xpcs, magnetic material, fluctuation

The influence of an applied magnetic field on the collective dynamics of novel anisotropic colloidal particles whose shape resembles peanuts is reported. Being made up of hematite cores and silica shells, these micrometer-sized particles align in a direction perpendicular to the applied external magnetic field, and assemble into chains along the field direction. The anisotropic dynamics of these particles is investigated using multispeckle ultrasmall-angle X-ray photon correlation spectroscopy (USA-XPCS). The results indicate that along the direction of the magnetic field, the particle dynamics strongly depends on the length scale probed. Here, the relaxation of the intermediate scattering function follows a compressed exponential behavior at large distances, while it appears diffusive at distances comparable or smaller than the particle size. Perpendicular to the applied field (and along the direction of gravity), the experimental data can be quantitatively reproduced by a combination of an advective term originating from sedimentation and a purely diffusive one that describes the thermal diffusion of the assembled chains and individual particles.

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Europium (Eu) metal has a body centered cubic crystal structure which, upon a paramagnetic-to-helical magnetic phase transition, undergoes a body centered tetragonal distortion. The magnetic helix appears below a Néel temperature (TN) of ∼90 K, and an applied magnetic field gives rise to conical magnet structure. We have prepared Eu metal thin films on Si (001) substrates using Eu metal as a target by pulsed laser deposition and studied the transport properties by a four-probe method. The resistance shows a sudden slope change at TN of 88 K. The magnetoresistance (MR) is positive at temperatures below 30 K and exhibits negative values above that. Our analyses show that the positive MR at low temperatures originates from magnetic field induced spin fluctuation, and the negative MR at higher temperature is a result of suppression of critical spin fluctuation of the Eu spins by the magnetic field. The Eu film also shows hysteretic MR behaviors in mid field range, which is a result of re-distribution of the helical antiferromagnetic domains by the magnetic fields. We have also studied the transverse magnetotransport in the Eu thin films. The observed anomalous Hall effect is believed to be associated with the magnetic moment induced by the field or due to the helical spin structure of Eu itself.

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In ice, it is well known that the orientation of H2O molecules is disordered by geometrical frustration. Ice-analogous materials having a pyrochlore lattice display interesting phenomena such as the spin-ice state and the magnetic monopole. In the spinel titanate MgTi2O4, the Ti ions have a quantum spin in the pyrochlore lattice. The Ti ions are displaced, accompanied by the spin-singlet formation. Since this displacement pattern follows the ice rule, the title compound is a material analogous to ice. When a small quantity of Ti ions are replaced with Mg ions, the ice-type structural fluctuation exists. In this structural ice-type state, the spins are also fluctuating at a nanosecond scale down to 0.3 K. We ascribed this phenomenon to the gapless frustrated random-singlet state, in which the spin-singlet pairs are resonating, and the orphan spins are hopping.

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Recently, altermagnetism (AM) has emerged as a new category of magnetism, alongside conventional antiferromagnetism (AFM) and ferromagnetism (FM). In an AM, superconductivity (SC) is faced with a dilemma that the spin-polarized bands, induced by the broken time reversal (T ) symmetry, dominantly supports spin-triplet pairing. In contrast, AM spin fluctuations routinely facilitate spin-singlet pairing as in AFM. Consequently, unconventional SC is either absent or weak in AM materials. Here, we propose that stacking 2D AM materials could resolve this dilemma. Stacked 2D materials have yielded a variety of new electronic properties by altering the symmetries inherent in the monolayer. In a 2D anisotropic Hubbard model, we investigate the general energy dispersions of both single-layer and stacked AM materials. We demonstrate that AM sheet stacking can alter the original symmetries, consequently affecting the energy dispersion. The interlayer magnetic coupling enhances the low q magnetic fluctuations. T symmetry is restored in the AA stacking with an antiferromagnetic interlayer coupling, and then both the energy dispersion and pairing interaction are in favor of spin-singlet SC. The ferromagnetic interlayer coupling in the AB stacking not only recovers T symmetry but also supports spin-triplet pairing. It is further anticipated that twisted bilayer AM sheets could exhibit additional novel electronic properties, including topology, flat bands, and collective excitations. Our work illustrates that stacking sheets of AM materials could open up a unique research domain in exploring novel quantum phenomena and offer a fertile ground for potential electronic applications.

While significant magnetic interactions exist in lithium transition metal oxides, commonly used as Li-ion cathodes, the interplay between magnetic couplings, disorder, and redox processes remains poorly understood. In this work, we focus on the high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) cathode as a model system on which to apply a computational framework that uses first principles-based statistical mechanics methods to predict the finite temperature magnetic properties of materials and provide insights into the complex interplay between magnetic and chemical degrees of freedom. Density functional theory calculations on multiple distinct Ni–Mn orderings within the LNMO system, including the ordered ground-state structure (space group P4332), reveal a preference for a ferrimagnetic arrangement of the Ni and Mn sublattices due to strong antiferromagnetic superexchange interactions between neighboring Mn4+ and Ni2+ ions and ferromagnetic Mn–Mn and Ni–Ni couplings, as revealed by magnetic cluster expansions. These results are consistent with qualitative predictions using the Goodenough-Kanamori-Anderson rules. Simulations of the finite temperature magnetic properties of LNMO are conducted using Metropolis Monte Carlo. We find that a “semiclassical” Monte Carlo sampling method based on the Heisenberg Hamiltonian accurately predicts experimental magnetic transition temperatures observed in magnetometry measurements. This study highlights the importance of a robust computational toolkit that accurately captures the complex chemomagnetic interactions and predicts finite temperature magnetic behavior to help analyze experimental magnetic and magnetic resonance spectroscopy data acquired ex situ and operando.

Competing interaction within magnetic materials leads to the formation of complex magnetic domain configurations such as stripe domains, labyrinthine domains, magnetic bubbles, and skyrmions, which are fingerprints of different magnetic materials. An effective tailoring of these competing interactions and the resultant magnetic domain configurations by extrinsically means is of current interest. Through depositing an interfacially asymmetric Ta/CoFeB/MgO/Ta multilayer on a rare-earth-doped yttrium iron garnet of composition SmLu: YIG film, we show that the magnetic domain configurations from the bottom SmLu: YIG film can be partly imprinted onto the top Ta/CoFeB/MgO/Ta multilayer. The formation of stochastic domain configurations, such as labyrinthine domains, parallel stripe domains, and magnetic bubbles in the Ta/CoFeB/MgO/Ta multilayer, is jointly studied by using polar magneto-optical Kerr effect microscope and anomalous Hall effect measurements. The underlying physics is attributed to the fluctuating interlayer dipole–dipole interaction that results in the partial imprinting, which could substantially modify the intrinsic magnetism of the top Ta/CoFeB/MgO/Ta multilayer and leading to the formation of stochastic domain configurations. Our results provide an effective approach for tailoring the competing interaction in magnetic materials and for application scenarios in which the formation of stochastic domain configurations is required.

Thulium iron garnet (Tm3Fe5O12, TmIG) is a promising material for next-generation spintronic and quantum technologies owing to its high Curie temperature and strong perpendicular magnetic anisotropy. However, conventional magnetometry techniques are limited by insufficient spatial resolution and sensitivity to probe local magnetic phase transitions and critical spin dynamics in thin films. In this study, we present the first quantitative investigation of local magnetic field fluctuations near the Curie temperature in TmIG thin films using nitrogen-vacancy (NV) center-based quantum sensing. By integrating optically detected magnetic resonance (ODMR) and NV spin relaxometry (T1 measurements) with macroscopic techniques such as SQUID magnetometry and Hall effect measurements, we systematically characterize both the static magnetization and dynamic spin fluctuations across the magnetic phase transition. Our results reveal a pronounced enhancement in NV spin relaxation rates near 360 K, providing direct evidence of critical spin fluctuations at the nanoscale. This work highlights the unique advantages of NV quantum sensors for investigating dynamic critical phenomena in complex magnetic systems and establishes a versatile, multimodal framework for studying local phase transition kinetics in high-temperature magnetic insulators.

Recent demonstrations of moiré magnetism, featuring exotic phases with noncollinear spin order in the twisted van der Waals (vdW) magnet chromium triiodide CrI3, have highlighted the potential of twist engineering of magnetic (vdW) materials. However, the local magnetic interactions, spin dynamics, and magnetic phase transitions within and across individual moiré supercells remain elusive. Taking advantage of a scanning single-spin magnetometry platform, here we report observation of two distinct magnetic phase transitions with separate critical temperatures within a moiré supercell of small-angle twisted double trilayer CrI3. By measuring temperature-dependent spin fluctuations at the coexisting ferromagnetic and antiferromagnetic regions in twisted CrI3, we explicitly show that the Curie temperature of the ferromagnetic state is higher than the Néel temperature of the antiferromagnetic one by ~10 K. Our mean-field calculations attribute such a spatial and thermodynamic phase separation to the stacking order modulated interlayer exchange coupling at the twisted interface of moiré superlattices. van der Waals magnets can be arranged into twisted heterostructures, with the twisting leading to the formation of new magnetic phases. Here, Li, Sun, and coauthors show via NVcentre based magnetometry small angle twisted double trilayer CrI3 exhibits a co-existing, hybrid magnetic phase with distinct phase transition temperatures.

Compounds of the new materials class LnTAl$_4$X$_2$ (Ln = lanthanide, X = tetrel, T = transition metal) host exotic magnetic phenomena due to geometric frustration induced by their triangular lattice. Complex spin arrangements, magnetic fluctuations and double magnetic transitions have been well observed by means of magneto-transport. Nevertheless, the experimental electronic structure of this family of materials has been poorly studied. We have investigated the experimental electronic structure of two members of this class of materials: SmAuAl$_4$Ge$_2$ and TbAuAl$_4$Ge$_2$. By means of Angle-Resolved PhotoEmission Spectroscopy (ARPES) accompanied by Density Functional Theory calculations (DFT), we reveal common trends and features, the important effect of localized spin moments on the electronic structure, the presence of surface-localized electronic states and the nature of the surface termination layer. Low-dimensionality, exchange interaction, and spin-orbit coupling are all important ingredients of the electronic structure.

Understanding the formation and dynamics of charge and spin-ordered states in low-dimensional transition metal oxide materials is crucial to understanding unconventional high-temperature superconductivity. La_{2-x}Sr_{x}NiO_{4+δ} (LSNO) has attracted much attention due to its interesting spin dynamics. Recent x-ray photon correlation spectroscopy studies have revealed slow dynamics of the spin order (SO) stripes in LSNO. Here, we applied resonant soft x-ray ptychography to map the spatial distribution of the SO stripe domain inhomogeneity in real space. The reconstructed images show the SO domains are spatially anisotropic, in agreement with previous diffraction studies. For the SO stripe domains, it is found that the correlation lengths along different directions are strongly coupled in space. Surprisingly, fluctuations were observed in the real space amplitude signal, rather than the phase or position. We attribute the observed slow dynamics of the stripe domains in LSNO to thermal fluctuations of the SO domain boundaries.

X-ray photon correlation spectroscopy (XPCS) is an emerging technique to characterize nanoscale dynamics in-situ. This is a powerful yet challenging technique, requiring many experimental conditions such as high coherence, high stability, and high coherent flux. These conditions are not directly available at most existing beamlines, such that XPCS can only be performed at a few beamlines with coherent x-ray beams. In this work, we performed a series of XPCS experiments at a scattering beamline not dedicated for coherent applications. By systematically study the effects of coherence, stability, and coherent flux, we demonstrate practical means to perform and optimize XPCS experiments. We hope this work can help to apply this emerging technique at more beamlines, in particular at existing beamlines with non-coherent x-ray beams.

Understanding and interpreting dynamics of functional materials in situ is a grand challenge in physics and materials science due to the difficulty of experimentally probing materials at varied length and time scales. X-ray photon correlation spectroscopy (XPCS) is uniquely well-suited for characterizing materials dynamics over wide-ranging time scales. However, spatial and temporal heterogeneity in material behavior can make interpretation of experimental XPCS data difficult. In this work, we have developed an unsupervised deep learning (DL) framework for automated classification of relaxation dynamics from experimental data without requiring any prior physical knowledge of the system. We demonstrate how this method can be used to accelerate exploration of large datasets to identify samples of interest, and we apply this approach to directly correlate microscopic dynamics with macroscopic properties of a model system. Importantly, this DL framework is material and process agnostic, marking a concrete step towards autonomous materials discovery. Editorial summary: Understanding non-equilibrium dynamics in materials is hindered by the difficulty of collecting and analyzing experimental data. Here, authors develop an machine learning framework for categorizing and tracking dynamics using time-resolved XPCS.

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The orientation behavior and the translational dynamics of spherical magnetic silica-nickel Janus colloids in an external magnetic field have been studied by small-angle X-ray scattering and X-ray photon correlation spectroscopy at ultra small-angles. For weak applied fields and at low volume fractions, the particle dynamics is dominated by Brownian motion even though the net magnetic moments of the individual particles are aligned in the direction of the field as indicated by the anisotropy in the small-angle scattering patterns. For higher fields the magnetic forces result in more complex structural changes with nickel caps of Janus particles pointing predominantly along the applied magnetic field. The alignment ultimately leads to chain-like configurations and the intensity-intensity autocorrelation functions, g2(q,t), show a second slower decay which becomes more pronounced at higher volume fractions. A direction dependent analysis of g2(q,t) revealed a faster than exponential decay perpendicular to the field which is related to the sedimentation of magnetically ordered domains. The corresponding velocity fluctuations could be decoupled from the diffusion of particles by decomposing g2(q,t) into advective and diffusive contributions. Finally, the particle dynamics becomes anisotropic at higher volume fractions and strong magnetic fields. The derived translational diffusion coefficients indicate slower particle dynamics perpendicular to the field as compared to the parallel direction.

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 for the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions among others. We report here on the behavior of chiral DWs at ultrashort timescale after optical pumping in perpendicularly magnetized asymmetric multilayers. The magnetization dynamics is 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 varying spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW texture shortly after the laser pulse as a distortion of the homochiral Néel shape toward a transient mixed Bloch-Néel-Bloch texture along a direction transverse to the DW. There is interest in encoding of information in complex spin structures present in magnetic systems, such as domain walls. Here, Léveillé et al study the ultrafast dynamics of chiral domain walls, and show the emergence of a transient spin chiral texture at the domain wall.

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The advance of magnetic nanotechnologies relies on detailed understanding of nanoscale magnetic mechanisms in materials. Magnetic domain memory (MDM), that is, the tendency for magnetic domains to repeat the same pattern during field cycling, is important for magnetic recording technologies. Here we demonstrate MDM in [Co/Pd]/IrMn films, using coherent X-ray scattering. Under illumination, the magnetic domains in [Co/Pd] produce a speckle pattern, a unique fingerprint of their nanoscale configuration. We measure MDM by cross-correlating speckle patterns throughout magnetization processes. When cooled below its blocking temperature, the film exhibits up to 100% MDM, induced by exchange-coupling with the underlying IrMn layer. The degree of MDM drastically depends on cooling conditions. If the film is cooled under moderate fields, MDM is high throughout the entire magnetization loop. If the film is cooled under nearly saturating field, MDM vanishes, except at nucleation and saturation. Our findings show how to fully control the occurrence of MDM by field cooling. The extent to which a material revisits previously accessed magnetization states under a cycling applied magnetic field has important implications for its use in memory technology. Here, the authors demonstrate the effects of field-cooling on magnetic domain memory in exchange-biased Co/Pd thin films.

Equilibrium phase transitions are influenced by fluctuations and often discussed within the framework of the Gibbs free energy, wherein the exchange of energy between system and thermal bath is stationary and all regions of the sample exhibit the same phase. Presence of spatial heterogeneity in the magnetic structures such as pinning centers, domain walls, topological defects, etc. may cause temporal heterogeneity that modifies the nature of the magnetic phase transition. This study reports that interplay of nanoscale thermodynamics with spatio‐temporal heterogeneity gives rise to complex phase transition pathways in amorphous FexGe1‐x thin films with temperature and Fe‐concentration (x). Coherent resonant soft X‐ray scattering experiments that have simultaneous spatial, temporal, and spectral sensitivity show that the origin of helical to paramagnetic phase transition in amorphous Fe‐Ge thin films lies in the appearance of enhanced‐fluctuation spots deep inside the ordered state. The fluctuations are heterogeneous, starting over a small fraction of the domains that increases and becomes isotropic over the entire film as the temperature increases or the Fe‐concentration decreases. The fluctuating‐fraction, when normalized to magnetization for different Fe‐concentrations, follows a single power law behavior, suggesting that the nature of the transition can be described in terms of the underlying spatio‐temporal fluctuations.

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We investigated surface nanostructures on an antiferromagnet MnBi2Te4 using a novel imaging technique, direct (real)-space and real time coherent x-ray imaging (direct-CXI). This technique has provided new insights into antiferromagnetic textures, including the formation of anti-phase antiferromagnetic (AFM) domains and thermal dynamics of AFM domains and domain walls. While this method produces real-space images of AFM textures without requiring a complex imaging retrieval process, its underlying imaging mechanism has not been fully understood, limiting a deep understanding of AFM textures and the information they contain. By investigating the well-defined structural characteristics of the nanostructures fabricated on MnBi2Te4, we elucidate the imaging principle of this novel technique. We find that the observed images can be well explained by the Fresnel diffraction integral. Using a simple model from classical optics, our calculations successfully reproduce the experimentally observed images of the nanostructures. This demonstrates that direct-CXI not only provides straightforward real-space imaging but also contains phase information through its Fresnel diffraction integral.

X-ray photoemission electron microscopy, one of the most successful imaging tools at synchrotrons, is known to have limitations related to the application of external fields and to the short electron mean free path. In order to overcome such issues, we adapt an existing XPEEM instrument to simultaneously perform coherent x-ray scattering measurements in reflectivity mode, thus adding a complementary method to XPEEM. Photon-in photon-out x-ray scattering provides the sensitivity to buried interfaces as well as the possibility to work under external fields, which is challenging when using charged particles for imaging. XPEEM, in turn, greatly alleviates the difficulties associated with the reconstruction methods used in coherent diffraction imaging. The combination of the two methods is demonstrated for an artifical spin-ice lattice showing both chemical and magnetic contrast.

Altermagnets, a unique class of magnetic materials that combines features of both ferromagnets and antiferromagnets, have garnered attention for their potential in spintronics and magnonics. While the electronic properties of altermagnets have been well studied, characterizing their magnon excitations is essential for fully understanding their behavior and enabling practical device applications. In this work, we introduce a measurement protocol combining resonant inelastic X-ray scattering with circular polarization and azimuthal scanning to probe the chiral nature of the altermagnetic split magnon modes in CrSb. This approach circumvents the challenges posed by domain averaging in macroscopic samples, allowing for precise measurements of the polarization and energy of the magnons in individual antiferromagnetic domains. Our findings demonstrate a pronounced circular dichroism in the magnon peaks, with an azimuthal dependence that is consistent with the theoretical predictions and the g-wave symmetry. By establishing a reliable and accessible method for probing altermagnetic magnons, this work opens new avenues for fundamental studies of these collective excitations and for developing next-generation magnonic device applications. Using circularly polarized inelastic X-ray scattering, the authors map spin-wave (magnon) excitations in the altermagnet CrSb and detect a reversible chiral signal for the first time, establishing a practical method to probe altermagnetic magnons.

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The 5d^{1} ordered double perovskites present an exotic playground for studying novel multipolar physics due to large spin-orbit coupling. We present Re L_{3} edge resonant inelastic x-ray scattering (RIXS) results that reveal the presence of the dynamic Jahn-Teller effect in the A_{2}MgReO_{6} (A=Ca, Sr, Ba) family of 5d^{1} double perovskites. The spin-orbit excitations in these materials show a strongly asymmetric line shape and exhibit substantial temperature dependence, indicating that they are dressed with lattice vibrations. Our experimental results are explained quantitatively through a RIXS calculation based on a spin-orbit-lattice entangled electronic ground state with the dynamic Jahn-Teller effect taken into consideration. We find that the spin-orbit-lattice entangled state is robust against magnetic and structural phase transitions as well as against significant static Jahn-Teller distortions. Our results illustrate the importance of including vibronic coupling for a complete description of the ground state physics of 5d^{1} double perovskites.

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In the present work a computational approach is applied to model and predict the results of X-ray resonant magnetic reflectometry – a non-destructive synchrotron-based technique to probe chemical composition, crystallographic environment and magnetization in multilayer epitaxial heterostructures with nanoscale depth resolution. The discussed 2D mapping approach is a step forward with respect to conventional resonant X-ray reflectometry and consists of collecting a fine step array of reflected intensity as a function of grazing angle and photon energy across the absorption edge of a particular chemical element. With the use of circularly polarized photons the method can be extended to magnetic systems to produce a map of dichroic reflectance directly related to the magnetization profile of the heterostructure. Studying the magnetic field dependence of dichroic reflectance maps can provide valuable information on the magnetization reversal of individual sublayers of a multilayer heterostructure. In the present paper modeling is performed for a bilayer system mimicking the behavior of a 30 nm ɛ-Fe2O3 thin film that is known to exhibit a pronounced two-component magnetic hysteresis. A technique to find optimal energy/angle combinations in order to sense magnetization of individual sublayers is proposed. Also discussed is the advantage of heavy-element capping, which leads to a substantial increase of the dichroic intensity oscillation contrast in the pre-edge region where the sensitivity to the magnetic behavior of the deeply buried interfaces is most pronounced.

The sensitivity of circularly polarized x-ray resonant magnetic scattering (CXRMS) to chiral asymmetry has been demonstrated. The study was performed on a 2D array of Permalloy (Py) square nanomagnets of 700 nm lateral size arranged in a chess pattern, in a square lattice of 1000 nm lattice parameter. Previous x-ray magnetic circular dichroism photoemission electron microscopy (XMCD-PEEM) images on this sample showed the formation of vortices at remanence and a preference in their chiral state. The magnetic hysteresis loops of the array along the diagonal axis of the squares indicate a non-negligible and anisotropic interaction between vortices. The intensity of the magnetic scattering using circularly polarized light along one of the diagonal axes of the square magnets becomes asymmetric in intensity in the direction transversal to the incident plane at fields where the vortex states are formed. The asymmetry sign is inverted when the direction of the applied magnetic field is inverted. The result is the expected in the presence of an unbalanced chiral distribution. The effect is observed by CXRMS due to the interference between the charge scattering and the magnetic scattering.

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The physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full‐field microscopy and resonant magnetic X‐ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin–orbit Mott insulator Sr2IrO4 with area over ≈0.1 mm2 and with spatial resolution as high as ≈150 nm. With the unprecedented wide field of views and high spatial resolution, an intertwining of two AF domains on a length comparable to the measured average AF domain wall width of 545 nm is revealed. This mesoscopic structure comprises a substantial portion of the sample surface, and thus can result in a macroscopic response unexpected from its microscopic magnetic structure. In particular, the symmetry analysis presented in this work shows that the inversion symmetry, which is preserved by the microscopic AF order, becomes ill‐defined at the mesoscopic length scale. This result underscores the importance of this novel technique for a thorough understanding of the physical properties of antiferromagnets.

X-ray resonant magnetic reflectometry is a synchrotron based non-destructive method of investigation of electronic and magnetic depth profiles in nanoscale multilayers. The choice of x-ray photon energy close to the absorption edges makes the method selective to oxidation state, crystallographic environment and magnetization of individual chemical elements. The enhanced technique dealing with acquisition and modelling of the angle-energy maps of resonant reflectance is discussed in the present contribution.

Altermagnets are a novel class of magnetic materials, where magnetic order is staggered both in coordinate and momentum space. The metallic rutile oxide RuO2, long believed to be a textbook Pauli paramagnet, recently emerged as a putative workhorse altermagnet when resonant X-ray and neutron scattering studies reported nonzero magnetic moments and long-range collinear order. While some experiments seem consistent with altermagnetism, magnetic order in RuO2 remains controversial. We show that RuO2 is nonmagnetic, both in bulk and thin film. Muon spectroscopy complemented by density-functional theory finds at most 1.14 × 10−4 μB/Ru in bulk and at most 7.5 × 10−4 μB/Ru in 11 nm epitaxial films, at our spectrometers’ detection limit, and dramatically smaller than previously reported neutron results that were used to rationalize altermagnetic behavior. Our own neutron diffraction measurements on RuO2 single crystals identify multiple scattering as the source for the false signal in earlier studies.

Significance The need for denser storage devices calls for new materials and nanostructures capable of confining single bits of information in a few nanometers. A new topological distribution of spins termed skyrmions is emerging, which promises to robustly confine a small magnetization in a few-nanometers-wide circular domain. A great deal of attention is being devoted to the understanding of these magnetic patterns and their manipulation. We manufactured a large nanoslice supporting over 70,000 skyrmions, and film their evolution in direct-space via cryo-Lorentz transmission electron microscopy. We reveal the octagonal distortion of the skyrmion lattice and show how these distortions and other defects impact its long-range order. These results pave the way to the control of a large two-dimensional array of skyrmions. Magnetic skyrmions are promising candidates as information carriers in logic or storage devices thanks to their robustness, guaranteed by the topological protection, and their nanometric size. Currently, little is known about the influence of parameters such as disorder, defects, or external stimuli on the long-range spatial distribution and temporal evolution of the skyrmion lattice. Here, using a large (7.3×7.3 μm2) single-crystal nanoslice (150 nm thick) of Cu2OSeO3, we image up to 70,000 skyrmions by means of cryo-Lorentz transmission electron microscopy as a function of the applied magnetic field. The emergence of the skyrmion lattice from the helimagnetic phase is monitored, revealing the existence of a glassy skyrmion phase at the phase transition field, where patches of an octagonally distorted skyrmion lattice are also discovered. In the skyrmion phase, dislocations are shown to cause the emergence and switching between domains with different lattice orientations, and the temporal fluctuation of these domains is filmed. These results demonstrate the importance of direct-space and real-time imaging of skyrmion domains for addressing both their long-range topology and stability.

Magnetic skyrmions in bulk materials are typically regarded as two-dimensional structures. However, they also exhibit three-dimensional configurations, known as skyrmion tubes, that elongate and extend in-depth. Understanding the configurations and stabilization mechanism of skyrmion tubes is crucial for the development of advanced spintronic devices. However, the generation and annihilation of skyrmion tubes in confined geometries are still rarely reported. Here, we present direct imaging of skyrmion tubes in nanostructured cuboids of a chiral magnet FeGe using Lorentz transmission electron microscopy (TEM), while applying an in-plane magnetic field. It is observed that skyrmion tubes stabilize in a narrow field-temperature region near the Curie temperature (Tc). Through a field cooling process, metastable skyrmion tubes can exist in a larger region of the field-temperature diagram. Combining these experimental findings with micromagnetic simulations, we attribute these phenomena to energy differences and thermal fluctuations. Our results could promote topological spintronic devices based on skyrmion tubes.

Magnetic skyrmion random switching and structural stability are critical limitations for storage data applications. Enhancing skyrmions’ magnetic properties could improve their thermal structural stability. Hence, micromagnetic calculation was carried out to explore the thermal nucleation and stability of skyrmions in magnetic nanodevices. Different magnetic properties such as uniaxial magnetic anisotropy energy (Ku), saturation magnetization (Ms) and Dzyaloshinskii—Moriya interaction (DMI) were used to assess the thermal stability of skyrmions in magnetic nanowires. For some values of Ms and Ku, the results verified that the skyrmion structure is stable at temperatures above 800 K, which is higher than room temperature. Additionally, manipulating the nanowire geometry was found to have a substantial effect on the thermal structural stability of the skyrmion in storage nanodevices. Increasing the nanowire dimensions, such as length or width, enhanced skyrmions’ structural stability against temperature fluctuations in nanodevices. Furthermore, the random nucleation of the skyrmions due to the device temperature was examined. It was shown that random skyrmion nucleation occurs at temperature values greater than 700 K. These findings make skyrmion devices suitable for storage applications.

A new magnetic phase is reported in the chiral magnet, Cu2OSeO3, which is predicted to affect its physical properties. The lack of inversion symmetry in the crystal lattice of magnetic materials gives rise to complex noncollinear spin orders through interactions of a relativistic nature, resulting in interesting physical phenomena, such as emergent electromagnetism. Studies of cubic chiral magnets revealed a universal magnetic phase diagram composed of helical spiral, conical spiral, and skyrmion crystal phases. We report a remarkable deviation from this universal behavior. By combining neutron diffraction with magnetization measurements, we observe a new multidomain state in Cu2OSeO3. Just below the upper critical field at which the conical spiral state disappears, the spiral wave vector rotates away from the magnetic field direction. This transition gives rise to large magnetic fluctuations. We clarify the physical origin of the new state and discuss its multiferroic properties.

The skyrmion lattice state (SkL), a crystal built of mesoscopic spin vortices, gains its stability via thermal fluctuations in all bulk skyrmion host materials known to date. Therefore, its existence is limited to a narrow temperature region below the paramagnetic state. This stability range can drastically increase in systems with restricted geometries, such as thin films, interfaces and nanowires. Thermal quenching can also promote the SkL as a metastable state over extended temperature ranges. Here, we demonstrate more generally that a proper choice of material parameters alone guarantees the thermodynamic stability of the SkL over the full temperature range below the paramagnetic state down to zero kelvin. We found that GaV4Se8, a polar magnet with easy-plane anisotropy, hosts a robust Néel-type SkL even in its ground state. Our supporting theory confirms that polar magnets with weak uniaxial anisotropy are ideal candidates to realize SkLs with wide stability ranges.

Co x Zn y Mn z (x + y + z = 20) compounds crystallizing in the chiral β-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, Co x Zn y Mn z compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid 8c site in the crystal structure, while Mn tends to share the geometrically frustrated 12d site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich 8c sublattice and the Mn on the 12d sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around 1μ B /atom and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about 3μ B ) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

No abstract available

Noncoplanar spin textures usually exhibit a finite scalar spin chirality (SSC) that can generate effective magnetic fields and lead to additional contributions to the Hall effect, namely topological or unconventional anomalous Hall effect (UAHE). Unlike topological spin textures (e.g., magnetic skyrmions), materials that exhibit fluctuation-driven SSC and UAHE are rare. So far, their realization has been limited to either low temperatures or high magnetic fields, both of which are unfavorable for practical applications. Identifying new materials that exhibit UAHE in a low magnetic field at room temperature is therefore essential. Here, we report the discovery of a large UAHE far above room temperature in epitaxial Fe3Ga4 films, where the fluctuation-driven SSC stems from the field-induced transverse-conical-spiral phase. Considering their epitaxial nature and the large UAHE stabilized at room temperature in a low magnetic field, Fe3Ga4 films represent an exciting, albeit rare, example of a promising material for spintronic devices.

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Proximity effect, which is the coupling between distinct order parameters across interfaces of heterostructures, has attracted immense interest owing to the customizable multifunctionalities of diverse 3D materials. This facilitates various physical phenomena, such as spin order, charge transfer, spin torque, spin density wave, spin current, skyrmions, and Majorana fermions. These exotic physics play important roles for future spintronic applications. Nevertheless, several fundamental challenges remain for effective applications: unavoidable disorder and lattice mismatch limits in the growth process, short characteristic length of proximity, magnetic fluctuation in ultrathin films, and relatively weak spin–orbit coupling (SOC). Meanwhile, the extensive library of atomically thin, 2D van der Waals (vdW) layered materials, with unique characteristics such as strong SOC, magnetic anisotropy, and ultraclean surfaces, offers many opportunities to tailor versatile and more effective functionalities through proximity effects. Here, this paper focuses on magnetic proximity, i.e., proximitized magnetism and reviews the engineering of magnetism‐related functionalities in 2D vdW layered heterostructures for next‐generation electronic and spintronic devices. The essential factors of magnetism and interfacial engineering induced by magnetic layers are studied. The current limitations and future challenges associated with magnetic proximity‐related physics phenomena in 2D heterostructures are further discussed.

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We construct the general free energy governing long-wavelength magnetism in two dimensional oxide heterostructures, which applies irrespective of the microscopic mechanism for magnetism. This leads, in the relevant regime of weak but non-negligible spin-orbit coupling, to a rich phase diagram containing in-plane ferromagnetic, spiral, cone, and Skyrmion lattice phases, as well as a nematic state stabilized by thermal fluctuations.

It is shown that columnar fluctuations, in conjunction with weak quenched disorder, lead to a T{3/2} temperature dependence of the electrical resistivity. This is proposed as an explanation of the observed non-Fermi-liquid behavior in the helimagnet MnSi, with one possible realization of the columnar fluctuations provided by Skyrmion lines that have independently been proposed to be present in this material.

Authors uncover room-temperature skyrmions in 2D van der Waals material, attributing their presence to Fe-deficiency-induced symmetry breaking, enabling topological Hall effect and small Néel-type skyrmions with femtosecond laser writability. Realizing room-temperature magnetic skyrmions in two-dimensional van der Waals ferromagnets offers unparalleled prospects for future spintronic applications. However, due to the intrinsic spin fluctuations that suppress atomic long-range magnetic order and the inherent inversion crystal symmetry that excludes the presence of the Dzyaloshinskii-Moriya interaction, achieving room-temperature skyrmions in 2D magnets remains a formidable challenge. In this study, we target room-temperature 2D magnet Fe_3GaTe_2 and unveil that the introduction of iron-deficient into this compound enables spatial inversion symmetry breaking, thus inducing a significant Dzyaloshinskii-Moriya interaction that brings about room-temperature Néel-type skyrmions with unprecedentedly small size. To further enhance the practical applications of this finding, we employ a homemade in-situ optical Lorentz transmission electron microscopy to demonstrate ultrafast writing of skyrmions in Fe_3- x GaTe_2 using a single femtosecond laser pulse. Our results manifest the Fe_3- x GaTe_2 as a promising building block for realizing skyrmion-based magneto-optical functionalities.

One of the important challenges in condensed matter science is to understand ultrafast, atomic-scale fluctuations that dictate dynamic processes in equilibrium and non-equilibrium materials. Here, we report an important step towards reaching that goal by using a state-of-the-art perfect crystal based split-and-delay system, capable of splitting individual X-ray pulses and introducing femtosecond to nanosecond time delays. We show the results of an ultrafast hard X-ray photon correlation spectroscopy experiment at LCLS where split X-ray pulses were used to measure the dynamics of gold nanoparticles suspended in hexane. We show how reliable speckle contrast values can be extracted even from very low intensity free electron laser (FEL) speckle patterns by applying maximum likelihood fitting, thus demonstrating the potential of a split-and-delay approach for dynamics measurements at FEL sources. This will enable the characterization of equilibrium and, importantly also reversible non-equilibrium processes in atomically disordered materials. X-ray photon correlation spectroscopy has been mainly used to measure slow dynamics using synchrotron sources. Here the authors demonstrate the split-and- delay pulse set-up to study nanosecond dynamics of gold nanoparticles using XPCS with free electron laser pulses.

Ferrimagnetic Fe/Gd multilayers host maze‐like stripe domains that transform into a disordered bubble/skyrmion lattice under out‐of‐plane magnetic fields at ambient temperature. Femtosecond magneto‐optics distinguishes these textures via their distinct coherent breathing dynamics. Crucially, applying a brief in‐plane “set” magnetic field to the stripe state enhances both frequency and amplitude of the bubble/skyrmion lattice breathing mode. Lorentz transmission electron microscopy, magnetic force microscopy, and micromagnetic simulations reveal that this enhancement arises from field‐aligned stripes nucleating a dense, near‐hexagonal bubble/skyrmion lattice upon out‐of‐plane field application, with strong indications for a pure skyrmion lattice. Thus, modifying the initial domain configuration by in‐plane fields enables precise control of coherent magnetization dynamics on picosecond to nanosecond timescales and potentially even of topology.

ABSTRACT Soft X-ray microscopic experiments, such as scanning X-ray microscopy (SXM) and coherent X-ray imaging (CDI), have emerged as powerful techniques for investigating magnetic materials. However, X-ray imaging experiments under external magnetic fields with flexible orientation remain limited. We have developed a soft X-ray microscope equipped with a four-pole electromagnet, enabling imaging experiments magnetic field orientations continuously adjustable from parallel to perpendicular to the incident X-ray beam. This unique capability allows versatile experimental configurations, facilitating static magnetic imaging and advanced time-resolved imaging. We performed time-resolved SXM measurements on two different types of NiFe (permalloy) samples, successfully capturing the collective magnetization dynamics induced by ferromagnetic resonance effect. Furthermore, CDI measurement is demonstrated on multiferroic BiFeO$_3$ 3, visualizing sinusoidal magnetic structures with a spatial periodicity on the order of ten nanometers. The results highlight the efficiency of the developed X-ray microscope as a platform for studying the dynamics and complex spin textures of magnetic materials and spintronic devices. GRAPHICAL ABSTRACT IMPACT STATEMENT We present the development of a soft X-ray microscope with a four-pole electromagnet, enabling versatile magnetic field orientations, and demonstrate advanced measurements for magnetization dynamics and nanoscale magnetic texture.

Scanning transmission X-ray magnetic resonant microscopy (STXMRM) is used to study the magnetization reversal mechanisms of its stripe domains in dipolar-coupled layers $\mathbf{Py}/\mathbf{Al/NdCo}_{7}$, used for reconfigurable spin wave transport. The element specificity of X-rays permitted to observe the evolution of the stripe-domains as a function of the external magnetic field in each layer and determine the correlations between layers. This is key for the understanding of the hysteretic behavior observed in the spin wave transmission of the permalloy (Py) when it is dipolar coupled to the stripe domain textured $\mathbf{NdCo}_{7}$ layer. The images obtained show that the stripe domain imprinting on the Py layer is undetected in the range of fields where hysteretic properties in the dynamics of the Py magnetic moments are present. Therefore, the spin dynamics in the Py should be governed by the stray field of the $\mathbf{NdCo}_{7}$ layer.

The present chapter reviews current neutron and x-ray scattering techniques employed to elucidate the magnetic structures and spin dynamics of magnetic materials. Both techniques provide measurements as a function of the energy and the momentum transferred from the spin system to the probe particles, in terms of five-dimensional data sets as a function of various thermodynamic fields at the control of the experimenter. These scattering techniques yield fundamental information about the equal-time correlations such the magnetic configuration and symmetry, as well as the dynamics that determine the exchange interactions for prototypical systems that behave as linear, planar, or three-dimensional systems. Historically, neutron scattering has been the magnetic scattering technique of choice for such investigations, but the extraordinary advances in resonant x-ray scattering techniques have enable new types of magnetic scattering measurements. The type of information obtained with the two techniques is largely complementary and depends on the interests of the investigators. We discuss these possibilities and provide numerous examples of the techniques applied to different classes of magnetic systems.

Chiral magnets have attracted a large amount of research interest in recent years because they support a variety of topological defects, such as skyrmions and bimerons, and allow for their observation and manipulation through several techniques. They also have a wide range of applications in the field of spintronics, particularly in developing new technologies for memory storage devices. However, the vast amount of data generated in these experimental and theoretical studies requires adequate tools, among which machine learning is crucial. We use a Convolutional Neural Network (CNN) to identify the relevant features in the thermodynamical phases of chiral magnets, including (anti-)skyrmions, bimerons, and helical and ferromagnetic states. We use a flexible multi-label classification framework that can correctly classify states in which different features and phases are mixed. We then train the CNN to predict the features of the final state from snapshots of intermediate states of a lattice Monte Carlo simulation. The trained model allows identifying the different phases reliably and early in the formation process. Thus, the CNN can significantly speed up the large-scale simulations for 3D materials that have been the bottleneck for quantitative studies so far. Moreover, this approach can be applied to the identification of mixed states and emerging features in real-world images of chiral magnets.

Characterizing quantum materials is essential for understanding their microscopic interactions and advancing quantum technology. X-ray photon correlation spectroscopy (XPCS) with coherent X-ray sources offers access to higher-order correlations, but its theoretical basis, the Siegert relation, is derived from dynamical light scattering with independent classical scatterers, and its validity for XPCS remains unexamined. Here we present a microscopic quantum theory of XPCS derived from elecron-photon interaction Hamiltonians, introducing four configurations tied to distinct fourth-order electron-density correlation functions. We examine the validity of the Siegert relation and derive a generalized Siegert relation. Notably, the Siegert relation breaks down even in non-interacting Fermi gas due to exchange interactions. Furthermore, density matrix renormalization group calculations on 1D Kitaev chain reveal oscillatary signatures that can distinguish topologically trivial phases from topological phases with Majorana zero modes. Our work provides a robust theoretical foundation for XPCS and highlights the value of higher-order correlations in advanced X-ray and neutron sources for probing quantum materials.

Split-pulse x-ray photon correlation spectroscopy has been proposed as one of the unique capabilities made possible with the x-ray free electron lasers. It enables characterization of atomic scale structural dynamics that dictates the macroscopic properties of various disordered material systems. Central to the experimental concept are x-ray optics that are capable of splitting individual coherent femtosecond x-ray pulse into two distinct pulses, introduce an adjustable time delay between them, and then recombine the two pulses at the sample position such that they generate two coherent scattering patterns in rapid succession. Recent developments in such optics showed that, while true 'amplitude splitting' optics at hard x-ray wavelengths remains a technical challenge, wavefront and wavelength splitting are both feasible, able to deliver two micron sized focused beams to the sample with sufficient relative stability. Here, we however show that the conventional approach to speckle visibility spectroscopy using these beam splitting techniques can be problematic, even leading to a decoupling of speckle visibility and material dynamics. In response, we discuss the details of the experimental approaches and data analysis protocols for addressing issues caused by subtle beam dissimilarities for both wavefront and wavelength splitting setups. We also show that in some scattering geometries, the Q-space mismatch can be resolved by using two beams of slightly different incidence angle and slightly different wavelengths at the same time. Instead of measuring the visibility of weak speckle patterns, the time correlation in sample structure is encoded in the 'side band' of the spatial autocorrelation of the summed speckle patterns, and can be retrieved straightforwardly from the experimental data. We demonstrate this with a numerical simulation.

The polarization of x-rays plays an outstanding role in experimental techniques such as non-resonant magnetic x-ray scattering and resonant x-ray scattering of magnetic and multipolar order. Different instrumental methods applied to synchrotron light can transform its natural polarization into an arbitrary polarization state. Several synchrotron applications, in particular in the field of magnetic and resonant scattering rely on the improvement in the signal/noise ratio or the deeper insight into the ordered state and the scattering process made possible through these polarization techniques. Here, we present the mathematical framework for the description of fully and partially polarized x-rays, with some applications such as linear x-ray polarization analysis for the determination of the scattered beam's polarization, and the Ge K-edge resonant scattering.

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultra-thin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this non-ergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.

We have used high-resolution resonant inelastic x-ray scattering (RIXS) to study a thin film of NdNiO$_3$, a compound whose unusual spin- and bond-ordered electronic ground state has been of long-standing interest. Below the magnetic ordering temperature, we observe well-defined collective magnon excitations along different high-symmetry directions in momentum space. The magnetic spectra depend strongly on the incident photon energy, which we attribute to RIXS coupling to different local electronic configurations of the expanded and compressed NiO$_6$ octahedra in the bond-ordered state. Both the noncollinear magnetic ground state and the observed site-dependent magnon excitations are well described by a model that assumes strong competition between the antiferromagnetic superexchange and ferromagnetic double-exchange interactions. Our study provides direct insight into the magnetic dynamics and exchange interactions of the rare-earth nickelates, and demonstrates that RIXS can serve as a site-selective probe of magnetism in these and other materials.

Chiral magnets support topological skyrmion textures due to the Dzyaloshinskii-Moriya spin-orbit interaction. In the presence of a sufficiently large applied magnetic field, such skyrmions are large amplitude excitations of the field-polarized magnetic state. We investigate analytically the interaction between such a skyrmion excitation and its small amplitude fluctuations, i.e., the magnons in a clean two-dimensional chiral magnet. The magnon spectrum is found to include two magnon-skyrmion bound states corresponding to a breathing mode and, for intermediate fields, a quadrupolar mode, which will give rise to subgap magnetic and electric resonances. Due to the skyrmion topology, the magnons scatter from a Aharonov-Bohm flux density that leads to skew and rainbow scattering, characterized by an asymmetric differential cross section with, in general, multiple peaks. As a consequence of the skew scattering, a finite density of skyrmions will generate a topological magnon Hall effect. Using the conservation law for the energy-momentum tensor, we demonstrate that the magnons also transfer momentum to the skyrmion. As a consequence, a magnon current leads to magnon pressure reflected in a momentum-transfer force in the Thiele equation of motion for the skyrmion. This force is reactive and governed by the scattering cross sections of the skyrmion; it causes not only a finite skyrmion velocity but also a large skyrmion Hall effect. Our results provide, in particular, the basis for a theory of skyrmion caloritronics for a dilute skyrmion gas in clean insulating chiral magnets.

Underlying disorder in skyrmion materials may both inhibit and facilitate skyrmion reorientations and changes in topology. The identification of these disorder-induced topologically active regimes is critical to realizing robust skyrmion spintronic implementations, yet few studies exist for disordered bulk samples. Here, we employ small-angle neutron scattering (SANS) and micromagnetic simulations to examine the influence of skyrmion order on skyrmion lattice formation, transition, and reorientation dynamics across the phase space of a disordered polycrystalline Co$_{8}$Zn$_{8}$Mn$_{4}$ bulk sample. Our measurements reveal a new disordered-to-ordered skyrmion square lattice transition pathway characterized by the novel promotion of four-fold order in SANS and accompanied by a change in topology of the system, reinforced through micromagnetic simulations. Pinning responses are observed to dominate skyrmion dynamics in the metastable triangular lattice phase, enhancing skyrmion stabilization through a remarkable and previously undetected skyrmion memory effect which reproduces previous ordering processes and persists in zero field. These results uncover the cooperative interplay of anisotropy and disorder in skyrmion formation and restructuring dynamics, establishing new tunable pathways for skyrmion manipulation.

In magnetism, skyrmions correspond to classical three-dimensional spin textures characterized by a topological invariant that keeps track of the winding of the magnetization in real space, a property that cannot be easily generalized to the quantum case since the orientation of a quantum spin is in general ill-defined. Moreover, as we show, the quantum skyrmion state cannot be directly observed in modern experiments that probe the local magnetization of the system. However, we show that this novel quantum state can still be identified and fully characterized by a special local three-spin correlation function defined on neighbouring lattice sites -- the scalar chirality -- which reduces to the classical topological invariant for large systems, and which is shown to be nearly constant in the quantum skyrmion phase.

Magnetic skyrmions are particle-like spin-swirling objects ubiquitously realized in magnets. They are topologically stable chiral kinks composed of multiple modulation waves of spiral spin structures, where the helicity of each spiral is usually selected by antisymmetric exchange interactions in noncentrosymmetric crystals. We report an experimental observation of a distorted triangular lattice of skyrmions in the polar tetragonal magnet EuNiGe$_3$, reflecting a strong coupling with the lattice. Moreover, through resonant x-ray diffraction, we find that the magnetic helicity of the original spiral at zero field is reversed when the skyrmion lattice is formed in a magnetic field. This means that the energy gain provided by the skyrmion lattice formation is larger than the antisymmetric exchange interaction. Our findings will lead us to a further understanding of emergent magnetic states.

Magnetic skyrmions are particle-like objects with topologically-protected stability which can be set into motion with an applied current. Using a particle-based model we simulate current-driven magnetic skyrmions interacting with random quenched disorder and examine the skyrmion velocity fluctuations parallel and perpendicular to the direction of motion as a function of increasing drive. We show that the Magnus force contribution to skyrmion dynamics combined with the random pinning produces an isotropic effective shaking temperature. As a result, the skyrmions form a moving crystal at large drives instead of the moving smectic state observed in systems with a negligible Magnus force where the effective shaking temperature is anisotropic. We demonstrate that spectral analysis of the velocity noise fluctuations can be used to identify dynamical phase transitions and to extract information about the different dynamic phases, and show how the velocity noise fluctuations are correlated with changes in the skyrmion Hall angle, transport features, and skyrmion lattice structure.

The recent discovery of tunable Dzyaloshinskii-Moriya interactions in layered magnetic materials with perpendicular magnetic anisotropy makes them promising candidates for stabilization and manipulation of skyrmions at elevated temperatures. In this article, we use Monte Carlo simulations to investigate the robustness of skyrmions in these materials against thermal fluctuations and finite-size effects. We find that in confined geometries and at finite temperatures skyrmions are present in a large part of the phase diagram. Moreover, we find that the confined geometry favors the skyrmion over the spiral phase when compared to infinitely large systems. Upon tuning the magnetic field through the skyrmion phase, the system undergoes a cascade of transitions in the magnetic structure through states of different number of skyrmions, elongated and half-skyrmions, and spiral states. We consider how quantum and thermal fluctuations lift the degeneracies that occur at these transitions, and find that states with more skyrmions are typically favored by fluctuations over states with less skyrmions. Finally, we comment on electrical detection of the various phases through the topological and anomalous Hall effects.

Traditionally, skyrmions are treated as continuum magnetic textures of classical spins with a well-defined topological skyrmion number. Owing to their topological protection, skyrmions have attracted great interest as building blocks in future memory technology. Smaller skyrmions offer greater memory density, however, quantum effects are not negligible for skyrmions with sizes of just a few lattice constants. In this paper we study quantum fluctuations around dense skyrmion crystal ground states, and focus on the utility of a discretized order parameter for quantum skyrmions. The order parameter is found to be a useful indicator of the existence of quantum skyrmions, and captures the quantum phase transition between two distinct skyrmion phases.

The stability of magnetic skyrmions has been investigated in the past, but mostly in the absence of thermal fluctuations. However, thermal spin fluctuations modify the magnetic properties (exchange stiffness, Dzyaloshinskii-Moriya interaction (DMI) and anisotropy) that define skyrmion stability. Thermal magnons also excite internal skrymion dynamics, deforming the skyrmion shape. Entropy has also been shown to modify skyrmion lifetimes in experiments, but is absent or approximated in previous studies. Here we use metadynamics to calculate the free energy surface of a magnetic thin film in terms of the topological charge and magnetization. We identify the free energy minima corresponding to different spin textures and the lowest energy paths between the ferromagnetic and single skyrmion states. We show that at low temperatures the lowest free energy barrier is a skyrmion collapse process. However, this energy barrier increases with temperature. An alternative path, where a singularity forms on the skrymion edge, has a larger free energy barrier at low temperatures but decreases with increasing temperature and eventually becomes the lowest energy barrier.

Many magnetic equilibrium states and phase transitions are characterized by fluctuations. Such magnetic fluctuation can in principle be detected with scattering-based x-ray photon correlation spectroscopy (XPCS). However, in the established approach of XPCS, the magnetic scattering signal is quadratic in the magnetic scattering cross section, which results not only in often prohibitively small signals but also in a fundamental inability to detect negative correlations (anticorrelations). Here, we propose to exploit the possibility of heterodyne mixing of the magnetic signal with static charge scattering to reconstruct the first-order (linear) magnetic correlation function. We show that the first-order magnetic scattering signal reconstructed from heterodyne scattering now directly represents the underlying magnetization texture. Moreover, we suggest a practical implementation based on an absorption mask rigidly connected to the sample, which not only produces a static charge scattering signal but also eliminates the problem of drift-induced artificial decay of the correlation functions. Our method thereby significantly broadens the range of scientific questions accessible by magnetic x-ray photon correlation spectroscopy.

The current driven magnetization dynamics of a thin-film, three magnetic terminal device (spin-flip transistor) is investigated theoretically. We consider a magnetization configuration in which all magnetizations are in the device plane, with source-drain magnetizations chosen fixed and antiparallel, whereas the third contact magnetization is allowed to move in a weak anisotropy field that guarantees thermal stability of the equilibrium structure at room temperature. We analyze the magnetization dynamics of the free layer under a dc source-drain bias current within the macrospin model and magneto-electronic circuit theory. A new tunable two-state behavior of the magnetization is found and the advantages of this phenomenon and potential applications are discussed.

We develop the hydrodynamical theory of collinear spin currents coupled to magnetization dynamics in metallic ferromagnets. The collective spin density couples to the spin current through a U(1) Berry-phase gauge field determined by the local texture and dynamics of the magnetization. We determine phenomenologically the dissipative corrections to the equation of motion for the electronic current, which consist of a dissipative spin-motive force generated by magnetization dynamics and a magnetic texture-dependent resistivity tensor. The reciprocal dissipative, adiabatic spin torque on the magnetic texture follows from the Onsager principle. We investigate the effects of thermal fluctuations and find that electronic dynamics contribute to a nonlocal Gilbert damping tensor in the Landau-Lifshitz-Gilbert equation for the magnetization. Several simple examples, including magnetic vortices, helices, and spirals, are analyzed in detail to demonstrate general principles.

We consider a hybrid structure consisting of superconducting or normal leads with a combined ferromagnet-3D topological insulator interlayer. We compare responses of a Josephson junction and a normal junction to magnetic texture dynamics. In both cases the electromotive force resulting from the magnetization dynamics generates a voltage between the junction leads. For an open circuit this voltage is the same for normal and superconducting leads and allows for electrical detection of magnetization dynamics and a structure of a given magnetic texture. However, under the applied current the electrical response of the Josephson junction is essentially different due to the strong dependence of the critical Josephson current on the magnetization direction and can be used for experimental probing of this dependence. We propose a setup, which is able to detect a defect motion and to provide detailed information about the structure of magnetic inhomogeneity. The discussed effect could be of interest for spintronics applications.

A theoretical study is made of the magnetization versus applied field curves of ferromagnetic/nonmagnetic multilayers constructed according to a Fibonacci quasiperiodic sequence. The ferromagnetic films are assumed to have uniaxial anisotropy and are coupled by both bilinear and biquadratic effective exchange. The effects of quasiperiodicity in the magnetic phases are illustrated numerically for Fe/Cr systems.

Electromagnetic brakes (EMBrs) are regularly used in continuous casting to control interface fluctuations in a mould. In the research, to appraise the damping effect of EMBr technique, namely EMBr-ruler technique, the influence of magnetic field intensity on behaviours of melt flow and interface fluctuation in a Compact Strip Production (CSP) thin slab funnel mould are numerically simulated. The numerical results illustrate that the existence of the EMBr-ruler technique is conducive to inhibiting the impact of the ascending circulation on the interface in the CSP funnel mould. With the gradual enhancement of magnetic intensity, the service efficiency of the EMBr-ruler on the fluctuation of two-phase steel/slag interface is enhanced. For instance, by applying a magnetic induction intensity of 0.5 T, the maximum fluctuation height of two-phase steel/slag interface is decreased to 6.4 mm. This can well prevent surface defects, such as slag entrapment.

Significance The flow and deformation behavior of colloidal glasses are important to a wide range of potential applications, but direct connections between the macroscopic flow/deformation and microscopic structure or dynamics have been difficult to come by. In this work, we utilize simultaneous stress-controlled rheology and x-ray scattering to bridge this gap. By probing the onset of yielding in a colloidal glass, we determine that the transition from recoverable to unrecoverable deformation is strongly linked to the loss of structural memory and the acceleration of the nanoscale fluctuations of the glass.

The dynamics and structure of mixed phases in a complex fluid can significantly impact its material properties, such as viscoelasticity. Small-angle X-ray Photon Correlation Spectroscopy (SA-XPCS) can probe the spontaneous spatial fluctuations of the mixed phases under various in situ environments over wide spatiotemporal ranges (10^−6–10^3 s /10^−10–10^−6 m). Tailored material design, however, requires searching through a massive number of sample compositions and experimental parameters, which is beyond the bandwidth of the current coherent X-ray beamline. Using 3.7-μs-resolved XPCS synchronized with the clock frequency at the Advanced Photon Source, we demonstrated the consistency between the Brownian dynamics of ~100 nm diameter colloidal silica nanoparticles measured from an enclosed pendant drop and a sealed capillary. The electronic pipette can also be mounted on a robotic arm to access different stock solutions and create complex fluids with highly-repeatable and precisely controlled composition profiles. This closed-loop, AI-executable protocol is applicable to light scattering techniques regardless of the light wavelength and optical coherence, and is a first step towards high-throughput, autonomous material discovery. Robot-compatible liquid sample setup for a fully-automated, μs-resolved Small-angle X-ray Photon Correlation Spectroscopy (SA-XPCS) workflow that can enable autonomous designs of complex fluids.