Element-specific X-ray Magnetic Circular Dichroism Research Articles

Residual Ferromagnetic Regions Affecting the First-Order Phase Transition in Off-Stoichiometric Fe-Rh.

Among the magnetocaloric materials featuring first-order phase transitions (FOPT), FeRh is considered as a reference system to study the FOPT because it is a "simple" binary system with a CsCl structure exhibiting a large adiabatic temperature change. Recently, ab initio theory predicted that changes in the Fe/Rh stoichiometry in the vicinity of equiatomic composition strongly influence the FOPT characteristics. However, this theoretical prediction was not clearly verified experimentally. Here, we investigated the composition dependence of the transitional hysteresis in FeRh. It is shown that a Fe excess of only 1 at. % induces a ferromagnetic state in the whole temperature range (from 5 K up to Tc) for a minor portion of the sample (≈10%), while 5 at. % is enough to completely eliminate the FOPT. Element-specific X-ray magnetic circular dichroism (XMCD) measurements suggest that this ferromagnetic contribution arises from residual FeRh ferromagnetic regions. We attribute the formation of such domains to Fe antisite defects, as Mössbauer spectroscopy demonstrates the presence of Fe atoms located at the 1b (Rh) sites in the CsCl-type structure. As a consequence, compared with the equiatomic composition, the slightly Fe-rich sample exhibits completely different FOPT properties, influencing the magnetocaloric performances. Thus, our study sheds light on the origin of the remarkable stoichiometric sensitivity of the FOPT behavior in FeRh. These insights have broader implications for understanding FOPT dynamics and the role of residual ferromagnetic domains.

Realizing Room-Temperature Ferromagnetism in Molecular-Intercalated Antiferromagnet VOCl.

2D van der Waals (vdW) magnets are gaining attention in fundamental physics and advanced spintronics, due to their unique dimension-dependent magnetism and potential for ultra-compact integration. However, achieving intrinsic ferromagnetism with high Curie temperature (TC) remains a technical challenge, including preparation and stability issues. Herein, an applicable electrochemical intercalation strategy to decouple interlayer interaction and guide charge doping in antiferromagnet VOCl, thereby inducing robust room-temperature ferromagnetism, is developed. The expanded vdW gap isolates the neighboring layers and shrinks the distance between the V-V bond, favoring the generation of ferromagnetic (FM) coupling with perpendicular magnetic anisotropy. Element-specific X-ray magnetic circular dichroism (XMCD) directly proves the source of the ferromagnetism. Detailed experimental results and density functional theory (DFT) calculations indicate that the charge doping enhances the FM interaction by promoting the orbital hybridization between t2 g and eg. This work sheds new light on a promising way to achieve room-temperature ferromagnetism in antiferromagnets, thus addressing the critical materials demand for designing spintronic devices.

Formation of Mn-rich interfacial phases in Co2FexMn1-xSi thin films

We report the formation of Mn-rich regions at the interface of Co2FexMn1-xSi thin films grown on GaAs substrates by molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) with electron energy loss (EEL) spectrum imaging reveals that each interfacial region: (1) is 1–2 nm wide, (2) occurs irrespective of the Fe/Mn composition ratio and in both Co-rich and Co-poor films, and (3) displaces both Co and Fe indiscriminately. We also observe a Mn-depleted region in each film directly above each Mn-rich interfacial layer, roughly 3 nm in width in the x = 0 and x = 0.3 films, and 1 nm in the x = 0.7 (less Mn) film. We posit that growth energetics favor Mn diffusion to the interface even when there is no significant Ga interdiffusion into the epitaxial film. Element-specific X-ray magnetic circular dichroism (XMCD) measurements show larger Co, Fe, and Mn orbital to spin magnetic moment ratios compared to bulk values across the Co2FexMn1-xSi compositional range. The values lie between reported values for pure bulk and nanostructured Co, Fe, and Mn materials, corroborating the non-uniform, layered nature of the material on the nanoscale. Finally, SQUID magnetometry demonstrates that the films deviate from the Slater-Pauling rule for uniform films of both the expected and the measured composition. The results inform a need for care and increased scrutiny when forming Mn-based magnetic thin films on III-V semiconductors like GaAs, particularly when films are on the order of 5 nm or when interface composition is critical to spin transport or other device applications.

Transfer of magnetic anisotropy in epitaxial Co/NiO/Fe trilayers

The magnetic properties of Co(10 Å)/NiO(40 Å)/Fe trilayer epitaxially grown on W(110) substrate were investigated with use of x-ray magnetic linear dichroism (XMLD) and x-ray magnetic circular dichroism (XMCD). We showed that magnetic anisotropy of Fe film that can be controlled by a thickness-driven spin reorientation transition is transferred via interfacial exchange coupling not only to NiO layer but further to ferromagnetic Co overlayer as well. Similarly, a temperature driven spin reorientation of Fe sublayer induces a reorientation of NiO spin orientation and simultaneous switching of the Co magnetization direction. Finally, by element specific XMCD and XMLD magnetic hysteresis loop measurements we proved that external magnetic field driven reorientation of Fe and Co magnetizations as well as NiO Néel vector are strictly correlated and magnetic anisotropy fields of Fe and Co sublayers are identical despite the different crystal structures.

Aluminum substitution in low damping epitaxial lithium ferrite films

Ferromagnetic insulators with ultra-low damping and tunable magnetic properties are of great interest for the study of spin wave-based phenomena as well as for applications in spin wave spintronics. We have developed epitaxial spinel structure ferrite thin films of Li0.5(AlxFe2.5−x)O4 (LAFO) with ultra-low magnetic damping. The degree of Al substitution tunes the saturation magnetization and in-plane magnetic anisotropy while maintaining excellent epitaxy and crystalline quality as well as low magnetic damping. All LAFO films, deposited on (001)-oriented MgAl2O4 (MAO), are under epitaxial compressive strain, giving rise to an easy magnetic axis in-plane and a hard out-of-plane axis. Element-specific x-ray magnetic circular dichroism measurements confirm that the magnetism originates from the majority of Fe3+ cations occupying the octahedral sites, which are displaced by increased Al3+ substitution. Broadband ferromagnetic resonance reveals the lowest Gilbert damping parameters of 5×10−4 for films 10–15 nm thick. Our films demonstrate the robustness of the low loss dynamic properties of Li0.5(AlxFe2.5−x)O4 that are promising for spin wave applications.

Evolution of Compensated Magnetism and Spin-Torque Switching in Ferrimagnetic Fe1−xTbx

Compensated ferrimagnets (FIMs) made of rare-earth transition-metal compounds have stimulated increasing interest for enabling fast spin dynamics. Taking $\mathrm{Fe}$-$\mathrm{Tb}$ compounds as an example, substantial efforts have been made on the study of compensated magnetism and its potential spintronic applications. Current-induced spin-orbit torque (SOT) switching and its evolution with compensated ferrimagnetism in these compounds, however, remain to be systematically explored, which motivates the present study. By combining magnetometry and anomalous Hall effect measurements, a compositional magnetization compensation point (${x}_{c}$ = 0.25) is determined for ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ films of a fixed thickness of 6.5 nm. The antiferromagnetic coupling between $\mathrm{Fe}$ and $\mathrm{Tb}$ sublattices is directly revealed by conducting element-specific x-ray magnetic circular dichroism measurements. The evolution of SOT switching as a function of $\mathrm{Tb}$ concentration (x) in $\mathrm{Pt}/{\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}/\mathrm{Ta}$ multilayers is subsequently investigated. An enhanced SOT efficiency (approximately 3 times) is obtained at ${x}_{c}$ = 0.25. By conducting an endurance test, reliable SOT switching is revealed for over ${10}^{4}$ cycles. Our work suggests that compensated FIMs of composition ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ could be implemented for realizing efficient and stable spin-orbitronic performances.

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.

Creation and observation of Hopfions in magnetic multilayer systems

Among topological solitons, magnetic skyrmions are two-dimensional particle-like objects with a continuous winding of the magnetization, and magnetic Hopfions are three-dimensional objects that can be formed from a closed loop of twisted skyrmion strings. Theoretical models suggest that magnetic Hopfions can be stabilized in frustrated or chiral magnetic systems, and target skymions can be transformed into Hopfions by adapting their perpendicular magnetic anisotropy, but their experimental verification has been elusive so far. Here, we present an experimental study of magnetic Hopfions that are created in Ir/Co/Pt multilayers shaped into nanoscale disks, known to host target skyrmions. To characterize three-dimensional spin textures that distinguish Hopfions from target skyrmions magnetic images are recorded with surface-sensitive X-ray photoemission electron microscopy and bulk-sensitive soft X-ray transmission microscopy using element-specific X-ray magnetic circular dichroism effects as magnetic contrast. These results could stimulate further investigations of Hopfions and their potential application in three-dimensional spintronics devices.

Ferromagnetic Cu3N Nanoparticles Demonstrated by X-ray Magnetic Circular Dichroism (XMCD) and the Density Functional Theory (DFT) Calculations

In recent years, ferromagnetism induced by natural defects of nonmagnetic semiconductors has been widely investigated and expected to be applied in spintronics. On this basis, we report the ferromagnetic behavior of copper (I) nitride (Cu3N) nanoparticles. A robust room temperature ferromagnetism is found in Cu3N nanoparticles with the saturated magnetization of 4 memu/g (300 K). Based on the element-specific X-ray magnetic circular dichroism (XMCD) and the density functional theory (DFT) analysis, it is concluded that the ferromagnetism of Cu3N nanoparticles originate from the surface Cu vacancies. Moreover, by increasing the surface area of Cu3N, the variation of magnetism is realized, and the surface states related to ferromagnetism is further revealed.

Magnetic Transition in Monolayer VSe2 via Interface Hybridization.

Magnetism in monolayer (ML) VSe2 has attracted broad interest in spintronics, while existing reports have not reached consensus. Using element-specific X-ray magnetic circular dichroism, a magnetic transition in ML VSe2 has been demonstrated at the contamination-free interface between Co and VSe2. Through interfacial hybridization with a Co atomic overlayer, a magnetic moment of about 0.4 μB per V atom in ML VSe2 is revealed, approaching values predicted by previous theoretical calculations. Promotion of the ferromagnetism in ML VSe2 is accompanied by its antiferromagnetic coupling to Co and a reduction in the spin moment of Co. In comparison to the absence of this interface-induced ferromagnetism at the Fe/ML MoSe2 interface, these findings at the Co/ML VSe2 interface provide clear proof that the ML VSe2, initially with magnetic disorder, is on the verge of magnetic transition.

Evidence of Spin Frustration in a Vanadium Diselenide Monolayer Magnet.

Monolayer VSe2 , featuring both charge density wave and magnetism phenomena, represents a unique van der Waals magnet in the family of metallic 2D transition-metal dichalcogenides (2D-TMDs). Herein, by means of in situ microscopy and spectroscopic techniques, including scanning tunneling microscopy/spectroscopy, synchrotron X-ray and angle-resolved photoemission, and X-ray absorption, direct spectroscopic signatures are established, that identify the metallic 1T-phase and vanadium 3d1 electronic configuration in monolayer VSe2 grown on graphite by molecular-beam epitaxy. Element-specific X-ray magnetic circular dichroism, complemented with magnetic susceptibility measurements, further reveals monolayer VSe2 as a frustrated magnet, with its spins exhibiting subtle correlations, albeit in the absence of a long-range magnetic order down to 2 K and up to a 7 T magnetic field. This observation is attributed to the relative stability of the ferromagnetic and antiferromagnetic ground states, arising from its atomic-scale structural features, such as rotational disorders and edges. The results of this study extend the current understanding of metallic 2D-TMDs in the search for exotic low-dimensional quantum phenomena, and stimulate further theoretical and experimental studies on van der Waals monolayer magnets.

Electric field effect on magnetism in a MgO/Pd/Co system with a solid-state capacitor structure

The electric field effect on the magnetism in a MgO/Pd/Co system, in which a magnetic moment is induced in the Pd layer owing to the ferromagnetic proximity effect, has been investigated using various experimental methods. An electric field was applied to the surface of the Pd layer through a solid-state HfO2/MgO dielectric bilayer by applying a gate voltage with a back-gating configuration. Changes in the magnetic properties of the system as a result of gate voltage application were detected using magnetization and polar-Kerr effect measurements as well as X-ray absorption and X-ray magnetic circular dichroism (XMCD) spectroscopies. A systematic change in the magnetic moment of the system by the application of a gate voltage is observed. The magnetic hysteresis loops obtained by the polar-Kerr effect measurement and the element-specific XMCD signal at the Co L3-edge clearly show a reproducible change in the coercivity that is dependent on the gate voltage.

Intrinsic and Extrinsic Ferromagnetism in Co-Doped Indium Tin Oxide Revealed Using X-Ray Magnetic Circular Dichroism

The effects of high-temperature annealing on ferromagnetic Co-doped Indium Tin Oxide (ITO) thin films have been investigated using X-ray diffraction (XRD), magnetometry, and X-Ray Magnetic Circular Dichroism (XMCD). Following annealing, the magnetometry results indicate the formation of Co clusters with a significant increase in the saturation magnetization of the thin films arising from defects introduced during cluster formation. However, sum rule analysis of the element-specific XMCD results shows that the magnetic moment at the Co sites is reduced after annealing. The effects of annealing demonstrate that the ferromagnetism observed in the as-deposited Co-doped ITO films arises from intrinsic defects and cannot be related to the segregation of metallic Co clusters.

Influence of the Fe-Co ratio on the exchange coupling in TbFeCo/[Co/Pt] heterostructures

We report on a systematic study of exchange coupled heterostructures, consisting of ferromagnetic [Co/Pt] multilayers and ferrimagnetic (FI) $\mathrm{T}{\mathrm{b}}_{x}\mathrm{F}{\mathrm{e}}_{100\ensuremath{-}x\ensuremath{-}y}\mathrm{C}{\mathrm{o}}_{y}$ (20 nm) alloy thin films with varying composition exhibiting strong perpendicular magnetic anisotropy. In particular, the impact of the Tb content and ratio of Fe and Co of the amorphous FI alloy on the exchange interaction at the interface was investigated. In this paper, the magnetic properties of single ternary TbFeCo thin films were analyzed in a broad composition range and compared to coupled TbFeCo/[Co/Pt] heterostructures. While a rather linear dependence of the exchange coupling strength was observed for Fe/Co-dominated ferrimagnets with increasing amount of Co, a nonlinear behavior is observed for Tb-dominated alloys. The latter behavior is governed by the variation of the exchange stiffness of the ferrimagnet. Additionally, by using element-specific x-ray magnetic circular dichroism measurements, the thickness of the interface domain wall (IDW) in the ferrimagnet, which is formed during the reversal of the ferromagnet, can be extracted. An inverse correlation between the IDW thickness and the exchange coupling strength at the interface was deduced.

Probing the Buried Magnetic Interfaces

Understanding magnetism in ferromagnetic metal/semiconductor (FM/SC) heterostructures is important to the development of the new-generation spin field-effect transistor. Here, we report an element-specific X-ray magnetic circular dichroism study of the interfacial magnetic moments for two FM/SC model systems, namely, Co/GaAs and Ni/GaAs, which was enabled using a specially designed FM1/FM2/SC superstructure. We observed a robust room temperature magnetization of the interfacial Co, while that of the interfacial Ni was strongly diminished down to 5 K because of hybridization of the Ni d(eg) and GaAs sp(3) states. The validity of the selected method was confirmed by first-principles calculations, showing only small deviations (<0.02 and <0.07 μB/atom for Co/GaAs and Ni/GaAs, respectively) compared to the real FM/SC interfaces. Our work proved that the electronic structure and magnetic ground state of the interfacial FM2 is not altered when the topmost FM2 is replaced by FM1 and that this model is applicable generally for probing the buried magnetic interfaces in the advanced spintronic materials..

The influence of Nd dopants on spin and orbital moments in Nd-doped permalloy thin films

Magnetic properties of NdX-Ni80Fe20(1−X) thin films have been investigated using x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at room temperature. With the Nd concentration increasing, the ratio of orbital-to-spin moment of Ni and Fe increases significantly, indicating that the spin-orbit coupling in permalloy thin films is enhanced due to the Nd impurities. The spin and orbital moments have been obtained by the sum rules analysis, which shows that the Nd impurities lead to a strong dispersion of spin moments of Fe and Ni while have no effect on orbital moments in Nd-doped permalloy thin films. Element-specific XMCD hysteresis loops suggest an antiferromagnetic coupling between the magnetic moments of Nd and permalloy at room temperature. The static magnetic properties have been studied by vibrating sample magnetometer for comparison, which shows a nice agreement with the XMCD results.

A hard X-ray nanospectroscopy station at SPring-8 BL39XU

We upgraded the hard X-ray spectroscopy beamline BL39XU at SPring-8 by installing Kirkpatrick-Baez mirrors to equip it with submicron-focusing capability. At the new experimental end-station located 74 m from the undulator source, an X-ray nanoprobe with a size of 120 × 100 nm2 has become available at 5–16 keV. This upgrade is fully compatible with the X-ray polarization tunability and helicity-switching between right- and left-circular polarizations; these are unique features of the beamline, which is equipped with a diamond X-ray phase retarder. A spatial resolution of ≈100 nm for X-ray absorption fine structure and X-ray magnetic circular dichroism (XMCD) spectroscopy has been achieved. We show a magnetization reversal process of an individual magnetic dot in bit-patterned perpendicular recording media using element-specific XMCD magnetization measurements to demonstrate the performance of the new station.

X-ray magnetic circular dichroism of ferromagnetic Co4N epitaxial films on SrTiO3(001) substrates grown by molecular beam epitaxy

5-nm thick Co4N layers capped with 3-nm thick Au layers were grown epitaxially on SrTiO3(001) substrates by molecular beam epitaxy using solid Co and a radio-frequency NH3 plasma. Spin and orbital magnetic moments of the Co4N layers were estimated using x-ray magnetic circular dichroism (XMCD) measurements at 300 K. The site-averaged Co 3d spin magnetic moment is evaluated to be about 1.4 μB, which is smaller than that predicted theoretically (1.58 μB). The element-specific XMCD intensities for the Co L3 edge and N K edge show that the magnetic moment is induced at the N atoms.

Interplay between Size, Composition, and Phase Transition of Nanocrystalline Cr3+-Doped BaTiO3 as a Path to Multiferroism in Perovskite-Type Oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.

Direct probing magnetization reversal of exchange-coupled-composite media by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) was used to directly probe the depth-dependent magnetization reversal of CoPtCr-SiO2-based exchange-coupled-composite media with laminated soft layers. A thin Fe-marker layer in the soft layer was used as the indicator of local magnetization. Element-specific XMCD loops of Fe-marker layers confirmed the transition of the magnetization reversal from rigid magnets to exchange-spring magnets with increasing thickness of the soft layer. The micromagnetic simulations revealed the importance of the reduced exchange constant (Asoft) by laminating the soft layer for domain-wall assisting reversal. By comparing XMCD loops with simulations, we can deduce the effective Asoft.