse-grained magnetite is traditionally characterized by weak coercivity and remanence, the specimen used in the present study exhibited high coercivity and an intense remanent magnetization. The presence of hematite lamellae observed on the partially oxidized magnetite specimen indicated an array of striped domains, intensifying a remanence and coercivity. We also conducted XAS and XMCD analyses on a natural lodestone permanent magnet produced by lightning strikes; while maghemite was found to be present, no magnetic domain structures were observed. Considering that the nucleation of hematite lamellae on magnetite/maghemite grains is due to high-temperature oxidation, we attribute the intense remanent magnetization and magnetic hardening of Vredefort granites to post-impact hydrothermal activity.

Perpendicular magnetic anisotropy at the interface between ultrathin Fe film and MgO studied by angular-dependent x-ray magnetic circular dichroism

Interface perpendicular magnetic anisotropy (PMA) in ultrathin Fe/MgO (001) has been investigated using angular-dependent x-ray magnetic circular dichroism (XMCD). We found that anisotropic orbital magnetic moments deduced from the analysis of XMCD contribute to the large PMA energies, whose values depend on the annealing temperature. The large PMA energies determined from magnetization measurements are related to those estimated from the XMCD and the anisotropic orbital magnetic moments through the spin-orbit interaction. The enhancement of anisotropic orbital magnetic moments can be explained mainly by the hybridization between the Fe 3dz2 and O 2pz states.

Enhancement of perpendicular magnetic anisotropy by compressive strain in alternately layered FeNi thin films

The effect of the lattice strain on magnetic anisotropy of alternately layered FeNi ultrathin films grown on a substrate, Cu(tCu = 0–70 ML)/Ni48Cu52(124 ML)/Cu(0 0 1) single crystal, is systematically studied by means of in situ x-ray magnetic circular dichroism (XMCD) and reflection high-energy electron diffraction (RHEED) analyses. To investigate the magnetic anisotropy of the FeNi layer itself, a non-magnetic substrate is adopted. From the RHEED analysis, the in-plane lattice constant, ain, of the substrate is found to shrink by 0.8% and 0.5% at tCu = 0 and 10 ML as compared to that of bulk Cu, respectively. Fe L-edge XMCD analysis is performed for n ML FeNi films grown on various ain, and perpendicular magnetic anisotropy (PMA) is observed at n = 3 and 5, whereas the film with n = 7 shows in-plane magnetic anisotropy. Moreover, it is found that PMA is enhanced with decreasing ain, in the case where a Cu spacer layer is inserted. We suppose that magnetic anisotropy in the FeNi films is mainly carried by Fe, and the delocalization of the in-plane orbitals near the Fermi level increases the perpendicular orbital magnetic moment, which leads to the enhancement of PMA.