X-ray Magnetic Circular Dichroism Spectroscopy Research Articles

X-ray magnetic circular dichroism spectroscopy at the Fe L edges with a picosecond laser-driven plasma source

Time-resolved x-ray magnetic circular dichroism (XMCD) enables a unique spectroscopic view on complex spin and charge dynamics in multi-elemental magnetic materials. So far, its application in the soft-x-ray range has been limited to synchrotron-radiation sources and free-electron lasers. By combining a laser-driven plasma source with a magnetic thin-film polarizer, we generate up to 30% circular polarization in the soft-x-ray regime, enabling the first XMCD spectroscopy at the Fe L edges in a laser laboratory, to our knowledge. Our approach can be readily adapted to other transition metal L and rare earth M absorption edges, and with a temporal resolution of <10ps, a wide range of ultrafast magnetization studies can be realized.

Enhancement of Spin Polarization in Two-Dimensional Electron Gases at Patterned LaAlO3/SrTiO3 Interfaces

Spin-polarized two-dimensional electron gases (2DEGs) at the interfaces of SrTiO3-based correlated oxides have attracted tremendous attention in electronics and spintronics. Hitherto, the transition temperature (TC) for such spin polarization remains very low at around 20 K, seriously restricting further spin-based applications. Here, we demonstrate a new strategy to greatly enhance the spin polarization at the interfaces of the prototypical LaAlO3/SrTiO3 by conveniently inserting a SrCoO2.5-patterned Hall-bar layer. In the modified interfacial heterostructure, signatures of spin polarization, such as the Kondo effect, hysteretic magnetoresistance, magnetic hysteresis loop, and anomalous Hall effect, are all unambiguously observed. The TC of spin polarization deduced from the anomalous Hall effect is promoted to a significantly high temperature of 100 K, much higher than any reported values for 2DEGs at oxide interfaces. Combining atomic-level resolution electron energy-loss spectroscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism spectroscopy, the origin of spin polarization is attributed to the occurrence of Ti3+ ions located around the interfaces. This work opens up a reliable interfacial engineering route to enhance the spin polarization in 2DEGs at oxide interfaces, which is applicable for practical spin-based logic and memory devices.

Design of broadband reflective quarter-wave plates in the water window region based on aperiodic Cr/Sc multilayer mirrors

The generation of a water window attosecond pulse with circular polarization is important for ultrafast research such as X-ray magnetic circular dichroism spectroscopy. In this paper, we propose a design method of broadband reflective quarter-wave plates in the water window region based on aperiodic Cr/Sc multilayer mirrors. Three broadband reflective quarter-wave plates are designed for achieving high circular polarization and circular reflection in the water window region with 10, 15, and 20 eV bandwidths. The results indicate that the reflected lights of the designed aperiodic Cr/Sc multilayer mirrors all show a more than 95% circular polarization degree and considerable circular reflection in their design band. The proposed broadband reflective quarter-wave plates can be used for controlling the polarization state of broadband water window region sources and generating the circularly polarized attosecond pulse by the linearly polarized water window region sources.

Synthesis of Nanostructured Ferrites and Cation Distribution Studies by X-ray Magnetic Circular Dichroism, Mössbauer Spectroscopy, and X-ray Absorption Spectroscopy

Synthesis of Nanostructured Ferrites and Cation Distribution Studies by X-ray Magnetic Circular Dichroism, Mössbauer Spectroscopy, and X-ray Absorption Spectroscopy

Electronic structure and magnetic anisotropy transition in Mn3−xCoxGa Heusler films investigated with soft x-ray spectroscopy

Employing soft x-ray magnetic circular dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS), we have investigated electronic structures and spin configurations of Co-substituted ${\mathrm{Mn}}_{3}\mathrm{Ga}$ (${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$) Heusler films, which are promising spintronic materials exhibiting perpendicular magnetic anisotropy (PMA). Co $2p$ XAS and XMCD measurements, combined with the charge-transfer multiplet analysis, show that the substituted Co ions in ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ consist of mainly ferromagnetic metallic Co clusters and a small amount of divalent Co ions with disordered magnetic moments. Mn $2p$ XAS and XMCD measurements show that Mn ions at both the octahedral and tetrahedral sites in ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ are nearly divalent with weak covalent-bonding nature, reflecting non-Jahn-Teller ions, and that the magnetic moments of Mn ions are polarized along the ordered magnetic moments of the substituted Co ions. Angle-dependent Co $2p$ XMCD data of ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ agree with the weak magnetic anisotropy transition from PMA to in-plane magnetic anisotropy (IMA) with increasing $x$. Spin and orbital magnetic moments of Co ions are estimated by using the sum-rule analysis, which reveal negligible Co-content and polar-angle dependencies of the Co orbital to spin magnetic moment ratio. This work reveals that the PMA-to-IMA magnetic transition in ${\mathrm{Mn}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Ga}$ is determined mainly by ferromagnetic metallic Co clusters.

Synchrotron X-ray spectroscopic study of the antiferromagnetic-ferromagnetic transition in Ni-doped FeRh epitaxial thin films

We investigate the change of magnetism and electronic structure across the antiferromagnetic (AFM) -ferromagnetic (FM) transition of Ni-doped FeRh epitaxial thin films by x-ray magnetic circular dichroism (XMCD) and hard x-ray photoemission spectroscopy (HAXPES). The Rh L edge XMCD results indicate that the remnant FM phase at low temperature possesses smaller Rh moment than the normal FM phase which can undergo AFM-FM transition. The HAXPES results confirm an increase of Rh 4d density of state at the Fermi level and a possible “well-screened” state at the Ni 2p photoemission in the FM phase. FM fluctuation of the inter-site Fe-Fe exchange coupling within the AFM phase can be interpreted from the temperature dependence of the Fe 2p HAXPES results.

As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering.

A novel mixed-valent hybrid chiral and polar compound, Fe7As3Se12(en)6(H2O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe5Se9] chains connected via [As3Se2]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)3]2+ complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and 57Fe Mössbauer spectroscopy showed the presence of mixed-valent Fe2+/Fe3+ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by 57Fe Mössbauer spectroscopy, showing that a fraction of the Fe3+/Fe2+ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe3+ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.

Control of perpendicular magnetic anisotropy at the Fe/MgO interface by phthalocyanine insertion

Perpendicular magnetic anisotropy at an interface between Fe and MgO with Co-phthalocyanine (CoPc) adsorption has been investigated. We find that the CoPc adsorption at an Fe/MgO interface increases the perpendicular magnetic anisotropy energy (PMA) while maintaining the voltage-controlled magnetic anisotropy effect. By performing x-ray magnetic circular dichroism spectroscopy measurements, we reveal that the CoPc adsorption induces hole accumulation at the Fe surface. The origin of the PMA enhancement is attributed to the increased orbital magnetic moment anisotropy of the Fe atoms.

Magnetic moments and spin structure in single-phase B20 Co1+xSi1−x (x = 0.043)

Neutron powder diffraction (NPD) and x-ray magnetic circular dichroism (XMCD) spectroscopy are employed to investigate the magnetism and spin structure in single-phase B20 Co1.043Si0.957. The magnetic contributions to the NPD data measured in zero fields are consistent with the helical order among the allowed spin structures derived from group theory. The magnitude of the magnetic moment is (0.3 ± 0.1)μB/Co according to NPD, while the surface magnetization probed by XMCD at 3 kOe is (0.18–0.31)μB/Co. Both values are substantially larger than the bulk magnetization of 0.11μB/Co determined from magnetometry at 70 kOe and 2 K. These experimental data indicate the formation of a helical spin phase and the associated conical states in high magnetic fields.

Picosecond time-resolved X-ray ferromagnetic resonance measurements at Shanghai synchrotron radiation facility

An experimental picosecond time-resolved X-ray ferromagnetic resonance (TR-XFMR) apparatus with a time resolution of 13 ps (RMS) or 31 ps (FWHM) was constructed and demonstrated in the 07U and 08U1A soft X-ray beamlines at the Shanghai Synchrotron Radiation Facility (SSRF) using pump-probe detection and X-ray magnetic circular dichroism (XMCD) spectroscopy. Element and time-resolved ferromagnetic resonance was excited by continuous microwave phase-locking of the bunch clock within the photon beam during synchrotron radiation and was characterized by detecting the magnetic circular dichroism signals of the elements of interest in the magnetic films. Using this equipment, we measured the amplitude of the element-specific moment precession during ferromagnetic resonance (FMR) at 2 GHz in a single Ni81Fe19 layer.

Observing the orbital angular momentum of Fe and Co in chiral magnet Fe0.75Co0.25Si using soft x-ray magnetic circular dichroism

The intermetallic compound Fe1−xCoxSi has a helical magnetic order for 0.05<x<0.8, whereas its orbital angular momentum contributing to the occurrence of a Dzyaloshinskii–Moriya interaction has not yet been verified. We applied soft x-ray magnetic circular dichroism spectroscopy on the Fe L2,3 and Co L2,3 edges below Tc for Fe0.75Co0.25Si such that their orbital magnetic moments morb were evaluated independently. The dichroic signals provide direct experimental evidence that morb for both Fe and Co is coupled in a parallel manner with each spin counterpart mspin. The ratio of morb to mspin is independently estimated as morb/mspin∼3% for Fe and 9% for Co. By comparing the average of the two mspin values with the saturated magnetization using a commercial superconducting quantum interference device (SQUID) magnetometer, the hole number in the d bands for both Fe and Co is roughly estimated as approximately 1.5. This suggests that Fe and Co should be arranged closely for stabilizing the ferromagnetic moments.

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.

Modification of interfacial spin-orbit torque in Co/Pt/oxide hybrid structures

We investigate current-induced spin-orbit torque (SOT) in Co/Pt/oxide systems by varying a sort of the oxide layer. When the Pt thickness is less than \ensuremath{\sim}2 nm, Pt at the interface of the oxide layer is magnetically polarized owing to the magnetic proximity effect. In these systems, fieldlike (FL) SOT depends significantly on the adjacent oxide material, whereas dampinglike SOT is almost irrelevant to oxides. The FL SOT efficiency of the $\mathrm{Pt}/\mathrm{Hf}{\mathrm{O}}_{2}$ sample is 2.2 times greater than that of the Pt/MgO sample at the maximum. X-ray magnetic circular dichroism spectroscopy reveals that the anisotropy of the Pt orbital magnetic moment varies with the oxide material, suggesting that the modulation of the electronic structure at the Pt/oxide interface contributes to SOT enhancement.

Strongly anisotropic antiferromagnetic coupling in EuFe2As2 revealed by stress detwinning

Of all parent compounds of iron-based high-temperature superconductors, EuFe2As2 exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic order and Fe structural/antiferromagnetic domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in-situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress-detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a new form of higher-order coupling between Eu and Fe magnetism.

Magnetic moment orientation and in-depth distribution of dysprosium near the surface of DyCo4.6 thin films from x-ray circularly polarized absorption

We have investigated the dysprosium distribution and its magnetic moment orientation at the region near the surface of ${\mathrm{DyCo}}_{4.4}$ and ${\mathrm{DyCo}}_{4.6}$ ferrimagnetic amorphous films with perpendicular magnetic anisotropy. X-ray magnetic circular dichroism spectroscopy of the films at the Dy ${M}_{4,5}$ and Co ${L}_{2,3}$ edges using total electron yield (TEY) detection was performed at 2 K and 300 K temperatures, and at sample orientations ranged from ${0}^{\ensuremath{\circ}}$ to ${70}^{\ensuremath{\circ}}$ with respect to the normal to the sample. The measurements showed an apparent partial decoupling between the cobalt and dysprosium magnetic sublattices. At RT, the magnetic moment per atom of dysprosium was below the minimum value expected if all dysprosium moments were Antiferromagnetic (AF) coupled to cobalt. At 2 K, the cobalt sublattice presented a surprisingly stronger magnetic anisotropy than the dysprosium sublattice. A detailed analysis of the circularly polarized spectra of the Dy ${M}_{5}$ edge, based on the deconvolution of the spectra in their related parallel, antiparallel, and transverse to ${J}_{z}$ spectral components, demonstrates that the spectra are composed by dysprosium with different magnetic moment distributions. The fit of the Dy ${M}_{5}$ spectra using the ${J}_{z}$ spectral components evidenced a gradation of dysprosium concentration due to segregation at the region probed by TEY. The topmost layer was magnetically uncoupled from cobalt. At RT, $25%$ of the dysprosium magnetic moments in the underlayer were found averaged oriented in the same direction of cobalt. The expected weak magnetic coupling of these dysprosium atoms to cobalt should explain the surprisingly lower magnetic anisotropy of the dysprosium sublattice compared to that of cobalt probed by TEY at 2 K.

Probing exchange bias at the surface of a doped ferrimagnetic insulator

With the realization of stress-induced perpendicular magnetic anisotropy, efficient spin-orbit torque switching, and room temperature topological Hall effect, interest in rare earth iron garnets has been revived in recent years for their potential in spintronic applications. In this study, we investigate the magnetic properties of micrometer-thick Bi and Ga substituted thulium iron garnets (BiGa:TmIG) grown by the liquid-phase epitaxy method. Above the magnetization compensation (MC) temperature, anomalous triple hysteresis is observed in BiGa:TmIG/Pt heterostructures by anomalous Hall effect measurements. X-ray magnetic circular dichroism and energy dispersive spectroscopy measurements reveal its origin as an internal exchange bias (EB) effect arising from inhomogeneities localized at the surface of the film. Possibly depending on the difference in thickness and defect realization of the EB layer, two types of magnetization reversal mechanisms, namely, the Stoner-Wohlfarth type and the reversible domain-wall motion type, are observed. Our results show that rich meta-magnetic phases exist in garnets close to MC, which can be robustly tuned by chemical composition engineering and conveniently probed by electrical transport measurements.

Alternation of Magnetic Anisotropy Accompanied by Metal-Insulator Transition in Strained Ultrathin Manganite Heterostructures

Fundamental understanding of interfacial magnetic properties in ferromagnetic heterostructures is essential to utilize ferromagnetic materials for spintronic device applications. In this paper, we investigate the interfacial magnetic and electronic structures of epitaxial single-crystalline LaAlO$_3$ (LAO)/La$_{0.6}$Sr$_{0.4}$MnO$_3$ (LSMO)/Nb:SrTiO$_3$ (Nb:STO) heterostructures with varying LSMO-layer thickness, in which the magnetic anisotropy strongly changes depending on the LSMO thickness due to the delicate balance between the strains originating from both the Nb:STO and LAO layers, using x-ray magnetic circular dichroism (XMCD) and photoemission spectroscopy (PES). We successfully detect the clear change of the magnetic behavior of the Mn ions concomitant with the thickness-dependent metal-insulator transition (MIT). Our results suggest that double-exchange interaction induces the ferromagnetism in the metallic LSMO film under tensile strain caused by the SrTiO$_3$ substrate, while superexchange interaction determines the magnetic behavior in the insulating LSMO film under compressive strain originating from the top LAO layer. Based on those findings, the formation of a magnetic dead layer near the LAO/LSMO interface is attributed to competition between the superexchange interaction via Mn 3$d_{3z^2-r^2}$ orbitals under compressive strain and the double-exchange interaction via the 3$d_{x^2-y^2}$ orbitals.

Rare-earth orbital moment contributions to the magnetic anisotropy in magnetostrictive Tb0.3Dy0.7Fe2

Soft x-ray Magnetic Circular Dichroism spectroscopy at the Dy and Tb M4,5 and the Fe L2,3 edges was performed on a sputter deposited polycrystalline Terfenol-D (Tb0.3Dy0.7Fe2) film on sapphire substrates at temperatures from 100 to 300 K to evaluate the elementwise contribution to the magnetocrystalline anisotropy and coercive field. The elemental spin and orbital magnetic moments were calculated using the x-ray Magnetic Circular Dichroism sum rules. As temperatures decreased, the Tb and Fe moments plateau at 200 K with values of 7.6 μB/atom and 1.8 μB/atom, respectively, while the Dy moment increases to 8.9 μB/atom at 100 K. Between 300 and 200 K, the change in magnetic anisotropy is dominated by thermally induced magnetoelastic effects while for temperatures below 200 K magnetocrystalline anisotropy (MCA) changes are dominant. The MCA changes below 200 K appear to be due to increases in the Dy orbital moment with decreasing temperature in this temperature regime.

Revealing noncollinear magnetic ordering at the atomic scale via XMCD

Mass-selected V and Fe monomers, as well as the heterodimer {text{Fe}}_1{text{V}}_1, were deposited on a Cu(001) surface. Their electronic and magnetic properties were investigated via X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopy. Anisotropies in the magnetic moments of the deposited species could be examined by means of angle resolving XMCD, i.e. changing the X-ray angle of incidence. A weak adatom-substrate-coupling was found for both elements and, using group theoretical arguments, the ground state symmetries of the adatoms were determined. For the dimer, a switching from antiparallel to parallel orientation of the respective magnetic moments was observed. We show that this is due to the existence of a noncollinear spin-flop phase in the deposited dimers, which could be observed for the first time in such a small system. Making use of the two magnetic sublattices model, we were able to find the relative orientations for the dimer magnetic moments for different incidence angles.

Bulk and element-specific magnetism of medium-entropy and high-entropy Cantor-Wu alloys

Magnetic Compton scattering, x-ray magnetic circular dichroism spectroscopy and bulk magnetometry measurements are performed on a set of medium (NiFeCo and NiFeCoCr) and high (NiFeCoCrPd and NiFeCoCrMn) entropy Cantor-Wu alloys. The bulk spin momentum densities determined by magnetic Compton scattering are remarkably isotropic, and this is a consequence of the smearing of the electronic structure by disorder scattering of the electron quasiparticles. Non-zero x-ray magnetic circular dichroism signals are observed for every element in every alloy indicating differences in the populations of the majority and minority spin states implying finite magnetic moments. When Cr is included in the solid solution, the Cr spin moment is unambiguously antiparallel to the total magnetic moment, while a vanishingly small magnetic moment is observed for Mn, despite calculations indicating a large moment. Some significant discrepancies are observed between the experimental bulk and surface magnetic moments. Despite the lack of quantitative agreement, the element specific surface magnetic moments seem to be qualitatively reasonable.