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X-ray magnetic circular dichroism (XMCD), which by virtue of the sum rules provides element-specific spin and orbital moments, is obtained from the difference between two polarized spectra by reversing the direction of either the light helicity or the applied magnetic field. Usually, it is tacitly assumed that these two spectra are obtained using the same absolute degree of light and magnetic polarization. This is, however, not always possible and depends on circumstances that can be beyond control. First, we recapitulate the conventional XMCD sum rule method to obtain the values of the moments and emphasize some of the complications in the case of the rare-earthM4,5edges, such as the presence of strong core-holejjoverlap, linear dichroism, and magnetic dipole term⟨Tz⟩. Instead, we propose an alternative method. Using the individual polarized x-ray absorption spectra obtained at the Ho and DyM5edges, where each of theΔJ=−1,0, and+1transitions are separated by∼2eV in photon energy, we are able to determine independently the degree of circular dichroism in a single spectrum. Since light is a transverse wave, we need to include, apart from the circular dichroism, also a linear dichroism contribution in order to fit the circularly polarized spectra. In the measurements on paramagnetic rare-earth dopants it was found that reversing the field produces the same degree of circular dichroism, while reversing the helicity yields a∼20% difference in the degree of circular dichroism.

The ability to control the in-plane magnetic anisotropy of a thin film is important for magnetic device applications. One way of accomplishing this task is by glancing angle deposition (GLAD). In this study, thin Co layers have been deposited using GLAD magnetron sputtering on MgO(001) and MgO(110) substrates. For Co films on MgO(001), the in-plane anisotropy direction can be directly controlled via the deposition angle. In contrast, for Co on MgO(110), the anisotropy due to the deposition angle is competing with the anisotropy induced by the substrate, while the growth parameters determine which contribution dominates. On the other hand, while on MgO(001) the deposition angle as well as the film thickness affect the strength of the Co in-plane anisotropy, no influence of these parameters on the magnetic properties is found for films on MgO(110).

Resonant inelastic x-ray scattering (RIXS) using an incident energy tuned to the uranium N 4,5 absorption edges is reported from epitaxial films of α-U3O8 and UN. Theory shows that for U3O8 the multiplets associated with a 5f 1 configuration with a ground state of 2 F 5/2 and the excited state of 2 F 7/2 are observed. However, the strong transition predicted at a transfer energy of 1.67 eV is not observed. We assume this is a consequence of the intermediate state lifetime broadening due to interaction with continuum states when the transferred energy exceeds the onset of the continuum in the presence of the core hole. This hypothesis is supported by the results obtained for the 5f-itinerant system UN, where no sharp transitions have been observed, although the broad scattering response centered at ∼ 1 eV is considered a signature of a predominantly 5f 3 configuration in this band-like semi-metallic system. These experiments and theory add important information on these materials, both of which have been investigated since the 1960s, as well as whether RIXS at the uranium N edge can become a valuable tool for actinide research.

We identify a three-dimensional skyrmion sideface state in chiral magnets that consists of a thin layer of modulated surface spirals and an array of phase-locked skyrmion screws. Such chiral spin structures lead to a characteristic ‘X’-shaped magnetic diffraction pattern in resonant elastic x-ray scattering, reminiscent of Photo 51 of the DNA double helix diffraction. By measuring both thin plates and bulk Cu2OSeO3 crystals in the field-in-plane geometry, we unambiguously identified the modulated skyrmion strings by retrieving their chirality and helix angle. The breaking of the translational symmetry along the sidefaces suppresses the bulk-favored conical state, providing a stabilization mechanism for the skyrmion lattice phase that has been overlooked so far.

Element-specific spectroscopies using synchrotron-radiation can provide unique insights into materials properties. The recently developed technique of X-ray detected ferromagnetic resonance (XFMR) allows studying the magnetization dynamics of magnetic spin structures. Magnetic sensitivity in XFMR is obtained from the X-ray magnetic circular dichroism (XMCD) effect, where the phase of the magnetization precession of each magnetic layer with respect to the exciting radio frequency is obtained using stroboscopic probing of the spin precession. Measurement of both amplitude and phase response in the magnetic layers as a function of bias field can give a clear signature of spin-transfer torque (STT) coupling between ferromagnetic layers due to spin pumping. Over the last few years, there have been new developments utilizing X-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz frequency range. The techniques of diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) provide novel ways for the probing of the dynamics of chiral and multilayered magnetic materials, thereby opening up new pathways for the development of high-density and low-energy consumption data processing solutions., 10 pages, 5 figures

Owing to the unique chemical and electronic properties arising from 3d‐electrons, substitution with transition metal ions is one of the key routes for engineering new functionalities into materials. While this approach has been used extensively in complex metal oxide perovskites, metal halide perovskites have largely resisted facile isovalent substitution. In this work, it is demonstrated that the substitution of Co2+ into the lattice of methylammonium lead triiodide imparts magnetic behavior to the material while maintaining photovoltaic performance at low concentrations. In addition to comprehensively characterizing its magnetic properties, the Co2+ ions themselves are utilized as probes to sense the local electronic environment of Pb in the perovskite, thereby revealing the nature of their incorporation into the material. A comprehensive understanding of the effect of transition metal incorporation is provided, thereby opening the substitution gateway for developing novel functional perovskite materials and devices for future technologies.

In recent years, interest in expanding from 2D to 3D systems has grown in the magnetism community, from exploring new geometries to broadening the knowledge on the magnetic textures present in thick samples, and with this arise the need for new characterization techniques, in particular tomographic imaging. Here, we present a new tomographic technique based on Fourier transform holography, a lensless imaging technique that uses a known reference in the sample to retrieve the object of interest from its diffraction pattern in one single step of calculation, overcoming the phase problem inherent to reciprocal-space-based techniques. Moreover, by exploiting the phase contrast instead of the absorption contrast, thicker samples can be investigated. We obtain a 3D full-vectorial image of a 800 nm-thick extended Fe/Gd multilayer in a 5$\mu$m-diameter circular field of view with a resolution of approximately 80 nm. The 3D image reveals worm-like domains with magnetization pointing mostly out of plane near the surface of the sample but that falls in-plane near the substrate. Since the FTH setup is fairly simple, it allows modifying the sample environment. Therefore, this technique could enable in particular a 3D view of the magnetic configuration's response to an external magnetic field., Comment: 11 pages, 8 figures

An escalating challenge in condensed-matter research is the characterization of emergent order-parameter nanostructures such as ferroelectric and ferromagnetic skyrmions. Their small length scales coupled with complex, three-dimensional polarization or spin structures makes them demanding to trace out fully. Resonant elastic x-ray scattering (REXS) has emerged as a technique to study chirality in spin textures such as skyrmions and domain walls. It has, however, been used to a considerably lesser extent to study analogous features in ferroelectrics. Here, we present a framework for modeling REXS from an arbitrary arrangement of charge quadrupole moments, which can be applied to nanostructures in materials such as ferroelectrics. With this, we demonstrate how extended reciprocal space scans using REXS with circularly polarized x rays can probe the three-dimensional structure and chirality of polar skyrmions. Measurements, bolstered by quantitative scattering calculations, show that polar skyrmions of mixed chirality coexist, and that REXS allows valuation of relative fractions of right- and left-handed skyrmions. Our quantitative analysis of the structure and chirality of polar skyrmions highlights the capability of REXS for establishing complex topological structures toward future application exploits. M. R. M. and R. R. were supported by the Quantum Materials program from the Office of Basic Energy Sciences, U.S. Department of Energy (DE-AC02-05CH11231). V. A. S., J. W. F., and L. W. M. acknowledge the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC-0012375 for support to study complex-oxide heterostructure with x-ray scattering. L. W. M. and R. R. acknowledge partial support from the Army Research Office under the ETHOS MURI via cooperative agreement W911NF-21-2-0162. J. Í. acknowledge financial support from the Luxembourg National Research Fund through project FNR/C18/MS/12705883/REFOX. Diamond Light Source, UK, is acknowledged for beam time on beam line I10 under proposal NT24797. K. T. K., S. Y. P., and D. R. L. acknowledge support from the National Research Foundation of Korea, under Grant No. NRF-2020R1A2C1009597, NRF-2019K1A3A7A09033387, and NRF-2021R1C1C1009494. M. A. P. G. acknowledges support by the Czech Science Foundation (Project No. 19-28594X). This research used resources of the Advanced Light Source, a U.S. DOE Office of Science User Facility under Contract No. DE-AC02-05CH11231. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility at Argonne National Laboratory and is based on research supported by the U.S. DOE Office of Science-Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. S. D. gratefully acknowledges a start-up grant from Indian Institute of Science, Bangalore, India. F. G.-O., P. G.-F., and J. J. acknowledge financial support from Grant No. PGC2018-096955-B-C41 funded by MCIN/AEI/10.13039/501100011033 and by ERDF “A way of making Europe,” by the European Union. F. G.-O. acknowledges financial support from Grant No. FPU18/04661 funded by MCIN/AEI/10.13039/501100011033

All-optical switching of magnetization has great potential for use in future ultrafast and energy efficient nanoscale magnetic storage devices. So far, research has been almost exclusively focused on rare-earth based materials, which limits device tunability and scalability. Here, we show that a perpendicularly magnetized synthetic ferrimagnet composed of two distinct transition metal ferromagnetic layers, Ni3Pt and Co, can exhibit helicity independent magnetization switching. Switching occurs between two equivalent remanent states with antiparallel alignment of the Ni3Pt and Co magnetic moments and is observable over a broad temperature range. Time-resolved measurements indicate that the switching is driven by a spin-polarized current passing through the subnanometer Ir interlayer. The magnetic properties of this model system may be tuned continuously via subnanoscale changes in the constituent layer thicknesses as well as growth conditions, allowing the underlying mechanisms to be elucidated and paving the way to a new class of data storage devices.

Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe

The study of magnetic order in few- and monolayer van der Waals materials poses a challenge to the most commonly employed magnetic characterization techniques as they normally lack magnetic sensitivity and/or lateral resolution enabling their thickness-dependent probing. Here we demonstrate the usefulness of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr L 2 , 3 and Te M 5 edges, for the study of the ferromagnetic semiconductor CrSiTe3 (CST) in the form of single- and few-layer flakes. By scanning the sample under the incident x-ray beam, a map of the exfoliated system was obtained, which reproduced the optical micrographs showing the detailed distribution and thicknesses of the flakes. In this way, XAS/XMCD was performed at selected sample areas, revealing the thickness-resolved spectroscopic and magnetic properties of the flakes, such as the spin and orbital magnetic moments. The spin moment, in line with the saturation field, is decreasing with film thickness, revealing a single-domain and out-of-plane magnetization for the thinnest films. For CST, the electronic properties are governed by the strong covalent bond between the Cr 3d(e g ) and Te 5p states, giving rise to a superexchange scenario. We observed a gradually increasing ratio of orbital to spin moment for thinner flakes, which could be due to a further increase of the covalent mixing. Hysteresis loops were recorded at the Cr L 3 edge, showing an open loop for 10 down to ∼3 layers, while the bulk shows a wasp-waist shaped loop. With the transition temperature from the soft to the hard ferromagnetic state decreasing with thickness, the monolayer shows a narrowed, closed loop at 10 K, suggesting its transition temperature > 10 K. Our study demonstrates the unique capabilities of XAS/XMCD for the study of few-layer van der Waals magnets, highlighting the interplay between electron correlation and ferromagnetism in CST.

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 NB 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.

The topological surface states (TSSs) in topological insulators (TIs) offer exciting prospects for dissipationless spin transport. Common spin-based devices, such as spin valves, rely on trilayer structures in which a non-magnetic layer is sandwiched between two ferromagnetic (FM) layers. The major disadvantage of using high-quality single-crystalline TI films in this context is that a single pair of spin-momentum locked channels spans across the entire film, meaning that only a very small spin current can be pumped from one FM to the other, along the side walls of the film. On the other hand, using nanocrystalline TI films, in which the grains are large enough to avoid hybridization of the TSSs, will effectively increase the number of spin channels available for spin pumping. Here, we used an element-selective, x-ray based ferromagnetic resonance technique to demonstrate spin pumping from a FM layer at resonance through the TI layer and into the FM spin sink.

A major challenge in topological magnetism lies in the three-dimensional (3D) exploration of their magnetic textures. A recent focus has been the question of how 2D skyrmion sheets vertically stack to form distinct types of 3D topological strings. Being able to manipulate the vertical coupling should therefore provide a route to the engineering of topological states. Here, we present a new type of axially bound magnetic skyrmion string state in which the strings in two distinct materials are glued together across their interface. Using quasi-tomographic resonant elastic x-ray scattering, the 3D skyrmion profiles before and after their binding across the interface were unambiguously determined and compared. Their attractive binding is accompanied by repulsive twisting, i.e., the coupled skyrmions mutually affect each other via a compensating twisting. This state exists in chiral magnet-magnetic thin film heterostructures, providing a new arena for the engineering of 3D topological phases.

Biogenic nanoscale vanadium magnetite is produced by converting V(V)-bearing ferrihydrites through reductive transformation using the metal-reducing bacterium Geobacter sulfurreducens. With increasing vanadium in the ferrihydrite, the amount of V-doped magnetite produced decreased due to V-toxicity which interrupted the reduction pathway ferrihydrite–magnetite, resulting in siderite or goethite formation. Fe L2,3 and V L2,3 X-ray absorption spectra and data from X-ray magnetic circular dichroism analysis revealed the magnetite to contain the V in the Fe(III) Oh site, predominately as V(III) but always with a component of V(VI), present a consistent V(IV)/V(III) ratio in the range 0.28 to 0.33. The bacteriogenic production of V-doped magnetite nanoparticles from V-doped ferrihydrite is confirmed and the work reveals that microbial reduction of contaminant V(V) to V(III)/V(IV) in the environment will occur below the Fe-redox boundary where it will be immobilised in biomagnetite nanoparticles.

Description, Nonlinear optics reveal how the chirality of ferroelectric vortices can be deterministically controlled using an electric field., Polar textures have attracted substantial attention in recent years as a promising analog to spin-based textures in ferromagnets. Here, using optical second-harmonic generation–based circular dichroism, we demonstrate deterministic and reversible control of chirality over mesoscale regions in ferroelectric vortices using an applied electric field. The microscopic origins of the chirality, the pathway during the switching, and the mechanism for electric field control are described theoretically via phase-field modeling and second-principles simulations, and experimentally by examination of the microscopic response of the vortices under an applied field. The emergence of chirality from the combination of nonchiral materials and subsequent control of the handedness with an electric field has far-reaching implications for new electronics based on chirality as a field-controllable order parameter.

Magnetic domain formation in two-dimensional (2D) materials gives perspectives into the fundamental origins of 2D magnetism and also motivates the development of advanced spintronics devices. However, the characterization of magnetic domains in atomically thin van der Waals (vdW) flakes remains challenging. Here, we employ X-ray photoemission electron microscopy (XPEEM) to perform layer-resolved imaging of the domain structures in the itinerant vdW ferromagnet Fe5GeTe2which shows near room temperature bulk ferromagnetism and a weak perpendicular magnetic anisotropy (PMA). In the bulk limit, we observe the well-known labyrinth-type domains. Thinner flakes, on the other hand, are characterized by increasingly fragmented domains. While PMA is a characteristic property of Fe5GeTe2, we observe a spin-reorientation transition with the spins canting in-plane for flakes thinner than six layers. Notably, a bubble phase emerges in four-layer flakes. This thickness dependence, which clearly deviates from the single-domain behavior observed in other 2D magnetic materials, demonstrates the exciting prospect of stabilizing complex spin textures in 2D vdW magnets at relatively high temperatures.

Collective spin excitations of ordered magnetic structures o er great potential for the development of novel spintronic devices. The present approach is to rely on micromagnetic models to explain the origins of dynamic modes observed by ferromagnetic resonance (FMR) studies, since experimental tools to directly reveal the origins of the complex dynamic behavior are lacking. Here we demonstrate a new approach which combines resonant magnetic x-ray diffraction with FMR, thereby allowing for a reconstruction of the real-space spin dynamics of the system. This new diffractive FMR (DFMR) technique builds on x-ray detected FMR (XFMR) that allows for element-selective dynamic studies, giving unique access to specific wave components of static and dynamic coupling in magnetic heterostructures. In combination with diffraction, FMR is elevated to the level of a modal spectroscopy technique, potentially opening new pathways for the development of spintronic devices.

A three-dimensional singular point that consists of two oppositely aligned emergent monopoles is identified in continuous CoTb thin films, as confirmed by complementary techniques of resonant elastic x-ray scattering, Lorentz transmission electron microscopy, and scanning transmission x-ray microscopy. This new type of topological defect can be regarded as a superposition of an emergent magnetic monopole and an antimonopole, around which the source and drain of the magnetic flux overlap in space. We experimentally prove that the observed spin twist seen in Lorentz transmission electron microscopy reveals the cross-section of the superimposed three-dimensional structure, providing a straightforward strategy for the observation of magnetic singularities. Such a quasi particle provides an excellent platform for studying the rich physics of emergent electromagnetism.

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., 7 pages, 5 figures

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral DWs after optical pumping in perpendicularly magnetized asymmetric multilayers, probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of hot spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW textures shortly after the laser pulse as a distortion of the homochiral N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a direction transverse to the DW. Our study highlights how time-resolved CD-XRMS can be a unique tool for studying the time evolution in other systems showing a non-collinear electric/magnetic ordering such as skyrmion lattices, conical/helical phases, as well as the recently observed antiskyrmion lattices, in metallic or insulating materials.

Author(s): Dąbrowski, M; Hicken, RJ; Frisk, A; Newman, DG; Klewe, C; N'Diaye, AT; Shafer, P; Van Der Laan, G; Hesjedal, T; Bowden, GJ | Abstract: Non-collinear spin structures in materials that combine perpendicular and in-plane magnetic anisotropies are of great technological interest for microwave and spin wave-assisted magnetization switching. [Co/Pt] multilayers are well-known perpendicular anisotropy materials that have the potential to pin the magnetization of a soft magnetic layer, such as permalloy (Py), that has in-plane anisotropy, thereby forming a magnetic exchange spring. Here we report on multilayered [Co/Pt]/Pt/Py films, where an additional ultrathin Pt spacer has been included to control the coupling between the sub-units with in-plane and perpendicular magnetic anisotropy. Vector network analyser (VNA)-ferromagnetic resonance (FMR) measurements were made to obtain a complete picture of the resonant conditions, while the dynamical response of the sub-units was probed by synchrotron-based element- and phase selective x-ray detected FMR (XFMR). For all samples, only slight pinning of the dynamic magnetization of the Py by the [Co/Pt] was noted, and the FMR results were dominated by the 50 nm thick Py layer. Out-of-plane VNA-FMR maps reveal the presence of additional modes, e.g. a perpendicular standing spin-wave (PSSW) state. However, as the magnetic field is reduced below the saturation field, the PSSW state morphs continuously through a series of canted standing spin-wave (CSSW) states into a horizontal standing spin-wave (HSSW) state. The PSSW, CSSW and HSSW states are well described using a multilayer model of the Py film. The observation of CSSW modes is of particular relevance to microwave assisted magnetic recording, where microwave excitation stimulates precession of a soft layer canted out of plane by a pulsed magnetic field.

A chiral bobber is a localized three-dimensional magnetization configuration, terminated by a singularity. Chiral bobbers coexist with magnetic skyrmions in chiral magnets, lending themselves to new types of skyrmion-complementary bits of information. However, the on-demand creation of bobbers, as well as their direct observation remained elusive. Here, we introduce a new mechanism for creating a stable chiral bobber lattice state via the proximity of two skyrmion species with comparable size. This effect is experimentally demonstrated in a Cu$_2$OSeO$_3$/[Ta/CoFeB/MgO]$_4$ heterostructure in which an exotic bobber lattice state emerges in the phase diagram of Cu$_2$OSeO$_3$. To unambiguously reveal the existence of the chiral bobber lattice state, we have developed a novel characterization technique, magnetic truncation rod analysis, which is based on resonant elastic x-ray scattering.

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their nontrivial topology through single skyrmions and ordered and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is the key to advancing topological magnetism. Here, a new, yet general, approach to the "printing" of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers is presented. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, a focused electron beam with a graphic pattern generator to "print" skyrmions is used, which is referred to as skyrmion lithography. This work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Engineering the anomalous Hall effect (AHE) in the emerging magnetic topological insulators (MTIs) has great potentials for quantum information processing and spintronics applications. In this letter, we synthesize the epitaxial Bi2Te3/MnTe magnetic heterostructures and observe pronounced AHE signals from both layers combined together. The evolution of the resulting hybrid AHE intensity with the top Bi2Te3 layer thickness manifests the presence of an intrinsic ferromagnetic phase induced by the topological surface states at the heterolayer-interface. More importantly, by doping the Bi2Te3 layer with Sb, we are able to manipulate the sign of the Berry phase-associated AHE component. Our results demonstrate the un-paralleled advantages of MTI heterostructures over magnetically doped TI counterparts, in which the tunability of the AHE response can be greatly enhanced. This in turn unveils a new avenue for MTI heterostructure-based multifunctional applications., Comment: 20 pages, 4 figures

Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension, and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic x-ray scattering measurements on Cu2OSeO3 with an in-plane magnetic field we find that a state of perpendicularly ordered skyrmions forms - in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined in a narrow region of the temperature-field phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface-guiding makes the robust state particular interesting for racetrack-like devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.

Insulating antiferromagnets are efficient and robust conductors of spin current. To realise the full potential of these materials within spintronics, the outstanding challenges are to demonstrate scalability down to nanometric lengthscales and the transmission of coherent spin currents. Here, we report the coherent transfer of spin angular momentum by excitation of evanescent spin waves of GHz frequency within antiferromagnetic NiO at room temperature. Using element-specific and phase-resolved x-ray ferromagnetic resonance, we probe the injection and transmission of ac spin current, and demonstrate that insertion of a few nanometre thick epitaxial NiO(001) layer between a ferromagnet and non-magnet can even enhance the flow of spin current. Our results pave the way towards coherent control of the phase and amplitude of spin currents at the nanoscale, and enable the realization of spin-logic devices and spin current amplifiers that operate at GHz and THz frequencies.

The combination of topological insulators, that is, bulk insulators with gapless, topologically protected surface states, with magnetic order is a love-hate relationship that can unlock new quantum states and exotic physical phenomena, such as the quantum anomalous Hall effect and axion electrodynamics. Moreover, the unusual coupling between topological insulators and ferromagnets can also result in the formation of topological spin textures in the ferromagnetic layer. Skyrmions are topologically protected magnetization swirls that are promising candidates for spintronics memory carriers. Here, we report on the observation of skyrmionium in thin ferromagnetic films coupled to a magnetic topological insulator. The occurrence of skyrmionium, which appears as a soliton composed of two skyrmions with opposite winding numbers, is tied to the ferromagnetic state of the topological insulator. Our work presents a new combination of two important classes of topological materials and may open the door to new topologically inspired information-storage concepts in the future.

X-ray detected ferromagnetic resonance (XFMR) has recently emerged as a powerful synchrotron-radiation-based tool able to study the element-selective magnetization dynamics. Magnetic and chemical contrast in XFMR is obtained by X-ray magnetic circular dichroism (XMCD), while the phase difference between the magnetization precessions is monitored using stroboscopic probing. A unique property of time-resolved XFMR is the visualization of the magnetization precession for each individual layer in a magnetic device. Measurement of the amplitude and phase response of the magnetic layers gives a clear signature of spin-transfer torque (STT) coupling between ferromagnetic layers due to spin pumping.

Magnetic skyrmions are topologically non-trivial nanoscale objects. Their topology, which originates in their chiral domain wall winding, governs their unique response to a motion-inducing force. When subjected to an electrical current, the chiral winding of the spin texture leads to a deflection of the skyrmion trajectory, characterised by an angle with respect to the applied force direction. This skyrmion Hall angle is predicted to be skyrmion diameter-dependent. In contrast, our experimental study finds that the skyrmion Hall angle is diameter-independent for skyrmions with diameters ranging from 35 to 825 nm. At an average velocity of 6 ± 1 ms−1, the average skyrmion Hall angle was measured to be 9° ± 2°. In fact, the skyrmion dynamics is dominated by the local energy landscape such as materials defects and the local magnetic configuration., Magnetic skyrmions are promising objects for future spintronic devices. However, a better understanding of their dynamics is required. Here, the authors show that in contrast to predictions the skyrmion Hall angle is independent of their diameter and motion is dominated by disorder and skyrmion-skyrmion interactions in the system.

Magnetite is a magnetic, Fe(II)-Fe(III)-mineral formed through abiogenic and biogenic pathways. It constitutes an attractive material for remediation due to its reactivity, large surface-area-to-volume ratio when present as nanoparticles, and magnetic recoverability. Magnetite can be repeatedly microbially oxidized or reduced, but it is unclear how this influences the reactivity of magnetite towards toxic metal or metalloid contaminants. In this study, magnetite (both abiogenic and biogenic) was exposed to microbial Fe(II) oxidation and Fe(III) reduction, before reacted with hexavalent chromium (Cr(VI)) or pentavalent arsenic (As(V)). Results showed microbial reduction of both magnetite types improved the removal rate of Cr(VI) from solution, though surprisingly microbial Fe(II)-oxidation also showed enhanced reactivity towards Cr(VI) compared to un-treated magnetite. Synchrotron based analysis confirmed the formation of Cr(III) at the surface of the magnetite. Reactivity with As was less dramatic and showed un-treated material was able to remove As(V) from solution faster than microbially Fe(III)-reduced and Fe(II)-oxidized magnetite. The presence of humic substances was also shown to lead to a decreased reactivity of biogenic and abiogenic magnetite towards As(V) and Cr(VI). Our results imply that Fe-metabolizing bacteria influence the immobilization of contaminants and should be considered when evaluating remediation schemes, especially where Fe-metabolizing bacteria are active.

Magnetism in monolayer (ML) VSe

A biskyrmion consists of two bound, topologically stable, skyrmion spin textures. These coffee-bean-shaped objects are observed in real space in thin plates using Lorentz transmission electron microscopy (LTEM). From LTEM imaging alone, it is not clear whether biskyrmions are surface-confined objects, or, analogous to skyrmions in noncentrosymmetric helimagnets, 3D tube-like structures in a bulk sample. Here, the biskyrmion form factor is investigated in single- and polycrystalline-MnNiGa samples using small-angle neutron scattering. It is found that biskyrmions are not long-range ordered, not even in single crystals. Surprisingly all of the disordered biskyrmions have their in-plane symmetry axis aligned along certain directions, governed by the magnetocrystalline anisotropy. This anisotropic nature of biskyrmions may be further exploited to encode information.

Ferromagnet/two-dimensional transition-metal dichalcogenide (FM/2D TMD) interfaces provide attractive opportunities to push magnetic information storage to the atomically thin limit. Existing work has focused on FMs contacted with mechanically exfoliated or chemically vapor-deposition-grown TMDs, where clean interfaces cannot be guaranteed. Here, we report a reliable way to achieve contamination-free interfaces between ferromagnetic CoFeB and molecular-beam epitaxial MoSe2. We show a spin reorientation arising from the interface, leading to a perpendicular magnetic anisotropy (PMA), and reveal the CoFeB/2D MoSe2 interface allowing for the PMA development in a broader CoFeB thickness-range than common systems such as CoFeB/MgO. Using X-ray magnetic circular dichroism analysis, we attribute generation of this PMA to interfacial d-d hybridization and deduce a general rule to enhance its magnitude. We also demonstrate favorable magnetic softness and considerable magnetic moment preserved at the interface and theoretically predict the interfacial band matching for spin filtering. Our work highlights the CoFeB/2D MoSe2 interface as a promising platform for examination of TMD-based spintronic applications and might stimulate further development with other combinations of FM/2D TMD interfaces.

Magnetic skyrmions are particle-like, topologically protected magnetization entities that are promising candidates for information carriers in racetrack-memory schemes. The transport of skyrmions in a shift-register-like fashion is crucial for their embodiment in practical devices. Recently, we demonstrated experimentally that chiral skyrmions in Cu2OSeO3 can be effectively manipulated by a magnetic field gradient, leading to a collective rotation of the skyrmion lattice with well-defined dynamics in a radial field gradient. Here, we employ a skyrmion particle model to numerically study the effects of resultant shear forces on the structure of the skyrmion lattice. We demonstrate that anisotropic peak broadening in experimentally observed diffraction patterns can be attributed to extended linear regions in the magnetic field profile. We show that topological (5-7) defects emerge to protect the six-fold symmetry of the lattice under the application of local shear forces, further enhancing the stability of proposed magnetic field driven devices.

Elemental tin in the α-phase is an intriguing member of the family of topological quantum materials. In thin films, with decreasing thickness, α-Sn transforms from a three-dimensional (3D) topological Dirac semimetal (TDS) to a two-dimensional (2D) topological insulator (TI). Getting access to and making use of their topological surface states is challenging and requires interfacing to a magnetically ordered material. Here, we report the successful epitaxial growth of α-Sn thin films on Co, forming the core of a spin-valve structure. We carried out time- and element-selective ferromagnetic resonance experiments to investigate the presence of spin pumping through the spin-valve structure. We applied a rigorous statistical analysis of the experimental data using a Landau-Lifshitz-Gilbert-Slonczewski equation based model. A strong exchange coupling contribution was found, however no unambiguous proof for spin pumping. Nevertheless, the incorporation of α-Sn into a spin-valve remains a promising approach given its simplicity as an elemental TI and its room temperature application potential.

International audience; Time-averaged images of the magnetization within single-layer spin-transfer oscillators have been obtained using the holography with extended reference by autocorrelation linear differential operator technique. Transport measurements on a Pd(5)-Cu(20)-Ni81Fe19(7)-Cu(2)-Pd(2) (in nanometers) stack with a 100 nm diameter nanocontact reveal the presence of vortex dynamics. Magnetic images of the device for injected current values of 24 and -24 mA suggest that a vortex has been ejected from the nanocontact and become pinned at the edge of the region that is visible through the Au mask.