X-ray Magnetic Circular Dichroism Study Research Articles

Ionic control of magnetism in all-solid-state CoOx/yttria-stabilized zirconia heterostructures

Magneto-ionic gating, a procedure that enables the modulation of materials' magnetic properties by voltage-driven ion motion, offers alternative perspectives for emerging low-power magnetic storage and spintronic applications. Most previous studies in all-solid-state magneto-ionic systems have focused on the control of interfacial magnetism of ultrathin (i.e., 1–3 nm) magnetic films, taking advantage of an adjacent ionic conducting oxide, usually GdOx or HfOx, that transports functional ionic species (e.g., H+ or O2−). Here, we report on room-temperature OFF–ON ferromagnetism by solid-state magneto-ionics in relatively thick (25 nm) patterned CoOx films grown on an yttria-stabilized zirconia (YSZ) layer, which acts as a dielectric to hold electric field and as an O2− ion reservoir. Upon negatively biasing, O2− ions from the CoOx tend to migrate toward the YSZ gate electrode, leading to the gradual generation of magnetization (i.e., OFF-to-ON switching of a ferromagnetic state). X-ray absorption and magnetic circular dichroism studies reveal subtle changes in the electronic/chemical characteristics, responsible for the induced magnetoelectric effects in such all-oxide heterostructures. Recovery of the initial (virtually non-magnetic) state is achieved by application of a positive voltage. The study may guide future development of all-solid-state low-power CMOS-compatible magneto-ionic devices.

X-Ray Magnetic Circular Dichroism in Altermagnetic α-MnTe.

Altermagnetism is a recently identified magnetic symmetry class combining characteristics of conventional collinear ferromagnets and antiferromagnets, that were regarded as mutually exclusive, and enabling phenomena and functionalities unparalleled in either of the two traditional elementary magnetic classes. In this work we use symmetry, abinitio theory, and experiments to explore x-ray magnetic circular dichroism (XMCD) in the altermagnetic class. As a representative material for our XMCD study we choose α-MnTe with compensated antiparallel magnetic order in which an anomalous Hall effect has been already demonstrated. We predict and experimentally confirm a characteristic XMCD line shape for compensated moments lying in a plane perpendicular to the light propagation vector. Our results highlight the distinct phenomenology in altermagnets of this time-reversal symmetry breaking response, and its potential utility for element-specific spectroscopy and microscopy.

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.

Doping dependent intrinsic magnetization in silicon in Ni/Si heterostructures

This investigation delves into the complex interaction at metal-semiconductor interfaces, highlighting the magnetic proximity effect in Ni/Si interfaces through systematic X-ray magnetic circular dichroism (XMCD) studies at Ni and Si edges. We analyzed two Ni/Si heterostructures with differing semiconductor doping, uncovering a magnetic proximity effect manifesting as equilibrium magnetization in the semiconductor substrate induced by the adjacent Ni layer. Our results display distinct magnetization signs corresponding to the doping levels: low-doped samples show parallel alignment to the Ni layer, while high-doped samples align antiparallel, indicating a nuanced interplay of underlying magnetization mechanisms. These findings pinpoint the roles of electron tunneling and exchange splitting modification in the magnetization behavior. The study enriches the understanding of ferromagnetic-semiconductor interface behavior, setting a precedent for the design of advanced spintronic devices that leverage the nuanced magnetic properties of these hybrid systems.

Room-temperature-persistent magnetic interaction between coordination complexes and nanoparticles in maghemite-based nanohybrids.

Maghemite nanoparticles functionalised with Co(II) coordination complexes at their surface show a significant increase of their magnetic anisotropy, leading to a doubling of the blocking temperature and a sixfold increase of the coercive field. Magnetometric studies suggest an enhancement that is not related to surface disordering, and point to a molecular effect involving magnetic exchange interactions mediated by the oxygen atoms at the interface as its source. Field- and temperature-dependent X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies show that the magnetic anisotropy enhancement is not limited to surface atoms and involves the core of the nanoparticle. These studies also point to a mechanism driven by anisotropic exchange and confirm the strength of the magnetic exchange interactions. The coupling between the complex and the nanoparticle persists at room temperature. Simulations based on the XMCD data give an effective exchange field value through the oxido coordination bridge between the Co(II) complex and the nanoparticle that is comparable to the exchange field between iron ions in bulk maghemite. Further evidence of the effectiveness of the oxido coordination bridge in mediating the magnetic interaction at the interface is given with the Ni(II) analog to the Co(II) surface-functionalised nanoparticles. A substrate-induced magnetic response is observed for the Ni(II) complexes, up to room temperature.

Effect of Cr-substitution on vanadium dioxide thin films studied by soft X-ray magnetic circular dichroism

The tunability of magnetic properties of VO 2 has fascinated substantial attention in the first generation of spintronics-related applications such as read head sensors for hard disk drives and magnetic random-access memory. In this work, we focus on Cr doped VO 2 , V 1− x Cr x O 2 (0 ≤ x ≤ 0 . 3) (VCO) thin films and study the effect of Cr-substitution on the structural, electronic and magnetic properties. The VCO films were deposited on R-sapphire substrates using a pulse laser deposition method. Soft X-ray absorption spectroscopy (XAS) and soft X-ray magnetic circular dichroism (XMCD) studies of VCO thin films were performed to reveal and understand the origin of maTablegnetization and their enhancement with Cr-substitution. The XAS spectra suggest that the valences of V ions are in tetra and pentavalent states and Cr ions in the trivalent state. The XMCD spectra imply that the V and Cr ions are ferromagnetic at room temperature. These results also indicate that the VCO films are in the ferromagnetic insulating state and charge-ordered at the V and Cr sites. These results are consistent with the first principle density functional theory calculations. • XRD results suggest that a gradual structural change from the monoclinic M1 phase (0 ≤ x < 0.05) to triclinic T phase (0.05 ≤ x ≤ 0.20 to monoclinic M2 phase ( x > 0.20). • First Study to understand the origin of magnetization. • The VCO films are in the ferromagnetic insulating state and charge-ordered at the V and Cr sites, consistent with DFT. • The XMCD spectra indicate that the V and Cr ions are ferromagnetic at room temperature.

On-Surface Design of a 2D Cobalt-Organic Network Preserving Large Orbital Magnetic Moment.

The design of antiferromagnetic nanomaterials preserving large orbital magnetic moments is important to protect their functionalities against magnetic perturbations. Here, we exploit an archetype H6HOTP species for conductive metal-organic frameworks to design a Co-HOTP one-atom-thick metal-organic architecture on a Au(111) surface. Our multidisciplinary scanning probe microscopy, X-ray absorption spectroscopy, X-ray linear dichroism, and X-ray magnetic circular dichroism study, combined with density functional theory simulations, reveals the formation of a unique network design based on threefold Co+2 coordination with deprotonated ligands, which displays a large orbital magnetic moment with an orbital to effective spin moment ratio of 0.8, an in-plane easy axis of magnetization, and large magnetic anisotropy. Our simulations suggest an antiferromagnetic ground state, which is compatible with the experimental findings. Such a Co-HOTP metal-organic network exemplifies how on-surface chemistry can enable the design of field-robust antiferromagnetic materials.

Correlation between spin–phonon coupling and magneto-electric effects in CoFe2O4/PMN-PT nanocomposite: Raman spectroscopy and XMCD study

We have investigated strain-induced spin lattice coupling in the CoFe2O4/0.65Pb (Mg1/3Nb2/3)O3–0.35PbTiO3(PMN-PT) composite system, evident by temperature-dependent Raman spectroscopy and magnetization measurements. The strain interactions lead to magneto-electric coupling, and the measured magneto-electric voltage coefficient is 40 mV/cm−1Oe−1 for the CoFe2O4/PMN-PT composite samples. X-ray magnetic circular dichroism (XMCD) analysis establishes modified spin structure in the electrically poled CoFe2O4/PMN-PT composite, further validating the coupling between magnetic and electric ordering in the composite. The magneto-electric coupling coefficient α vs dc magnetic field curves revealed hysteretic behavior and enhanced α values after electric poling, which originates from the strain-induced modifications in the magnetic structure of composite in the electrically poled samples. These findings suggest that the existence of spin lattice coupling may lead to the mechanism of strong magneto-electric effects via strain interactions in CoFe2O4/PMN-PT composite.

Structural, Optical, and Magnetic Properties of Erbium-Substituted Yttrium Iron Garnets.

We synthesized a series of slightly erbium-substituted yttrium iron garnets (Er:YIG), Y3–xErxFe5O12 at different Er concentrations (x = 0, 0.01, 0.05, 0.10, and 0.20) using a solid-state reaction and investigated their structural, magnetic, and optical properties as a function of Er concentration. The volume of the unit cell slightly increased with Er concentration and Er atoms predominately replaced Y atoms in the dodecahedrons of YIG. The optical properties exhibited certain decreases in reflectance in the 1500–1600 nm wavelength range due to the presence of Er3+. Despite the many unpaired 4f electrons in Er3+, the total magnetic moments of Er:YIG showed similar trends with temperatures and magnetic fields above 30 K. An X-ray magnetic circular dichroism study confirmed the robust Fe 3d magnetic moments. However, the magnetic moments suddenly decreased to below 30 K with Er substitution, and the residual magnetism (MR) and coercive field (HC) in the magnetic hysteresis loops decreased to below 30 K with Er substitution. This implies that Er substitution in YIG has a negligible effect on magnetic properties over a wide temperature range except below 30 K where the Er 4f spins are coupled antiparallel to the majority Fe 3d spins. Our studies demonstrated that above 30 K the magnetic properties of YIG are retained even with Er substitution, which is evidence that the Er doping scheme is applicable for YIG-based magneto-optical devices in the mid-infrared regime.

Development of magnetism in Fe-doped magnetic semiconductors: Resonant photoemission and x-ray magnetic circular dichroism studies of (Ga,Fe)As

Fe-doped III-V ferromagnetic semiconductors (FMSs) such as (In,Fe)As, (Ga,Fe)Sb, (In,Fe)Sb, and (Al,Fe)Sb are promising materials for spintronic device applications because of the availability of both $\mathrm{n}$- and $\mathrm{p}$-type materials and the high Curie temperatures. On the other hand, (Ga,Fe)As, which has the same zinc-blende crystal structure as the Fe-doped III-V FMSs, shows paramagnetism. The origin of the different magnetic properties in the Fe-doped III-V semiconductors remains to be elucidated. To address this issue, we use resonant photoemission spectroscopy (RPES) and x-ray magnetic circular dichroism (XMCD) to investigate the electronic and magnetic properties of the Fe ions in a paramagnetic $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ thin film. The observed Fe $2p\text{\ensuremath{-}}3d$ RPES spectra show that the Fe $3d$ states are similar to those of ferromagnetic (Ga,Fe)Sb. The estimated Fermi level is located in the middle of the band gap in (Ga,Fe)As. The Fe ${L}_{2,3}$ XMCD spectra of $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ show preedge structures, which are not observed in the Fe-doped FMSs, indicating that the minority-spin (\ensuremath{\downarrow}) ${e}_{\ensuremath{\downarrow}}$ states are vacant in $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$. The XMCD results suggest that the carrier-induced ferromagnetic interaction in $({\mathrm{Ga}}_{0.95},{\mathrm{Fe}}_{0.05})\mathrm{As}$ is short ranged and weaker than that in the Fe-doped FMSs. The experimental findings suggest that the electron occupancy of the ${e}_{\ensuremath{\downarrow}}$ states contributes to the appearance of ferromagnetism in the Fe-doped III-V semiconductors, for $\mathrm{p}$-type as well as $\mathrm{n}$-type compounds.

Magnetism and spin transport at permalloy/Cu1−xTbx interfaces

The significance of spin transport over an interface in energy-efficient spintronic devices has stimulated interest in the spintronic society during the last few decades. Here, interfaces of $\text{permalloy}/{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ (Py/Cu-Tb) were investigated in depth. As the Cu-Tb thickness increases, we found that the saturation magnetization of the bilayers falls and then plateaus. Element-specific x-ray magnetic circular dichroism studies suggest that the Tb moment aligns opposite to the Fe and Ni moments, forming a self-assembled antiferromagnetic interface. As a result, the Cu-Tb adjacent layer to Py and the interface have a significant impact on spin transport. Relevant parameters, such as spin mixing conductance, spin diffusion length, and damping, can be tuned by inserting a thin Cu layer between Py and Tb or varying the compositions of Cu-Tb alloys. Using rare-earth Tb, we provide an effective method for controlling the spin transport and magnetism of ferromagnet/normal-metal interfaces. This approach is expected to have a great deal of potential in spintronic applications.

Electron Correlation Enhances Orbital Polarization at a Ferromagnetic Metal/Insulator Interface: Depth-Resolved X-ray Magnetic Circular Dichroism and First-Principles Study

The Fe(CoB)/MgO interface is vital to spintronics as it exhibits the tunneling magnetoresistance (TMR) effect and interfacial perpendicular magnetic anisotropy (PMA) simultaneously. To further enhance TMR and PMA for the development of high-density magnetoresistive random access memory, it is essential to clarify the behavior of Fe atoms interfaced with MgO. This study reveals that the spin and orbital magnetic moments of Fe are enhanced at the Fe/MgO interface. The enhancement in the orbital magnetic moment is much more significant than that predicted by the standard density functional theory. Theoretical calculations based on the orbital polarization correction reproduce this enhancement, the origin of which is attributed to the electron–electron correlation resulting from electron localization at the Fe/MgO interface. The present findings highlight the importance of electron–electron correlation at ferromagnet/oxide interfaces, which has often been disregarded in spintronics.

Magnetic anisotropy of the van der Waals ferromagnet Cr2Ge2Te6 studied by angular-dependent x-ray magnetic circular dichroism

The van der Waals ferromagnet ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT) has a two-dimensional crystal structure where each layer is stacked through van der Waals force. We have investigated the nature of the ferromagnetism and the weak perpendicular magnetic anisotropy (PMA) of CGT by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) studies of CGT single crystals. The XMCD spectra at the Cr ${L}_{2,3}$ edge for different magnetic field directions were analyzed on the basis of the cluster-model multiplet calculation. The Cr valence is confirmed to be $3+$ and the orbital magnetic moment is found to be nearly quenched, as expected for the high-spin ${t}_{2g}^{3}$ configuration of the ${\mathrm{Cr}}^{3+}$ ion. A large ($\ensuremath{\sim}0.2$ eV) trigonal crystal-field splitting of the ${t}_{2g}$ level caused by the distortion of the $\mathrm{Cr}{\mathrm{Te}}_{6}$ octahedron has been revealed, while the single-ion anisotropy (SIA) of the Cr atom is found to have a sign opposite to the observed PMA and too weak compared to the reported anisotropy energy. The present result suggests that anisotropic exchange coupling between the Cr atoms through the ligand Te $5p$ orbitals having strong spin-orbit coupling has to be invoked to explain the weak PMA of CGT, as in the case of the strong PMA of ${\mathrm{CrI}}_{3}$.

XMCD and ab initio study of interface-engineered ultrathin Ru/Co/W/Ru films with perpendicular magnetic anisotropy and strong Dzyaloshinskii-Moriya interaction.

Understanding the nature of recently discovered spin-orbital induced phenomena and a definition of a general approach for "ferromagnet/heavy-metal" layered systems to enhance and manipulate spin-orbit coupling, spin-orbit torque, and the Dzyaloshinskii-Moriya interaction (DMI) assisted by atomic-scale interface engineering are essential for developing spintronics and spin-orbitronics. Here, we exploit X-ray magnetic circular dichroism (XMCD) spectroscopy at the L2,3-edges of 5d and 4d non-magnetic heavy metals (W and Ru, respectively) in ultrathin Ru/Co/W/Ru films to determine their induced magnetic moments due to the proximity to the ferromagnetic layer of Co. The deduced orbital and spin magnetic moments agree well with the theoretically predicted values, highlighting the drastic effect of constituting layers on the system's magnetic properties and the strong interfacial DMI in Ru/Co/W/Ru films. As a result, we demonstrate the ability to simultaneously control the strength of magnetic anisotropy and intermixing-enhanced DMI through the interface engineered inversion asymmetry in thin-film chiral ferromagnets, which are a potential host for stable magnetic skyrmions.

Magnetic Hysteresis in a Monolayer of Oriented 6 nm CsNiCr Prussian Blue Analogue Nanocrystals.

Prussian blue analogue nanocrystals of the CsINiII[CrIII(CN)6] cubic network with 6 nm size were assembled as a single monolayer on highly organized pyrolytic graphite (HOPG). X-ray magnetic circular dichroism (XMCD) studies, at the Ni and Cr L2,3 edges, reveal the presence of an easy plane of magnetization evidenced by an opening of the magnetic hysteresis loop (coercive field of ≈200 Oe) when the magnetic field, B, is at 60° relative to the normal to the substrate. The angular dependence of the X-ray natural linear dichroism (XNLD) reveals both an orientation of the nanocrystals on the substrate and an anisotropy of the electronic cloud of the NiII and CrIII coordination sphere species belonging to the nanocrystals' surface. Ligand field multiplet (LFM) calculations that reproduce the experimental data are consistent with an elongated tetragonal distortion of surface NiII coordination sphere responsible for the magnetic behavior of monolayer.

Direct evidence to control the magnetization in Fe3O4 thin films by N2 ion implantation: a soft X-ray magnetic circular dichroism study

Direct evidence to control the magnetization in Fe3O4 thin films by N2 ion implantation: a soft X-ray magnetic circular dichroism study

Element-specific field-induced spin reorientation and tetracritical point in MnCr2S4

The ferrimagnetic spinel MnCr2S4 shows a variety of magnetic-field-induced phase transitions owing to bond frustration and strong spin-lattice coupling. However, the site-resolved magnetic properties at the respective field-induced phases in high magnetic fields remain elusive. Our soft x-ray magnetic circular dichroism studies up to 40 T directly evidence element-selective magnetic-moment reorientations in the field-induced phases. The complex magnetic structures are further supported by entropy changes extracted from magnetocaloric-effect measurements. Moreover, thermodynamic experiments reveal an unusual tetracritical point in the H-T phase diagram of MnCr2S4 due to strong spin-lattice coupling.

L10 -ordered (Fe100−xCrx)Pt thin films: Phase formation, morphology, and spin structure

Chemically ordered $L{1}_{0}$ $({\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Cr}}_{x})\mathrm{Pt}$ thin films were expitaxially grown on MgO(001) substrates by magnetron sputter-deposition at $770{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. In this sample series, Fe was continuously substituted by Cr over the full composition range. The lattice parameter in the [001] growth direction steadily increases from $L{1}_{0}$-FePt toward $L{1}_{0}$-CrPt, confirming the incorporation of Cr in the lattice occupying Fe sites. With the observed high degree of chemical ordering and (001) orientation, strong perpendicular magnetic anisotropy is associated, which persists up to a Cr content of $x=20$ at. %. Similarly, the coercive field in the easy-axis direction is strongly reduced, which is, however, further attributed to a strong alteration of the film morphology with Cr substitution. The latter changes from a well-separated island microstructure to a more continuous film morphology. In the dilute alloy with low Cr content, isolated Cr magnetic moments couple antiferromagnetically to the ferromagnetic Fe matrix. In this case, all Cr moments are aligned parallel, thus forming a ferrimagnetic FeCrPt system. With increasing Cr concentration, nearest-neighbor Cr-Cr pairs start to appear, thereby increasing magnetic frustration and disorder, which lead to canting of neighboring magnetic moments, as revealed by atomistic spin-model simulations with model parameters based on first principles. At higher Cr concentrations, a frustrated ferrimagnetic order is established. With Cr substitution of up to 20 at. %, no pronounced change in Curie temperature, which is in the range of 700 K, was noticed. But with further addition the Curie temperature drops down substantially even down to room temperature at 47 at. % Cr. Furthermore, x-ray magnetic circular dichroism studies on dilute alloys containing up to 20 at. % of Cr revealed similar spin moments for Fe and Cr in the range between 2.1--2.5 ${\ensuremath{\mu}}_{B}$ but rather large orbital moments of up to 0.50 $\ifmmode\pm\else\textpm\fi{}0.10\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{B}$ for Cr. These results were also compared to ab initio calculations.

Insight into Kondo screening in the intermediate-valence compound SmOs4Sb12 uncovered by soft x-ray magnetic circular dichroism

Through a high-precision soft x-ray magnetic circular dichroism (XMCD) study of the intermediate-valence compound $\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$, we show our successful approach of revealing unprecedented details of Kondo screening below a characteristic temperature of ${T}_{0}\ensuremath{\sim}20$ K in the paramagnetic phase. The multiplet XMCD structure at the Sm ${M}_{5}$ edge sensitive to the $4f$ configuration enabled us to observe a clear difference between the temperature evolution above and below ${T}_{0}$ with one- and two-component behavior, respectively. Our findings are in strong contrast to the conventional Kondo crossover that coincides with the valence transition, but are qualitatively accounted for by theoretical XMCD predictions combined with the two-fluid phenomenology recently proposed. This work contributes to the large context of Kondo physics that is closely related to quantum criticality in heavy fermion systems.

Magnetic anisotropy of one-dimensional Co nanostructures

We report a combined scanning tunneling microscopy and x-ray magnetic circular dichroism study to investigate the structural properties and magnetic behavior of a Co ultrathin film composed of dimer nanolines. These Co nanolines, \ensuremath{\sim}6 nm in length, are grown on a Si nanotemplate composed of nanoribbons self-organized on Ag(110). The first two Co layers present a weak magnetic response while upper Co layers exhibit an enhanced magnetization. Orbital and spin moments are experimentally determined. We show that an in-plane magnetization is favored and the magnetic anisotropy energy associated with the directions parallel and perpendicular to the nanolines are measured. The Co ultrathin film is shown to behave as a superparamagnetic system composed of one-dimensional segments containing each \ensuremath{\sim}170 ferromagnetically coupled Co atoms, with a blocking temperature estimated to be between 20 and 40 K. Our set of experiments allows for a comprehensive description of the magnetic behavior of the Co nanoline ultrathin film grown on a functionalized metallic substrate.