X-ray Magnetic Circular Dichroism Spectra Research Articles

Near-surface electronic structure in strained Ni-ferrite films: An x-ray absorption spectroscopy study

We report on the x-ray absorption spectra (XAS) and x-ray magnetic circular dichroism (XMCD) of a series of NiFe2O4 (Ni ferrite) films grown on symmetry matched substrates and measured in two geometries: out-of-plane and near in-plane. The Ni ferrite films, grown by pulsed laser deposition, are epitaxial and the substrates used (ZnGa2O4, CoGa2O4, MgGa2O4, and MgAl2O4) introduce a systematic variation in the lattice mismatch between the substrate and the film. Modeling of the XAS and XMCD spectra, both measured with the surface sensitive total electron yield mode, indicates that the Ni2+ cations reside on the octahedrally coordinated lattice sites in the spinel structure. Analyses of the Fe XAS and XMCD spectra are consistent with Fe3+ cations occupying a subset of the octahedral and tetrahedral sites in the spinel oxide lattice with the addition of a small amount of Fe2+ located on octahedral sites. The Ni2+ orbital to spin moment ratio (μℓ/μs), derived from the application of XMCD sum rules, is enhanced for the substrates with a small lattice mismatch relative to NiFe2O4. The results suggest a path for increasing the orbital moment in NiFe2O4 by applying thin film growth techniques that can maintain a highly strained lattice for the NiFe2O4 film.

Assembling Fe4 single-molecule magnets on a TiO2 monolayer.

The decoration of technologically relevant surfaces, such as metal oxides, with Single-Molecule Magnets (SMMs) constitutes a persistent challenge for the integration of these molecular systems into novel technologies and, in particular, for the development of spintronic and quantum devices. We used UHV thermal sublimation to deposit tetrairon(III) propeller-shaped SMMs (Fe4) as a single layer on a TiO2 ultrathin film grown on Cu(001). The properties of the molecular deposit were studied using a multi-technique approach based on standard topographic and spectroscopic measurements, which demonstrated that molecules remain largely intact upon deposition. Ultralow temperature X-ray Absorption Spectroscopy (XAS) with linearly and circularly polarized light was further employed to evaluate both the molecular organization and the magnetic properties of the Fe4 monolayer. X-ray Natural Linear Dichroism (XNLD) and X-ray Magnetic Circular Dichroism (XMCD) showed that molecules in a monolayer display a preferential orientation and an open magnetic hysteresis with pronounced quantum tunnelling steps up to 900 mK. However, unexpected extra features in the XAS and XMCD spectra disclosed a minority fraction of altered molecules, suggesting that the TiO2 film may be chemically non-innocent. The observed persistence of SMM behaviour on a metal oxide thin film opens new possibilities for the development of SMM-based hybrid systems.

Measurement of the Induced Magnetic Polarisation of Rotated-Domain Graphene Grown on Co Film with Polarised Neutron Reflectivity.

In this paper, we determine the magnetic moment induced in graphene when grown on a cobalt film using polarised neutron reflectivity (PNR). A magnetic signal in the graphene was detected by X-ray magnetic circular dichroism (XMCD) spectra at the C K-edge. From the XMCD sum rules an estimated magnetic moment of 0.3 μB/C atom, while a more accurate estimation of 0.49 μB/C atom was obtained by carrying out a PNR measurement at 300 K. The results indicate that the higher magnetic moment in Co is counterbalanced by the larger lattice mismatch between the Co-C (1.6%) and the slightly longer bond length, inducing a magnetic moment in graphene that is similar to that reported in Ni/graphene heterostructures.

Sublattice spin reversal and field induced Fe3+ spin-canting across the magnetic compensation temperature in Y1.5Gd1.5Fe5O12 rare-earth iron garnet

In the present work Fe3+ sublattice spin reversal and Fe3+ spin-canting across the magnetic compensation temperature (T) are demonstrated in polycrystalline Y1.5Gd1.5Fe5O12 (YGdIG) by means of in-field Mössbauer spectroscopy measurements. Corroborating in-field Mössbauer measurements, both Fe3+ & Gd3+ sublattice spin reversal has also been manifested in hard x-ray magnetic circular dichroism (XMCD) measurements. From in-field 57 Fe Mössbauer measurements, estimation and analysis of effective internal hyperfine field (H), relative intensity of absorption lines in a sextet elucidated unambiguously the signatures of Fe3+ spin reversal and field induced spin-canting of Fe3+ sublattices across T. Gd L3-edge XMCD signal is observed to consist of an additional spectral feature, identified as Fe3+ magnetic contribution to XMCD spectra of Gd L3-edge, enabling us the extraction of both the sublattices (Fe3+ & Gd3+) information from a single absorption edge analysis. The evolution of the XMCD amplitudes, which is proportional to magnetic moments, as a function of temperature for both magnetic sublattices extracted at the Gd L3-edge reasonably match with values that are extracted from bulk magnetization data of YGdIG and YIG (Y3Fe5O12) and corresponding Fe K-edge XMCD amplitudes for Fe contribution. These measurements pave new avenues to investigate how the magnetic behavior of such complex system acts across the compensation point.

Anisotropic hybridization probed by polarization dependent x-ray absorption spectroscopy in VI3 van der Waals Mott ferromagnet

Polarization dependent x-ray absorption spectroscopy was used to study the magnetic ground state and the orbital occupation in bulk-phase VI3 van der Waals crystals below and above the ferromagnetic and structural transitions. X-ray natural linear dichroism and x-ray magnetic circular dichroism spectra acquired at the V edges are compared against multiplet cluster calculations within the frame of the ligand field theory to quantify the intra-atomic electronic interactions at play and evaluate the effects of symmetry reduction occurring in a trigonally distorted VI6 unit. We observed a non zero linear dichroism proving the presence of an anisotropic charge density distribution around the V3+ ion due to the unbalanced hybridization between the vanadium and the ligand states. Such hybridization acts as an effective trigonal crystal field, slightly lifting the degeneracy of the ground state. However, the energy splitting associated to the distortion underestimates the experimental band gap, suggesting that the insulating ground state is stabilized by Mott correlation effects rather than via a Jahn–Teller mechanism. Our results clarify the role of the distortion in VI3 and establish a benchmark for the study of the spectroscopic properties of other van der Waals halides, including emerging 2D materials with mono and few-layers thickness, whose fundamental properties might be altered by reduced dimensions and interface proximity.

Ferromagnetism induced by hybridization of Fe 3d orbitals with ligand InSb bands in the n -type ferromagnetic semiconductor (In,Fe)Sb

Fe-doped III-V ferromagnetic semiconductor (FMS) (In,Fe)Sb is a promising material for spintronic device applications because of the n-type carrier conduction and the ferromagnetism with high Curie temperature (TC > 300 K). To clarify the mechanism of the high-TC ferromagnetism, we have investigated the electronic structure and magnetic properties of an (In,Fe)Sb thin film by performing x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Fe L2,3 edges. The magnetic-field dependence of the XMCD spectra reveals that there are ferromagnetic-like Fe and paramagnetic-like Fe components in the (In,Fe)Sb thin film. The XAS and XMCD spectra of the ferromagnetic-like and paramagnetic-like Fe components resemble those of other Fe-doped FMSs and extrinsic oxides, respectively. The finite value of the ratio between the orbital and spin magnetic moments estimated by applying the XMCD sum rules indicates that the valence state of the Fe ions substituting for the In sites in (In,Fe)Sb is not purely ionic Fe3+, but intermediate between Fe3+ and Fe2+. The qualitative correspondence between the magnetic-field dependence of the visible-light magnetic circular dichroism intensity and that of the XMCD intensity demonstrates that the Zeeman splitting of the InSb band is proportional to the net magnetization of the doped Fe. These results suggest that the ferromagnetism of (In,Fe)Sb originates from the Fe 3d orbitals hybridized with the host InSb bands.

Determining the Proximity Effect-Induced Magnetic Moment in Graphene by Polarized Neutron Reflectivity and X-ray Magnetic Circular Dichroism.

We report the magnitude of the induced magnetic moment in CVD-grown epitaxial and rotated-domain graphene in proximity with a ferromagnetic Ni film, using polarized neutron reflectivity (PNR) and X-ray magnetic circular dichroism (XMCD). The XMCD spectra at the C K-edge confirm the presence of a magnetic signal in the graphene layer, and the sum rules give a magnetic moment of up to ∼0.47 μB/C atom induced in the graphene layer. For a more precise estimation, we conducted PNR measurements. The PNR results indicate an induced magnetic moment of ∼0.41 μB/C atom at 10 K for epitaxial and rotated-domain graphene. Additional PNR measurements on graphene grown on a nonmagnetic Ni9Mo1 substrate, where no magnetic moment in graphene is measured, suggest that the origin of the induced magnetic moment is due to the opening of the graphene's Dirac cone as a result of the strong C pz-Ni 3d hybridization.

Local structure and magnetic properties of a nanocrystalline Mn-rich Cantor alloy thin film down to the atomic scale

The huge atomic heterogeneity of high-entropy materials along with a possibility to unravel the behavior of individual components at the atomic scale suggests a great promise in designing new compositionally complex systems with the desired multi-functionality. Herein, we apply multi-edge X-ray absorption spectroscopy (extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES), and X-ray magnetic circular dichroism (XMCD)) to probe the structural, electronic, and magnetic properties of all individual constituents in the single-phase face-centered cubic (fcc)-structured nanocrystalline thin film of Cr20Mn26Fe18Co19Ni17 (at.%) high-entropy alloy on the local scale. The local crystallographic ordering and component-dependent lattice displacements were explored within the reverse Monte Carlo approach applied to EXAFS spectra collected at the K absorption edges of several constituents at room temperature. A homogeneous short-range fcc atomic environment around the absorbers of each type with very similar statistically averaged interatomic distances (2.54–2.55 Å) to their nearest-neighbors and enlarged structural relaxations of Cr atoms were revealed. XANES and XMCD spectra collected at the L2,3 absorption edges of all principal components at low temperature from the oxidized and in situ cleaned surfaces were used to probe the oxidation states, the changes in the electronic structure, and magnetic behavior of all constituents at the surface and in the sub-surface volume of the film. The spin and orbital magnetic moments of Fe, Co, and Ni components were quantitatively evaluated. The presence of magnetic phase transitions and the co-existence of different magnetic phases were uncovered by conventional magnetometry in a broad temperature range.

First-principles study to probe the effect of substitution at X and Z sites on the electronic, magnetic and transport properties of Co2X(V, Nb, Ta)Z(Al, Ga, In, Si, Ge, Sn) Heusler alloys

In the present work, we have used density functional theory based electronic structure calculations to probe the role of isovalent substitution at X@(0.5, 0.5, 0.5) and Z@(0, 0, 0) site of Co2X(= Ta, Nb, V)Z(= Al, Ga, In). Further, we probed the role of electron-rich substitution at Z site on the electronic, magnetic and transport properties of half-metallic(HM) Co2TaZ( = Al,Ga,In) full Heusler alloys(FHA). One of our main aims is to study the effect of electron-rich substitution at Z site on the total and partial moments of X and Z sites. We have used the modified Becke–Johnson(mBJ) exchange–correlation term to calculate the HM band-gap. We observe that, isovalent substitution at X and Z sites provides a way to tune the lattice constant in range of ∼5.75 Å to ∼6.16 Å with sustained HM character. Further, it is noticed that the HM band-gap decreases with isovalent substitution (with low atomic number) at X site with maximum and minimum values of 1.09 eV and 0.5 eV, respectively. Isovalent substitution does not affect the total magnetic moment, however, it affects the atomic magnetic moment values. Further, we observe that 25% of substitution of tetravalent atoms (Si, Ge, Sn) at atomic site Z (having Al, Ga, In) enhances the total magnetic moment with a small decrease in the minority band-gap. However, 75 to 100 % of substitutions destroy the half-metallicity, showing an enhancement in the magnetic moment. Temperature dependent spin Seebeck coefficients corresponding to the 25% and 75% of substitution of tetravalent element at Z site exhibit values close to 150 μV/K and 25–75 μV/K, respectively, for temperatures above 400 K. Interestingly, electron rich substitution at Z cite, changes the orientation of the magnetic moment corresponding to the X (Ta) site element. In order to understand in detail the impact of substitution on the local magnetic moment of Co and X site atoms (V, Nb, Ta), we have performed calculations of respective X-ray magnetic circular dichroism(XMCD) spectra and analyzed the results in terms of unoccupied density of states. XMCD spectra of L2,3 edges of Co atom show only small deviation (in the intensity) with the changes in the X and Z sites. Unlike the case of Co atom, we find that the XMCD spectra of M2,3 edges of Ta atom exhibit substantial changes when tetravalent atoms replace the trivalent atoms at the Z site.

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.

Cr doping-induced ferromagnetism in the spin-glass Cd1−xMnxTe studied by x-ray magnetic circular dichroism

The prototypical diluted magnetic semiconductor Cd1−xMnxTe is a spin glass (x < 0.6) or an antiferromagnet (x > 0.6) but becomes ferromagnetic upon doping with a small amount of Cr atoms. To investigate the origin of the ferromagnetism in Cd1−x−yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). Measured XAS and XMCD spectra indicate that both Cr and Mn atoms are divalent and that the magnetic moments of Cr and Mn are aligned parallel. The magnetization of Mn increases with increasing Cr content, suggesting that ferromagnetic interaction exists between neighboring Mn and Cr ions despite the largely antiferromagnetic interaction between Mn atoms. By analyzing the element-specific magnetization curves, we conclude that Cr doping leads to the formation of ferromagnetic or superparamagnetic clusters consisting of several Cr ions surrounded by a much larger number of Mn ions.

Magnetic Properties and Electronic Configurations of Mn Ions in the Diluted Magnetic Semiconductor Ba1−xKx(Zn1−yMny)2As2 Studied by X-ray Magnetic Circular Dichroism and Resonant Inelastic X-ray Scattering

The magnetic properties and the electronic excitations of the new diluted magnetic semiconductor Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ have been studied by x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) at the Mn $L_{2,3}$ edge. The sum rule analysis of the XMCD spectra yields the net spin moment of $0.45\mu_{\text{B}}$/Mn and the small orbital moment of $0.05\mu_{\text{B}}$/Mn. This indicates that the Mn atoms are in the high-spin configurations of $d^{5}$, whereas the presence of competing ferromagnetic and antiferromagnetic interactions between the Mn ions reduces the net spin moment. RIXS spectra show broad peaks from 1 to 6 eV energy loss, which originate from the $d$-$d$ crystal field excitations of the Mn ions. Based on a comparison of the RIXS line shapes with those of Ga$_{1-x}$Mn$_{x}$As, we conclude that the ground state of Mn in Ba$_{1-x}$K$_{x}$(Zn$_{1-y}$Mn$_{y}$)$_{2}$As$_{2}$ consists not only of the charge-transferred $3d^{5}\underline{L}$ electron configuration ($\underline{L}$: ligand hole) with weakly bound holes as in Ga$_{1-x}$Mn$_{x}$As, but also of the pure $3d^{5}$ configuration with free holes.

Resonant inelastic x-ray scattering in Ca3LiOsO6 from first-principles

We have investigated the electronic and magnetic properties of the transition metal oxide (TMO) Ca3LiOsO6 within the density functional theory (DFT) using the generalized gradient approximation while taking into account strong Coulomb correlations (GGA+U) in the framework of the fully relativistic spin-polarized Dirac linear muffin-tin orbital band-structure method. Ca3LiOsO6 is an insulator and forms the K4CdCl6-type crystal structure with relatively high Néel temperature TN = 117 K. Despite of the large strength of spin–orbit coupling, it has only a small effect on the electronic and magnetic properties of Ca3LiOsO6. We have also investigated theoretically the x-ray absorption spectra (XAS), x-ray magnetic circular dichroism (XMCD) and resonant inelastic x-ray scattering (RIXS) spectra at the Os L3 edge. The experimentally measured RIXS spectrum of Ca3LiOsO6 in addition to the elastic scattering peak at 0 eV possesses a sharp feature below 2 eV corresponding to transitions within the Os t2g levels. The excitation located at 4.5 eV is due to t2g→eg transitions. The third wide structure situated at 6–8 eV appears due to ligand-to-metal charge transfer excitations: d−d transitions to the Os t2g manifold from the Os 5dO states derived from the ”tails” of oxygen 2p states. The high energy structure at 10–11.5 eV is due to Os 5dO→eg interband transitions. At higher energy above 11 eV transitions from the Os 5dO states to the empty Ca 5d states are located. We have investigated the effect of SO coupling, Coulomb correlations and core-hole effect on the energy band structure, XAS, XMCD, and RIXS spectra in Ca3LiOsO6.

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.

Anisotropy of 4f states in 3d−4f single-molecule magnets

We have measured angular-dependent fluorescence-yield x-ray magnetic circular dichroism spectra on single crystals of the heterometallic $3d\ensuremath{-}4f$ 12-metallacrown-4 ${\mathrm{TbMn}}_{4}$ and ${\mathrm{DyMn}}_{4}$ complexes. Simulated spectra using crystal-field multiplet calculations reproduce the experimentally observed spectra. The orientation of the molecules causes linear dichroism spectra of the $4f$ absorption spectra. This natural linear dichroism shows the anisotropic charge distribution of the rare-earth $4f$ state in the tetragonal crystal field despite the small $4f$ crystal-field splitting. The magnetic moment of the molecule is dominated by the rare-earth moment revealing a considerably large contribution of orbital moment. From a sum-rule analysis of experimental and simulated x-ray magnetic circular dichroism, we determined corrected spin and orbital Dy moments at low temperature (14 K) within a magnetic field of 7 T. We find a significant angular dependence of the Dy magnetic moments, indicating the presence of fourth-order magnetic anisotropy.

Control of site occupancy by variation of the Zn and Al content in NiZnAl ferrite epitaxial films with low magnetic damping

The structural and magnetic properties of Zn/Al doped nickel ferrite thin films can be adjusted by changing the Zn and Al content. The films are epitaxially grown by reactive magnetron sputtering using a triple cluster system to sputter simultaneously from three different targets. Upon the variation of the Zn content the films remain fully strained with similar structural properties, while the magnetic properties are strongly affected. The saturation magnetization and coercivity as well as resonance position and linewidth from ferromagnetic resonance (FMR) measurements are altered depending on the Zn content in the material. The reason for these changes can be elucidated by investigation of the x-ray magnetic circular dichroism spectra to gain site and valence specific information with elemental specificity. Additionally, from a detailed investigation by broadband FMR a minimum in g-factor and linewidth could be found as a function of film thickness. Furthermore, the results from a variation of the Al content using the same set of measurement techniques is given. Other than for Zn, the variation of Al affects the strain and even more pronounced changes to the magnetic properties are apparent.

Magnetoelastic anisotropy in Heusler-type Mn2−δCoGa1+δ films

Perpendicular magnetization is essential for high-density memory application using magnetic materials. High-spin polarization of conduction electrons is also required for realizing large electric signals from spin-dependent transport phenomena. Heusler alloy is a well-known material class showing the half-metallic electronic structure. However, its cubic lattice nature favors in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy (PMA), in general. This study focuses on an inverse-type Heusler alloy, Mn$_{2-\delta}$CoGa$_{1+\delta}$ (MCG) with a small off-stoichiometry ($\delta$) , which is expected to be a half-metallic material. We observed relatively large uniaxial magnetocrystalline anisotropy constant ($K_\mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG films with a small tetragonal distortion of a few percent. A positive correlation was confirmed between the $c/a$ ratio of lattice constants and $K_\mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly demonstrated a change in the domain patterns along with $K_\mathrm{u}$. X-ray magnetic circular dichroism (XMCD) was employed using synchrotron radiation soft x-ray beam to get insight into the origin for PMA. Negligible angular variation of orbital magnetic moment ($\Delta m_\mathrm{orb}$) evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to $K_\mathrm{u}$. Our first principles calculation reasonably explained the small $\Delta m_\mathrm{orb}$ and the positive correlation between the $c/a$ ratio and $K_\mathrm{u}$. The origin of the magnetocrystalline anisotropy was discussed based on the second-order perturbation theory in terms of the spin--orbit coupling, claiming that the mixing of the occupied $\uparrow$- and the unoccupied $\downarrow$-spin states is responsible for the PMA of the MCG films.

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 ${\mathrm{CrI}}_{3}$.

Probing the tunability of magnetism with external pressure in metastable Sr2NiIrO6 double perovskite

In ${\mathrm{Sr}}_{2}{\mathrm{NiIrO}}_{6}$ long-range Ir-Ir antiferromagnetic exchange interactions have been reported to overcome the ferromagnetic Ni-Ir interactions hampering the otherwise expected ferromagnetic behavior. Prompted by this, a combination of x-ray absorption spectroscopy and x-ray diffraction at high pressure is used here to investigate the interplay between the magnetic structure of the Ir sublattice and lattice degrees of freedom. The compression of ${\mathrm{Sr}}_{2}{\mathrm{NiIrO}}_{6}$ drives an unexpected nonmonotonic change of the x-ray magnetic circular dichroism (XMCD) spectra: The intensity first decreases in the 0- to 18-GPa range, then shows an increase in the 18- to 30-GPa range and again decreases for higher pressures. The XMCD intensity, a measure of the net magnetization in the Ir sublattice, however, is found to remain very low in the whole pressure range so the observed changes do not correspond with a transition from antiferromagnetic to ferromagnetic or ferrimagnetic order. The evolution of the XMCD is better explained in terms of a weakening/strengthening of the long-range antiferromagnetic (AFM) Ir-Ir interaction between ferromagnetic planes associated with the reduction of the lattice parameters. In particular, a correlation can be established between the evolution of the $b$/$a$ ratio and the weakening/strengthening of the AFM interaction.

Observation of spontaneous x-ray magnetic circular dichroism in a chiral antiferromagnet

X-ray magnetic circular dichroism (XMCD) signals are usually absent in antiferromagnets. In this paper, we report the observation of spontaneous XMCD spectra originating from the inverse triangular spin structure, or the polarization of the cluster magnetic octupole, in the chiral antiferromagnet ${\mathrm{Mn}}_{3}\mathrm{Sn}$ thin film. The result is consistent with the recent theoretical predictions that the inverse triangular spin structure can give rise to finite XMCD signals in the absence of net magnetization [J. Phys. Soc. Jpn 89, 083703 (2020) and Phys. Rev. Lett. 126, 157402 (2021)].