X-ray Magnetic Dichroism Research Articles

Signatures of canted antiferromagnetism in infinite-layer nickelates studied by x-ray magnetic dichroism

Signatures of canted antiferromagnetism in infinite-layer nickelates studied by x-ray magnetic dichroism

Coherent diffractive imaging with twisted X-rays: Principles, applications, and outlook

Recent technological breakthroughs in synchrotron and x-ray free electron laser facilities have revolutionized nanoscale structural and dynamic analyses in condensed matter systems. This review provides a comprehensive overview of the advancements in coherent scattering and diffractive imaging techniques, which are now at the forefront of exploring materials science complexities. These techniques, notably Bragg coherent diffractive imaging and x-ray photon correlation spectroscopy, x-ray magnetic dichroism, and x-ray correlation analysis leverage beam coherence to achieve volumetric three-dimensional imaging at unprecedented sub-nanometer resolutions and explore dynamic phenomena within sub-millisecond timeframes. Such capabilities are critical in understanding and developing advanced materials and technologies. Simultaneously, the emergence of chiral crystals—characterized by their unique absence of standard inversion, mirror, or other roto-inversion symmetries—presents both challenges and opportunities. These materials exhibit distinctive interactions with light, leading to phenomena such as molecular optical activity, chiral photonic waveguides, and valley-specific light emissions, which are pivotal in the burgeoning fields of photonic and spintronic devices. This review elucidates how novel x-ray probes can be leveraged to unravel these properties and their implications for future technological applications. A significant focus of this review is the exploration of new avenues in research, particularly the shift from conventional methods to more innovative approaches in studying these chiral materials. Inspired by structured optical beams, the potential of coherent scattering techniques utilizing twisted x-ray beams is examined. This promising direction not only offers higher spatial resolution but also opens the door to previously unattainable insights in materials science. By contextualizing these advancements within the broader scientific landscape and highlighting their practical applications, this review aims to chart a course for future research in this rapidly evolving field.

Enhanced orbital magnetic moment of Co film grown on Fe3O4(001)

We investigate the magnetic and electronic properties of Co films on Fe3O4(001) achieved through epitaxial growth using magnetron sputtering. X-ray magnetic circularly dichroism measurements characterize the atomic magnetism. Compared to Co films on the MgO substrate, Co on Fe3O4 exhibits a 96% enhancement in orbital magnetic moment (from 0.25 to 0.49 µB/atom) and an increase in spin magnetic moment (from 1.37 to 1.53 µB/atom), resulting in an increased mratio(ml/ms) from 0.18 to 0.32. This enhancement of the orbital moment emerges as a consequence of the interface interaction between Co and Fe3O4. Density functional theory calculations attribute this heightened orbital magnetic moment to the robust electronic exchange interactions. Our findings not only offer insights into the modulation of magnetic and electronic characteristics in Co-based magnetic heterostructures but also provide valuable implications for the potential application of magnetic oxide/ferromagnetic heterostructures in future spintronic devices.

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: Structural, strain, magnetic, and spin transport properties

Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} {\Omega}^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.

Structure and magnetism in ultra-thin hcp Fe films on Re(0001)

We have characterized the structure and the magnetic properties of ultra-thin Fe films (1–6 atomic layers) grown on Re(0001) at different temperatures. For deposition temperatures above 475 K, low-energy electron microscopy shows a layer-by-layer growth up to the third layer, after which 3D growth is observed. The first Fe layer is pseudomorphic with the Re substrate, continuing its hcp stacking sequence. Subsequent atomic layers were found by low-energy electron diffraction (LEED) and IV-LEED to exhibit a quasi-hexagonal, or distorted bcc lattice, with this in-plane distortion progressively reducing with thickness till reaching the bulk bcc lattice at 10 Fe atomic layers. X-ray magnetic dichroism showed no contrast for the first two layers, whereas films of three or more atomic layers display in-plane ferromagnetic order.

X-ray magnetic dichroism and tunnel magneto-resistance study of the magnetic phase in epitaxial CrVO x nanoclusters

Epitaxial clusters of chromium and chromium–vanadium oxides are studied by tunnel magneto-resistivity measurements, x-ray absorption spectrometry and circular magnetic circular dichroism. They turn out to carry a small magnetic moment that follows a super-paramagnetic behavior. The chromium ion contribution to this magnetization is mainly due to an original magnetic Cr2O3-like phase, whereas usual Cr2O3 is known to be anti-ferromagnetic in the bulk. For mixed clusters, vanadium ions also contribute to the total magnetization and they are coupled to the chromium ion spins. By measuring the dichroic signal at different temperatures, we get insight into the possible spin configurations of vanadium and chromium ions: we propose that the magnetic dipoles observed in the clusters assembly could be related to ionic spins that couple at a very short range, as for instance in short one-dimensional spins chains.

Advanced magnetic X-ray spectro-microscopies to characterize mesoscopic magnetic materials

This article provides a brief overview of advanced magnetic X-ray spectro-microscopies that are widely used in characterizing mesoscopic magnetic materials. Common to those techniques are various X-ray magnetic dichroism effects that are used as magnetic contrast mechanism. A particular interest is to use those techniques to image the underlying microscopic spin structures in magnetic materials with high spatial resolution and to ultimately resolve their full 3D mesoscale characteristics with elemental sensitivity, as well as their ultrafast dynamics upon excitations with field and current pulses.Recent research examples using various magnetic X-ray spectro-microscopies are presented to showcase their specific features with a focus on imaging novel topological spin textures, such as vortices, skyrmions and Hopfions, which are considered as potential building blocks towards low power, high-speed and high-density magnetic devices that could transform information technologies and potentially be used in biomedical applications.Future developments with magnetic X-ray spectro-microscopies harness the full coherence of next generation X-ray sources and could open the path towards single shot imaging with spatial and temporal resolutions down to fundamental magnetic length and time scales.

High-temperature superconductivity and its robustness against magnetic polarization in monolayer FeSe on EuTiO3

Spin degree of freedom generally plays an important role in unconventional superconductivity. In many of the iron-based compounds, superconductivity is found in close proximity to long-range antiferromagnetic order, whereas monolayer FeSe grown on SrTiO3, with enhanced superconductivity, exhibits no magnetic or nematic ordering. Here we grow monolayer and multilayer FeSe on antiferromagnetic EuTiO3(001) layers, in an effort to introduce a spin polarization in proximity to the superconductivity of FeSe. By X-ray magnetic dichroism, we observe an antiferromagnet–ferromagnet switching on Eu and Ti sites in EuTiO3 driven by the applied magnetic field, with no concomitant spin polarization on the Fe site of FeSe. Transport measurements show enhanced superconductivity of monolayer FeSe on EuTiO3 with a transition temperature of ~30 K. The band structure revealed by photoemission spectroscopy is analogous to that of FeSe/SrTiO3. Our work creates a platform for the interplay of spin and unconventional superconductivity in the two-dimensional limit.

X-Ray Spectroscopy of Cobaltites

The ability of cobalt to take, depending on the temperature, pressure, and doping, various charge and spin states in oxide compounds periodically becomes the topic of scientific controversy and is a frequent cause of ambiguity in the interpretation of experimental results. In this review, we will discuss cobaltites, i.e., oxide compounds of cobalt, in which cobalt is in an oxidation state (charge state) of 3+. In these compounds, Co3+ ions can take the following spin configurations: high-spin, low-spin, and intermediate-spin configurations. The conditions for the formation of various spin states are given on the basis of the Tanabe–Sugano diagrams. It is shown how X-ray spectral methods, such as X-ray photoelectron spectroscopy, X-ray emission spectroscopy, X-ray absorption spectroscopy, and X-ray magnetic dichroism, can be used to study cobaltites. By using Co L2,3 X-ray absorption spectra that have a rich structure, one can determine the charge and spin states of cobalt ions. These spectra can be reproduced by atom-like multiplet calculations that involve many electrons and use the energy splitting of atomic orbitals by the ligand field as a free parameter. Absorption K spectra of oxygen are of a band nature, i.e., these spectra can be used, by mixing the 3d states of cobalt with the 2p states of oxygen, to estimate the spin state of cobalt ions and to analyze cobaltite phases after external influences. It is shown that the spin states of cobalt ions can be determined from Co Kβ X‑ray emission spectra (electronic transition $$3p \to 1s$$ ) that are sensitive to the spin state of 3d electrons because of the strong exchange interaction between the 3p hole and 3d orbitals in the final state of the emission process. A classic example of cobaltites with a low-spin configuration of cobalt ions is lithium cobaltite LiCoO2 that is a well-known material for power source cathodes. The nature of holes in defective cobaltites of the LixCoO2 (x < 1) type is considered. It is shown that holes arising from defects in the lithium sublattice are of an oxygen nature. In cobaltite LaCoO3 that also contains trivalent cobalt ions, a transition from a low-spin state to a high-spin state occurs at temperatures above 90 K. The feasibility of an intermediate-spin state in this compound is also considered. The results of X-ray spectral studies of double substitution perovskites Ln1 – xAxCo1 – yMyO3 (Ln is a lanthanide, A is Ca or Sr, and M is a transition element) are given, and changes in the charge states of cobalt ions upon doping of perovskites are shown. The results of experimental determination of spin states in octahedrons and pyramids of cobaltites LnBaCo2O6 – δ ( $$0 \leqslant \delta \leqslant 1$$ ) at temperatures below and above the temperature of the metal-insulator transition are systematized. Methods for determining spin states on the basis of X-ray spectral data are discussed. It is shown in the last section of the article how the charge state and concentration of cobalt ions in doped layered cobaltites LnBaCo4O7 can be determined using X-ray absorption spectroscopy. In these compounds, cobalt ions are in tetrahedral positions and, consequently, trivalent cobalt ions should be exclusively in the high-spin state.

Tensor description of X-ray magnetic dichroism at the Fe L2,3-edges of Fe3O4.

A procedure to build the optical conductivity tensor that describes the full magneto-optical response of the system from experimental measurements ispresented. Applied to the Fe L2,3-edge of a 38.85 nm Fe3O4/SrTiO3 (001) thin-film, it is shown that the computed polarization dependence using the conductivity tensor is in excellent agreement with that experimentally measured. Furthermore, the magnetic field angular dependence is discussed using a set of fundamental spectra expanded on spherical harmonics. It is shown that the convergence of this expansion depends on the details of the ground state of the system in question and in particular on the valence-state spin-orbit coupling. While a cubic expansion up to the third order explains the angular-dependent X-ray magnetic linear dichroism of Fe3+ well, higher-order terms are required for Fe2+ when the orbital moment is not quenched.

Revealing 3D magnetization of thin films with soft X-ray tomography: magnetic singularities and topological charges

The knowledge of how magnetization looks inside a ferromagnet is often hindered by the limitations of the available experimental methods which are sensitive only to the surface regions or limited in spatial resolution. Here we report a vector tomographic reconstruction based on soft X-ray transmission microscopy and magnetic dichroism data, which has allowed visualizing the three-dimensional magnetization in a ferromagnetic thin film heterostructure. Different non-trivial topological textures have been resolved and the determination of their topological charge has allowed us to identify a Bloch point and a meron-like texture. Our method relies only on experimental data and might be of wide application and interest in 3D nanomagnetism.

Orbital-dependent electric field effect on magnetism in ultrathin cobalt

Electric field modulations of magnetism in ferromagnetic metals are in general ascribed to the electric field effects on the orbitals. However, a direct observation of the electric-field-induced changes in the orbital states of a ferromagnetic metal is lacking. In this Rapid Communication, we demonstrate that an electric field applied to an ultrathin Co film can change the electron occupations according to the orbitals, combining x-ray absorption and x-ray magnetic circularly dichroism spectroscopies with the sum rules. Under a positive electric field, the ${d}_{xz(yz)}$-occupied states decrease mainly via the modification of orbital hybridizations, while the ${d}_{{z}^{2}}$-occupied state increases. The changes in the ${d}_{xz}$ and ${d}_{yz}$ states dominantly alter the Curie temperature.

Weak Stripe Angle Determination by Quantitative x-ray Magnetic Microscopy

High-resolution scanning transmission x-ray microscopy (STXM) can provide quantitative access to local magnetization canting angles relative to the surface. We use this cutting-edge microscopy technique to probe the weak stripe magnetic texture hosted by a 180-nm-thick ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ layer. We report a full set of measurements of the weak stripe canting angle, as well as a method that uses only a single-impinging-direction x-ray beam to extract the angle. This method also allows the spatial profile to be extracted with 30 nm resolution, and the canting angle versus applied field can be measured. Combining these information, the macroscopic magnetization versus applied field loop on a complex spin texture can be reconstructed in an unprecedented way and shows undoubtedly that flux closure domains exist. These results show that the presence of flux closure domains, at the surface of such a thick film, is difficult to demonstrate by means of the STXM technique. Beyond the characterization of the weak stripe angle, this quantitative x-ray magnetic microscopy can be used to study the local textures hosted in all materials exhibiting some circular x-ray magnetic dichroism.

Sensing alterations of the local environment of 3d, 4d, and 4f central ions in polyoxopalladates with soft X-ray magnetic dichroisms

Polyoxopalladates hosting 3d, 4d or 4f central ions form a new class of molecular materials with well-defined, tunable morphology and ions coordination to be considered as model systems for basic research. We investigated the isotropic X-ray absorption near-edge structure (XANES), X-ray magnetic circular dichroism (XMCD), and X-ray magnetic linear dichroism (XMLD) to monitor the influence of symmetry changes on electronic and magnetic properties. After a brief introductory review about recent results on 3d and 4d transition metal ions, we (i) show that a change of the coordination symmetry from cubic eightfold (tetrahedral) to sixfold (octahedral) reduces the magnetic moment of Co2+ significantly and (ii) investigate whether axial distortions can modify the 4f states of trivalent lanthanide ions. While for Gd3+ there is no effect visible within experimental uncertainties, changes were observed for Dy3+ and Ho3+. The results are compared to atomic multiplet calculations including the intra-atomic spin-dipole term 〈Tz〉. Furthermore, several experimental pitfalls are discussed that may lead to misinterpretation of X-ray absorption data but can be excluded in the present study.

Low-temperature MnBi alloys: Electronic and magnetic properties, constitution, morphology and fabrication (Review article)

The article reviews the rich phenomena of physical properties of MnBi. The diverse phenomena include strong spin-orbit interaction, anomalous temperature dependence of the coercivity and the magneto-crystalline anisotropy field, unique magneto-optical properties. Issues addressed include the nature of the electronic ground states of MnBi, the electronic and magnetic structures, Fermi surface, magneto-crystalline anisotropy, x-ray magnetic dichroism. The discussion includes key experiments, such as optical and magneto-optical spectroscopic measurements, de Haas-van Alphen (dHvA) measurements, x-ray photoemission and x-ray absorption spectroscopy measurements as well as x-ray magnetic circular dichroism. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U were found to be crucial for the Fermi surface, cyclotron masses, magneto-optical properties, and x-ray magnetic circular dichroism. The microscopic origin of unique magneto-crystalline anisotropy and giant Kerr effect in MnBi is analyzed in detail. The huge Kerr effect in MnBi is caused by the combination of a sizable magnetic moment on manganese, the large spin-orbit coupling of bismuth, and a strong hybridization between the manganese 3d and the bismuth 6p states. The magneto-optically active states are mainly the 6p states of Bi. We show that the observed temperature dependence of the magneto-crystalline anisotropy can be explained taking into account the spin-orbit interaction together with strong Coulomb electron-electron interaction. The SO coupling of Bi is equally responsible for the large magneto-crystalline anisotropy energy as is the exchange splitting of Mn. The fabrication, morphology, and constitution of low-temperature MnBi alloys in bulk, thin films, and nanoparticles are discussed. The nanocomposite permanent magnetic materials based on MnBi, (Co, Fe) and Nd2Fe14B are also discussed.

Different spin axis orientation and large antiferromagnetic domains in Fe-doped NiO/Ru(0001) epitaxial films.

We present a spatially resolved X-ray magnetic dichroism study of high-quality, in situ grown nickel oxide films. NiO thin films were deposited on a Ru(0001) substrate by high temperature oxygen-assisted molecular beam epitaxy. We found that by adding a small amount of Fe, the growth mode can be modified in order to promote the formation of micron-sized, triangular islands. The morphology, shape, crystal structure and composition are determined by low-energy electron microscopy and diffraction, and synchrotron based X-ray absorption spectromicroscopy. The element specific XMLD measurements reveal strong antiferromagnetic contrast at room temperature and domains with up to micron sizes, reflecting the high structural quality of the islands. By means of vectorial magnetometry, the spin axis orientation was determined with nanometer spatial resolution, and found to depend on the relative orientation of the film and substrate lattices.

Magneto-optic and transverse-transport properties of noncollinear antiferromagnets

Previous studies on the anomalous Hall effect in coplanar non-collinear antiferromagnets are revisited and extended to magneto-optic properties, namely magneto-optic Kerr effect (MOKE) and X-ray magnetic dichroism (XMCD). Starting from group-theoretical considerations the shape of the frequency-dependent conductivity tensor for various actual and hypothetical spin configurations in cubic and hexagonal Mn$_3X$ compounds is determined. Calculated MOKE and X-ray dichroism spectra are used to confirm these findings and to give estimates of the size of the effects. For Mn$_3$IrPt and Mn$_3$PtRh alloys the concentration dependence of the anomalous and spin Hall conductivity is studied in addition.

Magnetism and transport in transparent high-mobility BaSnO3 films doped with La, Pr, Nd, and Gd

We have explored the effect of magnetic rare-earth dopants substitutionally incorporated on the Ba sites of ${\mathrm{BaSnO}}_{3}$ in terms of electronic transport, magnetism, and optical properties. We show that for ${\mathrm{Ba}}_{0.92}{R}_{0.08}{\mathrm{SnO}}_{3}$ thin films (where $R=\text{La,}\phantom{\rule{4.pt}{0ex}}\text{Pr,}\phantom{\rule{4.pt}{0ex}}\text{Nd,}\phantom{\rule{4.pt}{0ex}}\text{Gd}$), there is a linear increase of mobility with carrier concentration across all doping schemes. La-doped films have the highest mobilities, followed by Pr- and Nd-doped films. Gd-doped samples have the largest ionic size mismatch with the Ba site and correspondingly the lowest carrier concentrations and electron mobilities. However, crystallinity does not appear to be a strong predictor of transport phenomena; our results suggest that point defects more than grain boundaries are key ingredients in tuning the conduction of ${\mathrm{BaSnO}}_{3}$ films grown by pulsed laser deposition. Pronounced, nonhysteretic x-ray magnetic dichroism signals are observed for Pr-, Nd-, and Gd-doped samples, indicating paramagnetism. Finally, we probe the optical constants for each of the ${\mathrm{BaSnO}}_{3}$ doping schemes and note that there is little change in the transmittance across all samples. Together these results shed light on conduction mechanisms in ${\mathrm{BaSnO}}_{3}$ doped with rare-earth cations.

Giant Magnetoelectric Coupling in Multiferroic PbTi1-x V x O3 from Density Functional Calculations.

Giant magnetoelectric coupling is a very rare phenomenon that has gained much attention in the past few decades due to fundamental interest as well as practical applications. Here, we have successfully achieved giant magnetoelectric coupling in PbTi1–xVxO3 (x = 0–1) using a series of generalized gradient-corrected GGA (generalized gradient approximation), including on-site Coulomb repulsion (U)-corrected spin-polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi1–xVxO3 stabilizes in C-type antiferromagnetic ground state for x > 0.123. With the substitution of V into PbTiO3, the tetragonal distortion is highly enhanced accompanied by a linear increase in polarization. In addition, our band structure analysis shows that for lower x values, the tendency to form two-dimensional magnetism of PbTi1–xVxO3 decreases. The orbital magnetic polarization was calculated with self-consistent field method by including orbital polarization correction in the calculation as well as from the computed X-ray magnetic dichroism spectra. A nonmagnetic metallic ground state is observed for the paraelectric phase for V concentration (x) = 1 competing with a volume change of 10% showing a large magnetovolume effect. Our orbital-projected density of states as well as orbital ordering analysis suggest that the orbital ordering plays a major role in the magnetic-to-nonmagnetic transition when going from ferroelectric to paraelectric phase. The calculated magnetic anisotropic energy shows that the direction [110] is the easy axis of magnetization for x = 1 composition. The partial polarization analysis shows that the Ti/V–O hybridization majorly contributes to the total electrical polarization. The present study adds a new series of compounds to the magnetoelectric family with rarely existing giant coupling between electric- and magnetic-order parameters. These results show that such kind of materials can be used for novel practical applications where one can change the magnetic properties drastically (magnetic to nonmagnetic, as shown here) with external electric field and vice versa.

Magnetic properties of the ultrathin film of Zn0.50Mn0.50S/Rh (001) for spintronics applications

We have used the full potential linearized augmented plane waves (FP-LAPW) method to investigate the magnetic properties of Zn0.50Mn0.50S/Rh (001) system. Spin-orbit coupling and different spin-polarized states for the electronic structure are considering in LSDA potential. The L23-edges X-ray absorption (XAS) and X-ray magnetic dichroism (XMD) are calculated locally at Mn–Mn sites. The results show no circular dichroism spectra (leading to zero in sum for a spin up and spin down conspicuously of the independent Mn atoms), which is a behavior specific to an antiferromagnetic material. The system shows a perpendicular magnetic anisotropy of (664 ± 25) µeV. Hence, the magnetic moments are arranged antiparallel and reach the value of ±3.72 µB especially between nearest neighbor Mn atoms. This can be explaining by a large number of d-electrons available to be spin-polarized in the low-dimensional regime. We propose that the magnetic properties are resulting from the hybridization between Mn–d and sulfur–p states at a given distance.