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We describe 'RingSim', a phenomenological agent-based model that allows numerical simulation of magnetic nanowire networks with areas of hundreds of micrometers squared for durations of hundreds of seconds; a practical impossibility for general-purpose micromagnetic simulation tools. In RingSim, domain walls (DWs) are instanced as mobile agents which respond to external magnetic fields, and their stochastic interactions with pinning sites and other DWs are described via simple phenomenological rules. We first present a detailed description of the model and its algorithmic implementation for simulating the behaviours of arrays of interconnected ring-shaped nanowires, which have previously been proposed as hardware platforms for unconventional computing applications. The model is then validated against a series of experimental measurements of an array's static and dynamic responses to rotating magnetic fields. The robust agreement between the modelled and experimental data demonstrates that agent-based modelling is a powerful tool for exploring mesoscale magnetic devices, enabling time scales and device sizes that are inaccessible to more conventional magnetic simulation techniques.

Resistive switching is the fundamental process that triggers the sudden change of the electrical properties in solid-state devices under the action of intense electric fields. Despite its relevance for information processing, ultrafast electronics, neuromorphic devices, resistive memories and brain-inspired computation, the nature of the local stochastic fluctuations that drive the formation of metallic nuclei out of the insulating state has remained hidden. Here, using operando X-ray nano-imaging, we have captured the early-stages of resistive switching in a V2O3-based device under working conditions. V2O3 is a paradigmatic Mott material, which undergoes a first-order metal-to-insulator transition coupled to a lattice transformation that breaks the threefold rotational symmetry of the rhombohedral metal phase. We reveal a new class of volatile electronic switching triggered by nanoscale topological defects of the lattice order parameter of the insulating phase. Our results pave the way to the use of strain engineering approaches to manipulate topological defects and achieve the full control of the electronic Mott switching. The concept of topology-driven reversible electronic transition is of interest for a broad class of quantum materials, comprising transition metal oxides, chalcogenides and kagome metals, that exhibit first-order electronic transitions coupled to a symmetry-breaking order.

In synthetic antiferromagnets (SAFs) the combination of antiferromagnetic order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically (AF) aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field $\sim 100$~mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AF and FM states, at fields $\sim 40$~mT, by imaging the spin textures using complementary methods: photo-emission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where first skyrmionic nuclei form, which broaden into regions containing a FM-aligned labyrinth pattern that eventually occupies the whole film. This process remarkably occurs without any significant change in the net magnetic moment of the multilayer. The mix of AF- and FM-aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices using chiral magnetic textures made from SAFs.

Antiferromagnetic materials hold potential for use in spintronic devices with fast operation frequencies and field robustness. Despite the rapid progress in proof-of-principle functionality in recent years, there has been a notable lack of understanding of antiferromagnetic domain formation and manipulation, which translates to either incomplete or non-scalable control of the magnetic order. Here, we demonstrate simple and functional ways of influencing the domain structure in CuMnAs and Mn2Au, two key materials of antiferromagnetic spintronics research, using device patterning and strain engineering. Comparing x-ray microscopy data from two different materials, we reveal the key parameters dictating domain formation in antiferromagnetic devices and show how the non-trivial interaction of magnetostriction, substrate clamping and edge anisotropy leads to specific equilibrium domain configurations. More specifically, we observe that patterned edges have a significant impact on the magnetic anisotropy and domain structure over long distances, and we propose a theoretical model that relates short-range edge anisotropy and long-range magnetoelastic interactions. The principles invoked are of general applicability to the domain formation and engineering in antiferromagnetic thin films at large, which will pave the way towards realizing truly functional antiferromagnetic devices., Comment: 31 pages, 7 figures

The understanding of antiferromagnetic domain walls, which are the interface between domains with different N\'eel order orientations, is a crucial aspect to enable the use of antiferromagnetic materials as active elements in future spintronic devices. In this work, we demonstrate that in antiferromagnetic NiO/Pt bilayers circular domain structures can be generated by switching driven by electrical current pulses. The generated domains are T-domains, separated from each other by a domain wall whose spins are pointing toward the average direction of the two T-domains rather than the common axis of the two planes. Interestingly, this direction is the same for the whole circular domain indicating the absence of strong Lifshitz invariants. The domain wall can be micromagnetically modeled by strain distributions in the NiO thin film induced by the MgO substrate, deviating from the bulk anisotropy. From our measurements we determine the domain wall width to have a full width at half maximum of $\Delta = 98 \pm 10$ nm, demonstrating strong confinement., Comment: 9 pages, 5 figures

Using light to control transient phases in quantum materials is an emerging route to engineer new properties and functionality, with both thermal and non-thermal phases observed out of equilibrium. Transient phases are expected to be heterogeneous, either through photo-generated domain growth or by generating topological defects, and this impacts the dynamics of the system. However, this nanoscale heterogeneity has not been directly observed. Here we use time- and spectrally resolved coherent X-ray imaging to track the prototypical light induced insulator-to-metal phase transition in vanadium dioxide on the nanoscale with femtosecond time resolution. We show that the early-time dynamics are independent of the initial spatial heterogeneity and observe a 200 fs switch to the metallic phase. A heterogeneous response emerges only after hundreds of picoseconds. Through spectroscopic imaging, we reveal that the transient metallic phase is a highly orthorhombically strained rutile metallic phase, an interpretation that is in contrast to those based on spatially averaged probes. Our results demonstrate the critical importance of spatially and spectrally resolved measurements for understanding and interpreting the transient phases of quantum materials.

Efficient manipulation of antiferromagnetic (AF) domains and domain walls has opened up new avenues of research towards ultrafast, high-density spintronic devices. AF domain structures are known to be sensitive to magnetoelastic effects, but the microscopic interplay of crystalline defects, strain and magnetic ordering remains largely unknown. Here, we reveal, using photoemission electron microscopy combined with scanning X-ray diffraction imaging and micromagnetic simulations, that the AF domain structure in CuMnAs thin films is dominated by nanoscale structural twin defects. We demonstrate that microtwin defects, which develop across the entire thickness of the film and terminate on the surface as characteristic lines, determine the location and orientation of 180 degree and 90 degree domain walls. The results emphasize the crucial role of nanoscale crystalline defects in determining the AF domains and domain walls, and provide a route to optimizing device performance.

Mott transitions in real materials are first order and almost always associated with lattice distortions, both features promoting the emergence of nanotextured phases. This nanoscale self-organization creates spatially inhomogeneous regions, which can host and protect transient non-thermal electronic and lattice states triggered by light excitation. Here, we combine time-resolved X-ray microscopy with a Landau-Ginzburg functional approach for calculating the strain and electronic real-space configurations. We investigate V$_2$O$_3$, the archetypal Mott insulator in which nanoscale self-organization already exists in the low-temperature monoclinic phase and strongly affects the transition towards the high-temperature corundum metallic phase. Our joint experimental-theoretical approach uncovers a remarkable out-of-equilibrium phenomenon: the photo-induced stabilisation of the long sought monoclinic metal phase, which is absent at equilibrium and in homogeneous materials, but emerges as a metastable state solely when light excitation is combined with the underlying nanotexture of the monoclinic lattice.

The interest in understanding scaling limits of magnetic textures such as domain walls spans the entire field of magnetism from its relativistic quantum fundamentals to applications in information technologies. The traditional focus of the field on ferromagnets has recently started to shift towards antiferromagnets which offer a rich materials landscape and utility in ultra-fast and neuromorphic devices insensitive to magnetic field perturbations. Here we report the observation that domain walls in an epitaxial crystal of antiferromagnetic CuMnAs can be atomically sharp. We reveal this ultimate domain wall scaling limit using differential phase contrast imaging within aberrationcorrected scanning transmission electron microscopy, which we complement by X-ray magnetic dichroism microscopy and ab initio calculations. We highlight that the atomically sharp domain walls are outside the remits of established spin-Hamiltonian theories and can offer device functionalities unparalleled in ferromagnets., Comment: 8 pages, 4 figures, Supplementary information

We report room temperature long-distance spin transport of magnons in antiferromagnetic thin film hematite doped with Zn. The additional dopants significantly alter the magnetic anisotropies, resulting in a complex equilibrium spin structure that is capable of efficiently transporting spin angular momentum at room temperature without the need for a well-defined, pure easy-axis or easy-plane anisotropy. We find intrinsic magnon spin-diffusion lengths of up to 1.5 {\mu}m, and magnetic domain governed decay lengths of 175 nm for the low frequency magnons, through electrical transport measurements demonstrating that the introduction of non-magnetic dopants does not strongly reduce the transport length scale showing that the magnetic damping of hematite is not significantly increased. We observe a complex field dependence of the non-local signal independent of the magnetic state visible in the local magnetoresistance and direct magnetic imaging of the antiferromagnetic domain structure. We explain our results in terms of a varying and applied-field-dependent ellipticity of the magnon modes reaching the detector electrode allowing us to tune the spin transport.

TmFeO$_3$ (TFO) is a canted antiferromagnet that undergoes a spin reorientation transition (SRT) with temperature between 82 K and 94 K in single crystals. In this temperature region, the N\'eel vector continuously rotates from the crystallographic $c$-axis (below 82 K) to the $a$-axis (above 94 K). The SRT allows for a temperature control of distinct antiferromagnetic states without the need for a magnetic field, making it apt for applications working at THz frequencies. For device applications, thin films of TFO are required as well as an electrical technique for reading out the magnetic state. Here we demonstrate that orthorhombic TFO thin films can be grown by pulsed laser deposition and the detection of the SRT in TFO thin films can be accessed by making use of the all electrical spin Hall magnetoresistance (SMR), in good agreement for the temperature range where the SRT occurs. Our results demonstrate that one can electrically detect the SRT in insulators.

Understanding the electrical manipulation of antiferromagnetic order is a crucial aspect to enable the design of antiferromagnetic devices working at THz frequency. Focusing on collinear insulating antiferromagnetic NiO/Pt thin films as a materials platform, we identify the crystallographic orientation of the domains that can be switched by currents and quantify the N\'eel vector direction changes. We demonstrate electrical switching between different T-domains by current pulses, finding that the N\'eel vector orientation in these domains is along $[\pm5\ \pm5\ 19]$, different compared to the bulk $<11\bar{2}>$ directions. The final state of the N\'eel vector $\textbf{n}$ switching after current pulses $\textbf{j}$ along the $[1\ \pm1\ 0]$ directions is $\textbf{n}\parallel \textbf{j}$. By comparing the observed N\'eel vector orientation and the strain in the thin films, assuming that this variation arises solely from magnetoelastic effects, we quantify the order of magnitude of the magnetoelastic coupling coefficient as $b_{0}+2b_{1}=3*10^7 J\ m^{-3}$ . This information is key for the understanding of current-induced switching in antiferromagnets and for the design and use of such devices as active elements in spintronic devices.

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.

In the quest for post-CMOS technologies, ferromagnetic skyrmions and their anti-particles have shown great promise as topologically protected solitonic information carriers in memory-in-logic or neuromorphic devices. However, the presence of dipolar fields in ferromagnets, restricting the formation of ultra-small topological textures, and the deleterious skyrmion Hall effect when driven by spin torques have thus far inhibited their practical implementations. Antiferromagnetic analogues, which are predicted to demonstrate relativistic dynamics, fast deflection-free motion and size scaling have recently come into intense focus, but their experimental realizations in natural antiferromagnetic systems are yet to emerge. Here, we demonstrate a family of topological antiferromagnetic spin-textures in $\alpha$-Fe$_2$O$_3$ - an earth-abundant oxide insulator - capped with a Pt over-layer. By exploiting a first-order analogue of the Kibble-Zurek mechanism, we stabilize exotic merons-antimerons (half-skyrmions), and bimerons, which can be erased by magnetic fields and re-generated by temperature cycling. These structures have characteristic sizes of the order ~100 nm that can be chemically controlled via precise tuning of the exchange and anisotropy, with pathways to further scaling. Driven by current-based spin torques from the heavy-metal over-layer, some of these AFM textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature., Comment: 18 pages, 4 figures

The strain dependent functional properties of epitaxial transition metal oxide films can be significantly modified via substrate selection. However, large lattice mismatches preclude dislocation-free epitaxial growth on ferroelectric substrates, whose strain states are modified by applied electric fields. Here we overcome this mismatch problem by depositing an epitaxial film of ferromagnetic La0.7Sr0.3MnO3 on a single crystal substrate of well lattice matched SrTiO3 via a film of SrRuO3 that we subsequently dissolved, permitting the transfer of unstrained La0.7Sr0.3MnO3 to a ferroelectric substrate of 0.68Pb(Mg1/3Nb2/3)O3 0.32PbTiO3 in a different crystallographic orientation. Ferroelectric domain switching, and a concomitant ferroelectric phase transition, produced large non volatile changes of magnetization that were mediated by magnetic domain rotations at locations defined by the microstructure - as revealed via high resolution vector maps of magnetization constructed from photoemission electron microscopy data, with contrast from x-ray magnetic circular dichroism. In future, our method may be exploited to control functional properties in dislocation free epitaxial films of any composition.

Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of aniferromagnetic domains is one of the key prerequisites for understading physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20 nm epilayer of uniaxial CuMnAs we observe reversible 180 deg switching of the N\'eel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent 90 deg switching of the N\'eel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron microscopy using X-ray magnetic linear dichroism.

Materials suitable for magnonic crystals demand low magnetic damping and long spin wave (SW) propagation distance. In this context Co based Heusler compounds are ideal candidates for magnonic based applications. In this work, antidot arrays (with different shapes) of epitaxial $\mathrm{Co}_2\mathrm{Fe}_{0.4}\mathrm{Mn}_{0.6}\mathrm{Si}$ (CFMS) Heusler alloy thin films have been prepared using e-beam lithography and sputtering technique. Magneto-optic Kerr effect and ferromagnetic resonance analysis have confirmed the presence of dominant cubic and moderate uniaxial magnetic anisotropies in the thin films. Domain imaging via x-ray photoemission electron microscopy on the antidot arrays reveals chain like switching or correlated bigger domains for different shape of the antidots. Time-resolved MOKE microscopy has been performed to study the precessional dynamics and magnonic modes of the antidots with different shapes. We show that the optically induced spin-wave spectra in such antidot arrays can be tuned by changing the shape of the holes. The variation in internal field profiles, pinning energy barrier, and anisotropy modifies the spin-wave spectra dramatically within the antidot arrays with different shapes. We further show that by combining the magnetocrystalline anisotropy with the shape anisotropy, an extra degree of freedom can be achieved to control the magnonic modes in such antidot lattices.

Magnetic skyrmions are chiral spin textures that hold great promise as nanoscale information carriers. Since their first observation at room temperature, progress has been made in their current-induced manipulation, with fast motion reported in stray-field-coupled multilayers. However, the complex spin textures with hybrid chiralities and large power dissipation in these multilayers limit their practical implementation and the fundamental understanding of their dynamics. Here, we report on the current-driven motion of N\'eel skyrmions with diameters in the 100-nm range in an ultrathin Pt/Co/MgO trilayer. We find that these skyrmions can be driven at a speed of 100 m/s and exhibit a drive-dependent skyrmion Hall effect, which is accounted for by the effect of pinning. Our experiments are well substantiated by an analytical model of the skyrmion dynamics as well as by micromagnetic simulations including material inhomogeneities. This good agreement is enabled by the simple skyrmion spin structure in our system and a thorough characterization of its static and dynamical properties., Comment: 9 pages, 4 figures + Supplementary Information (12 pages, 9 figures)

We probe the current-induced magnetic switching of insulating antiferromagnet/heavy metals systems, by electrical spin Hall magnetoresistance measurements and direct imaging, identifying a reversal occurring by domain wall (DW) motion. We observe switching of more than one third of the antiferromagnetic domains by the application of current pulses. Our data reveal two different magnetic switching mechanisms leading together to an efficient switching, namely the spin-current induced effective magnetic anisotropy variation and the action of the spin torque on the DWs., Comment: 26 pages, including supplementary material

Unveiling the physics that governs the intertwining between the nanoscale self-organization and the dynamics of insulator-to-metal transitions (\textit{IMT}) is key for controlling on demand the ultrafast switching in strongly correlated materials and nano-devices. A paradigmatic case is the \textit{IMT} in V$_2$O$_3$, for which the mechanism that leads to the nucleation and growth of metallic nano-droplets out of the supposedly homogeneous Mott insulating phase is still a mystery. Here, we combine X-ray photoemission electron microscopy and ultrafast non-equilibrium optical spectroscopy to investigate the early stage dynamics of isolated metallic nano-droplets across the \textit{IMT} in V$_2$O$_3$ thin films. Our experiments show that the low-temperature monoclinic antiferromagnetic insulating phase is characterized by the spontaneous formation of striped polydomains, with different lattice distortions. The insulating domain boundaries accommodate the birth of metallic nano-droplets, whose non-equilibrium expansion can be triggered by the photo-induced change of the 3$d$-orbital occupation. We address the relation between the spontaneous nanotexture of the Mott insulating phase in V$_2$O$_3$ and the timescale of the metallic seeds growth. We speculate that the photoinduced metallic growth can proceed along a non-thermal pathway in which the monoclinic lattice symmetry of the insulating phase is partially retained., Comment: 17 pages, text+figures+methods+supplementary

We report on the effect of the lateral confinement and a perpendicular magnetic field on isolated room-temperature magnetic skyrmions in sputtered Pt/Co/MgO nanotracks and nanodots. We show that the skyrmions size can be easily tuned by playing on the lateral dimensions of the nanostructures and by using external magnetic field amplitudes of a few mT, which allow to reach sub-100 nm diameters. Our XMCD-PEEM observations also highlight the important role of the pinning on the skyrmions size and stability under an out-of-plane magnetic field. Micromagnetic simulations reveal that the effect of local pinning can be well accounted for by considering the thin film grain structure with local anisotropy variations and reproduce well the dependence of the skyrmion diameter on the magnetic field and the geometry., Comment: 17 pages, 4 figures

Nucleation processes of mixed-phase states are an intrinsic characteristic of first-order phase transitions, typically related to local symmetry breaking. Direct observation of emerging mixed-phase regions in materials showing a first-order metal-insulator transition (MIT) offers unique opportunities to uncover their driving mechanism. Using photoemission electron microscopy, we image the nanoscale formation and growth of insulating domains across the temperature-driven MIT in NdNiO3 epitaxial thin films. Heteroepitaxy is found to strongly determine the nanoscale nature of the phase transition, inducing preferential formation of striped domains along the terraces of atomically flat stepped surfaces. We show that the distribution of transition temperatures is an intrinsic local property, set by surface morphology and stable across multiple temperature cycles. Our data provides new insights into the MIT of heteroepitaxial nickelates and points to a rich, nanoscale phenomenology in this strongly correlated material.