X-ray Magnetic Circular Dichroism Signal Research Articles

Apparent ferrimagnetism in Sr(Fe0.2Mn0.2Co0.2Ti0.2V0.2)O3 high-entropy oxide perovskite thin films

We present a study on the structural and magnetic properties of Sr(Fe0.2Mn0.2Co0.2Ti0.2V0.2)O3 (S5BO) high-entropy oxide perovskite thin films. We use synchrotron-based x-ray absorption spectroscopy employing x-ray magnetic circular dichroism (XMCD) and reveal an enhanced presence of high-spin Co2+, which appears to feature a magnetic response opposing that of the two other magnetic transition metal elements, Fe and Mn. This is marked by both opposite XMCD signals and an inverted XMCD hysteresis loop for Co, while Fe and Mn show regularly shaped hysteresis curves, as the picture of a ferrimagnetic ground state emerges for S5BO.

Modulating above-room-temperature magnetism in Ga-implanted Fe5GeTe2 van der Waals magnets

The creation of van der Waals (vdW) ferromagnets with tunable Curie temperature (TC) and magnetic anisotropy is essential in developing vdW magnet-based devices. Here, we report an effective and reliable method for modulating the magnetic properties of vdW Fe5GeTe2 by site-specific Ga+ implantation. In this study, we report an easy axis in the ab-plane for bulk Fe5GeTe2 (TC = 310 K) and an axis out of the plane for thin Fe5GeTe2 flakes (TC = 290 K). Combining element-resolved photoemission electron microscopy and spatially resolved magneto-optic Kerr microscopy, we find that the implantation of a tiny amount of 10−3 Ga+·Å−3 in Fe5GeTe2 greatly enhances the TC from 290 to 360 K and switches the magnetic easy axis from the out-of-plane c axis to the ab-plane. The room-temperature x-ray magnetic circular dichroism signal is enhanced from 0% to 9% at an implantation level of 10−2 Ga+·Å−3. These results provide new opportunities for tailoring the magnetic properties of vdW materials beyond room temperature.

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.

Intrinsic Magnetic Proximity Effect at the Atomically Sharp Interface of CoxFe3-xO4/Pt Grown by Molecular Beam Epitaxy.

CoxFe3-xO4(CFO)/Pt bilayers prepared by molecular beam epitaxy were investigated for the anomalous Hall effect and X-ray magnetic circular dichroism (XMCD). We found that the anomalous Hall effect originates from a magnetic proximity effect at the CFO/Pt interface. The XMCD signal in the Pt L-edge was obtained only for the sample deposited at 600 °C, indicating that the magnetic proximity effect is sensitive to the interface structure. Transmission electron microscopy images of the CFO/Pt interface and XMCD measurements of Co and Fe L-edges do not provide direct evidence for interfacial atomic diffusion or alloying. In summary, these results suggest that the magnetic proximity effect is robust for transport properties, such as the anomalous Hall effect, while the induced magnetic moment depends on slight differences in the interfacial structure, such as the presence or absence of interfacial oxygen ions.

Surface Deposition Induced Reduction of the Ground State Spin in Cr10 Wheel

AbstractWe report the deposition of monolayers and multilayers of {Cr10(OMe)20(O2CCMe3)10} wheels, hereafter {Cr10}, onto Au(111) and Cu(111) single‐crystal substrates, and their characterization combining scanning tunneling microscopy, X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, X‐ray natural linear dichroism, and X‐ray magnetic circular dichroism (XMCD). {Cr10} in bulk shows axial magnetic anisotropy and a cluster total spin S = 9 ground state, stemming from an interaction scheme of two semi‐crowns containing four Cr3+ ions interacting ferromagnetically, separated by two Cr3+ antiferromagnetically coupled. The one‐monolayer (1ML) samples of {Cr10} sublimated on Ag(111) and Cu(111) show slightly different applied magnetic field dependence of XMCD signal. The field‐dependence of the magnetization evolves from a lower curve for the 1ML {Cr10} samples to a curve resembling the bulk one as the number of layers is increased, as shown in a 14ML sample. Monte Carlo simulations allow rationalizing the magnetization curves of the 1ML samples in terms of a reduction of the cluster ground‐state total spin, S = 3 or S = 6, as a consequence of variations in the intra‐wheel coupling interactions induced by the on‐surface deposition. The sensitivity of the magnetic configuration of {Cr10} to minor distortions of the intramolecular conformation might be exploited to control magnetism by external stimuli for applications.

Direct X-Ray Detection of the Spin Hall Effect in CuBi

The spin Hall effect and its inverse are important spin-charge conversion mechanisms. The direct spin Hall effect induces a surface spin accumulation from a transverse charge current due to spin orbit coupling even in non-magnetic conductors. However, most detection schemes involve additional interfaces, leading to large scattering in reported data. Here we perform interface free x-ray spectroscopy measurements at the Cu L_{3,2} absorption edges of highly Bi-doped Cu (Cu_{95}Bi_{5}). The detected X-ray magnetic circular dichroism (XMCD) signal corresponds to an induced magnetic moment of (2.7 +/- 0.5) x 10-12 {\mu}_{B} A^{-1} cm^{2} per Cu atom averaged over the probing depth, which is of the same order as for Pt measured by magneto-optics. The results highlight the importance of interface free measurements to assess material parameters and the potential of CuBi for spin-charge conversion applications.

Enhanced d−p hybridization intertwined with anomalous ground state formation in the van der Waals itinerant magnet Fe5GeTe2

${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$ is a van der Waals (vdW)-coupled unconventional ferromagnetic metal with a high Curie temperature (${T}_{\mathrm{C}}$) exceeding 300 K. The formation of an anomalous ground state significantly below ${T}_{\mathrm{C}}$ has received considerable attention, resulting in increased interest in understanding the spin-polarized electronic state evolution near the Fermi energy (${E}_{\mathrm{F}}$) as a function of temperature. Despite recent extensive studies, a microscopic understanding of the spin-polarized electronic structure around ${E}_{\mathrm{F}}$ has not yet been established owing to the intrinsic complexity of both the crystal and band structures. In this study, we investigate the temperature dependence of element-specific soft x-ray magnetic circular dichroism (XMCD). A systematic temperature evolution in the XMCD signal from both magnetic Fe and its ligand Te is clearly observed. More importantly, the enhancement in the hybridization between the Fe $3d$ and Te $5p$ states at low temperature in the zero-magnetic field limit is revealed. We discuss the implications of our observation in line with possible emergence of an exotic magnetic ground state in ${\mathrm{Fe}}_{5}{\mathrm{GeTe}}_{2}$.

Structural, magnetic and Magneto-transport properties of Mn-doped SiGe thin films

Mn-doped polycrystalline SiGe thin films are fabricated by radio frequency magnetron sputtering and subsequent rapid thermal annealing. Structural characterizations show that the Mn atoms are distributed uniformly in the SiGe matrix and mainly occupy the substitutional sites with 2+ state. X-ray magnetic circular dichroism signals reveal the spin and orbital magnetic moment information of the samples. Magnetic measurements show that the Curie temperature is up to ∼ 290 K and the effective magnetic moment is about 1.57 μB/Mn. Magneto-transport measurements reveal anomalous Hall effect, another signal of ferromagnetism in the samples. Our results provide insights into the structure and the ferromagnetism of the Mn-doped SiGe thin films and may be useful for related spintronic applications.

Toward Quantitative Magnetic Information from Transition Metal K-Edge XMCD of Prussian Blue Analogs.

Two series of Prussian blue analogs (PBA) were used as model compounds in order to disentangle the information contained in X-ray magnetic circular dichroism (XMCD) at the K-edges of transition metals. The number of 3d electrons on one site of the bimetallic cyanide polymer has been varied by associating to the [Fe(CN)6]3- or the [Cr(CN)6]3- precursors various divalent metal ions A2+ (Mn2+, Fe2+, Co2+, Ni2+, and Cu2+). The compounds were studied by X-ray diffraction and SQUID magnetometry, as well as by X-ray absorption spectroscopy and XMCD at the K-edges of the A2+ transition metal ion. The study shows that the 1s → 4p contribution to the A K-edge XMCD signal can be related to the electronic structure and the magnetic behavior of the probed A2+ ion: the shape of the signal to the filling of the 3d orbitals, the sign of the signal to the direction of the magnetic moment with respect to the applied magnetic field, the intensity of the signal to the total spin number SA, and the area under curve to the Curie constant CA. The whole study hence demonstrates that PBAs are particularly well-adapted for understanding the information contained in the transition metals K-edge XMCD signals. It also offers new perspectives toward the full disentangling of the information contained in these signals and access to new insights into materials magnetic properties.

Short-range magnetic interaction in a monolayer 1T−VSe2 film revealed by element-specific x-ray magnetic circular dichroism

We investigated the intrinsic magnetic properties of the two-dimensional ferromagnetic candidate monolayer $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ films using x-ray magnetic circular dichroism (XMCD), an element-specific magnetic probe. By performing high-resolution measurements, we succeeded in detecting a clear XMCD signal from the atomically thin $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ films under an external magnetic field. Through manipulation of the x-ray incidence angle, we were able to disentangle the in-plane and out-of-plane magnetic properties and found a strong magnetic anisotropy. Moreover, magnetic-field- and temperature-dependent XMCD revealed that there is no long-range ferromagnetic ordering even at 6 K, but short-range ferromagnetic and antiferromagnetic interactions between neighboring vanadium ions exist. Such low-temperature magnetic behavior signifies that the monolayer $1T\text{\ensuremath{-}}{\mathrm{VSe}}_{2}$ is on the verge of ferromagnetism, and this fact accounts for the reported ferromagnetism in ${\mathrm{VSe}}_{2}$-based heterostructures.

Covalent Mixing in the 2D Ferromagnet CrSiTe3 Evidenced by Magnetic X‐Ray Circular Dichroism

The low‐temperature electronic structure of the van der Waals ferromagnet is investigated. This ferromagnetic semiconductor has a magnetic bulk transition temperature of 33 K, which can reach up to 80 K in single‐ and few‐layer flakes. X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr and Te edges in vacuo‐cleaved single crystals, give strong evidence for hybridization‐mediated superexchange between the Cr atoms. The observed chemical shift in the XAS, as well as the comparison of XMCD with the calculated Cr multiplet spectra, confirms a strong covalent bond between the Cr () and Te states. Application of the XMCD sum rules gives a nonvanishing orbital moment, supporting a partial occupation of the states, apart from . Also, the presence of a nonzero XMCD signal at the Te edge confirms a Te spin polarization due to mixing with the Cr bonding states. The results strongly suggest that superexchange, instead of the previously suggested single‐ion anisotropy, is responsible for the low‐temperature ferromagnetic ordering of 2D materials such as and . This demonstrates the interplay between electron correlation and ferromagnetism in insulating 2D materials.

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)].

Magnetization relaxation and search for the magnetic gap in bulk-insulating V-doped (Bi, Sb)2Te3

V-doped (Bi,Sb)2Te3 has a ten times higher magnetic coercivity than its Cr-doped counterpart and, therefore, is believed to be advantageous for the quantum anomalous Hall effect (QAHE). The QAHE requires the opening of a magnetic bandgap at the Dirac point. We do not find this gap by angle-resolved photoelectron spectroscopy down to 1 K. By x-ray magnetic circular dichroism (XMCD), we directly probe the magnetism at the V site and derive spin and orbital magnetic moments of 1.69 and –0.22 μB/atom. Hysteresis curves of the XMCD signal show a strong dependence of the coercivity on the ramping velocity of the magnetic field. The XMCD signal decays on a timescale of minutes, which we conclude contributes to the absence of a detectable magnetic gap at the Dirac point.

Determination of spin chirality using x-ray magnetic circular dichroism

A 3-fold symmetric kagome lattice that has negative spin chirality can give a non-zero x-ray magnetic circular dichroism (XMCD) signal, despite that the total spin moment amounts to zero. This is explained by a hitherto unnoticed rule for the rotational symmetry invariance of the XMCD signal. A necessary condition is the existence of an anisotropic XMCD signal for the local magnetic atom, which can arise from a spin anisotropy either in the ground state or the final state. The angular dependence of the XMCD as a function of the beam direction has an unusual behavior. The maximum dichroism is not aligned along the spin direction, but depends on the relative orientation of the spin with respect to the atomic orientation. Therefore, different geometries can result in the same angular dependence, and the spin direction can only be determined if the atomic orientation is known. The consequences for the x-ray magneto-optical sum rules are given. The integrated XMCD signals are proportional to the anisotropy in the orbital moment and the magnetic dipole term, where the isotropic spin moment drops out.

Magnetic order of tetragonal CuO ultrathin films

We present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultra-thin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200K. The XMCD signal obtained around the Cu $L_{2,3}$ edges indicates the presence of pinned Cu$^{2+}$ moments that are parallel to the sample surface, and additionally, isotropic paramagnetic moments. The pinning of some of the Cu moments is caused by an AFM ordering consisting of moments that lie most likely in the plane of the film. Moreover, pinned moments show a larger orbital magnetic moment contribution with an approximate ratio of $m_{orb}/m_{spin} = 2$, indicating that these spins are located at sites with reduced symmetry. Some fractions of the pinned moments remain pinned from an AFM background even at 360K, indicating that $T_N >$ 360K. A simple model could explain qualitatively these experimental findings; however, it is in contrast to theoretical predictions, showing that the magnetic properties of ultra-thin T-CuO films differ from bulk expectations and is more complex.

A cookbook for the investigation of coordination polymers by transition metal K-edge XMCD.

In order to disentangle the physical effects at the origin of transition metal K-edge X-ray magnetic circular dichroism (XMCD) in coordination polymers and quantify small structural distortions from the intensity of these signals, a systematic investigation of Prussian blue analogs as model compounds is being conducted. Here the effects of the temperature and of the external magnetic field are tackled; none of these external parameters modify the shape of the XMCD signal but they both critically modify its intensity. The optimized experimental conditions, as well as a reliable and robust normalization procedure, could thus be determined for the study of the intrinsic parameters. Through an extended discussion on measurements on other XMCD-dedicated beamlines and for other coordination compounds, we finally provide new transition metal K-edge XMCD users with useful information to initiate and successfully carry out their projects.

Presence of X-Ray Magnetic Circular Dichroism Signal for Zero-Magnetization Antiferromagnetic State.

X-ray magnetic circular dichroism (XMCD) is generally not observed for antiferromagnetic (AFM) states because XMCD signals from the antiparallelly coupled spins cancel each other. In this Letter, we theoretically show the presence of an XMCD signal from compensated two-dimensional triangle AFM structures on a Kagome lattice. The calculation reveals the complete correspondence between the XMCD spectra and the sign of the spin chirality: the XMCD signal only appears when the spin chirality is negative. This XMCD signal originates from the different absorption coefficients of the three sublattices reflecting different charge density anisotropies and directions of spin and orbital magnetic moments.

Evidence for largest room temperature magnetic signal from Co2+ in antiphase-free & fully inverted CoFe2O4 in multiferroic-ferrimagnetic BiFeO3-CoFe2O4 nanopillar thin films

The ongoing quest for defect-free thin films systems that are apt for being used as spin filtering materials for spintronic applications did yet not deliver satisfying results regarding materials that would be up to the pertinent requirements. Using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Co-L2,3 and Fe-L2,3 absorption edges, we have investigated the magnetic properties of a nanostructured thin film with self-assembled CoFe2O4 nanopillars embedded in BiFeO3, the latter being a well-known system for its combined multiferroic and spintronic properties. In this BiFeO3-CoFe2O4 heterostructure we observed a significant XMCD signal at the Co-L2,3 edges which turns out to be the largest among the presently reported for Co ions at room temperature. A quantitative analysis of the Co-L2,3 spectra unveils that such a large Co-L2,3 XMCD signal stems from the impeccable fully inverted spinel ordering of the A- and B-sites in antiphase-free CoFe2O4 nanopillars. This twofold perfect CoFe2O4 ordering feature yields an unprecedented optimization within a multifunctional ferrimagnetic-multiferroic thin film system highly relevant for spintronic applications, also resulting in an equally unprecedented macroscopic magnetic moment for such material as compared to its pure form as well as to technologically relevant thin film compound systems.

Itinerant ferromagnetism mediated by giant spin polarization of the metallic ligand band in the van der Waals magnet Fe5GeTe2

We investigate near-Fermi-energy (EF) element-specific electronic and spin states of ferromagnetic van der Waals (vdW) metal Fe5GeTe2. The soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) measurement provides spectroscopic evidence of localized Fe 3d band. We also find prominent hybridization between the localized Fe 3d band and the delocalized Ge/Te p bands. This picture is strongly supported from direct observation of the remarkable spin polarization of the ligand p bands near EF, using x-ray magnetic circular dichroism (XMCD) measurements. The strength of XMCD signal from ligand element Te shows the highest value, as far as we recognize, among literature reporting finite XMCD signal for none-magnetic element in any systems. Combining SX-ARPES and elemental selective XMCD measurements, we collectively point an important role of giant spin polarization of the delocalized ligand Te states for realizing itinerant long-range ferromagnetism in Fe5GeTe2. Our finding provides a fundamental elemental selective view-point for understanding mechanism of itinerant ferromagnetism in low dimensional compounds, which also leads insight for designing exotic magnetic states by interfacial band engineering in heterostructures.

Evidence of ZnCO3 interstitial phase in carbon implanted ZnO(002) thin films and room temperature ferromagnetism

High resolution X-ray diffraction (HRXRD) pattern of highly c-oriented (002) thin film provide a direct evidence of the formation of R-3c orthorhombic ZnCO3 structural phase. Room temperature ferromagnetism (RTFM) is measured using bulk vibrating sample magnetometery (VSM). A strong orbital anisotropy has been observed by X-ray linear dichroism (XLD) experiment, which also supports that ZnCO3 phase is structurally non-distortive after carbon-implantation. The absence of X-ray magnetic circular dichroism (XMCD) signal at O K- and C K-edges suggest that bulk VSM ferromagnetism is not related to the magnetic polarization of O 2p/C 2p orbitals hybridized with Zn-states. These experiments clearly indicate that the complex C–ZnO/ZnCO3 phase should be ferromagnetic having out-of-plane/in-plane orbital reconstruction.