We studied the displacement of magnetic domains under temperature gradients in perpendicularly magnetized Ta/[Pt/Co68B32/Ir]x10/Pt multilayer tracks with microfabricated Pt heaters and thermometers by magnetic force microscopy. Subtracting out the effects of the Oersted field from the heating current reveals the pure temperature gradient driven motion, which is always toward the heater. The higher the thermal gradient along the track is (owing to the proximity to the heater or larger heater currents), the greater the observed displacements of the domains are, up to a velocity of around 1 nm/s in a temperature gradient of 20 K/mu m. This velocity lies in the creep regime. Quantitative estimates of the strength of different driving mechanisms for the effect that have been proposed theoretically show that entropic forces dominate over those arising from the spin Seebeck and spin-dependent Seebeck effects in driving the domain motion.

In this Perspective article, we explore some of the promising spin and topology material platforms (e.g. spins in semi- and superconductors, skyrmionic, topological and 2D materials) being developed for such quantum components as qubits, superconducting memories, sensing, and metrological standards and discuss their figures of merit. Spin- and topology-related quantum phenomena have several advantages, including high coherence time, topological protection and stability, low error rate, relative ease of engineering and control, simple initiation and read-out. However, the relevant technologies are at different stages of research and development, and here we discuss their state-of-the-art, potential applications, challenges and solutions.

Thin films of polycrystalline $\beta$-Mn structure CoZn have been grown on thermally oxidized Si substrates by co-sputtering from elemental targets followed by annealing. A range of films grown with variable Co deposition power and fixed Zn deposition power were produced, so as to vary the proportions of the two elements reaching the substrate, which were annealed post-growth. Whilst all films exhibited a (211) $\beta$-Mn structure CoZn texture in X-ray diffraction, transmission electron microscopy showed that the composition with the highest integrated intensity for that Bragg peak contained large vacancies and was covered by a thick ZnO cap owing to being Co-deficient overall. CoZn films deposited at ratios tuned to give the optimal volume fraction of $\beta$-Mn were continuous, with crystallites up to 200~nm in size, with a much thinner ZnO cap layer. Magnetic measurements show that such optimal CoZn films have a Curie temperature $T_\mathrm{C} \sim 420$~K and saturation magnetization of 120~emu/cm$^3$, properties close to those reported for bulk crystals. The $\beta$-Mn structure is chiral (P4$_{1}$32/P4$_{3}$32 space group) and is known to give rise to a Dzyaloshinkii-Moriya interaction (DMI) that stabilizes room-temperature skyrmions in the bulk. Our thin films are thus a potential materials platform, compatible with planar processing technology, for magnetic skyrmions arising from a bulk DMI.

Magnetic skyrmions are nanoscale spin textures that exhibit topological stability, which, along with novel thermal and electrical transport properties, make them the ideal candidates for a variety of novel technological applications. Accessing the skyrmion spin texture at the nanoscale and understanding its interaction with local thermal gradients is essential for engineering skyrmion-based transport phenomena. However, direct experimental insight into the local thermoelectric response of single skyrmions remains limited. To address this, we employ scanning thermoelectric microscopy (SThEM) to probe the nanoscale thermoelectric response from a single skyrmion. By mapping the local thermoelectric voltage with nanoscale precision, we reveal a unique spatially resolved response that is the convolution of the underlying spin texture of the skyrmion and its interaction with the highly localised thermal gradient originating from the heated probe. We combine this with thermoelectric modelling of a range of skyrmion spin textures to reveal unique thermoelectric responses and allow the possibility of SThEM to be used as a tool to distinguish nanoscale spin textures. These findings provide fundamental insights into the interaction of topologically protected spin textures with local thermal gradients and the resultant spin transport. We demonstrate a novel route to characterise nanoscale spin textures, accelerating the material optimisation cycle, while also opening the possibility to harness skyrmions for spin caloritronics.

Skyrmions have been proposed as new information carriers in racetrack memory devices. To realise such devices, a small size; high speed of propagation; and minimal skyrmion Hall angle are required. Synthetic antiferromagnets (SAFs) present the ideal materials system to realise these aims. In this work, we use micromagnetic simulations to propose a new method for manipulating them using exclusively global magnetic fields. An out-of-plane microwave field induces oscillations in the skyrmions radius which in turn emits spin waves. When a static in-plane field is added, this breaks the symmetry of the skyrmions and causes asymmetric spin wave emission. This in turn drives motion of the skyrmions, with the fastest velocities observed at the frequency of the intrinsic out-of-phase breathing mode of the pair of skyrmions. This behaviour is investigated over a range of experimentally realistic antiferromagnetic interlayer exchange coupling strengths, and the results compared to previous works studying similar motion driven with an oscillating electric field. Through this the true effect of varying the exchange coupling strength is determined, and greater insight is gained into the mechanism of skyrmion motion. These results will help to inform the design of future novel computing architectures based on the dynamics of skyrmions in synthetic antiferromagnets.

N\'eel magnetic domain walls that are stabilized by achiral energy terms instead of the usual Dzyaloshinskii-Moriya interaction will be bistable, with the two possible chiral forms being degenerate. Here we focus on the theoretical study of the spin-orbit torque driven dynamics of such bistable N\'eel domain walls. We find that, for a given domain wall, two propagation directions along a nanowire are possible, depending on its initial state. These dynamics also exhibit complex dependence on the spin-orbit torque magnitude, leading to important transient regimes. Finally, a few ways are proposed for controlled or random reversal of the domain wall propagation direction. A robust analytical model which handles all the observed behaviors of such domain walls is developed and validated by comparing with numerical simulations. The obtained new dynamics open the way for new uses of domain walls in information storage and processing devices.

Spintronics and magnetic materials exhibit many physical phenomena that are promising for implementing neuromorphic computing natively in hardware. Here, we review the current state-of-the-art, focusing on the areas of spintronic synapses, neurons, and neural networks. Many current implementations are based on the paradigm of reservoir computing, where the details of the network do not need to be known but where significant post-processing is needed. Benchmarks are given where possible. We discuss the scientific and technological advances needed to bring about spintronic neuromorphic computing that could be useful to an end-user in the medium term.

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) 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 aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field -100 mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields - 40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically 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 whose operation is based on spin textures in perpendicularly magnetized SAFs.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

An antiferromagnet emits spin currents when time-reversal symmetry is broken. This is typically achieved by applying an external magnetic field below and above the spin-flop transition or by optical pumping. In this work we apply optical pump-THz emission spectroscopy to study picosecond spin pumping from metallic FeRh as a function of temperature. Intriguingly we find that in the low-temperature antiferromagnetic phase the laser pulse induces a large and coherent spin pumping, while not crossing into the ferromagnetic phase. With temperature and magnetic field dependent measurements combined with atomistic spin dynamics simulations we show that the antiferromagnetic spin-lattice is destabilised by the combined action of optical pumping and picosecond spin-biasing by the conduction electron population, which results in spin accumulation. We propose that the amplitude of the effect is inherent to the nature of FeRh, particularly the Rh atoms and their high spin susceptibility. We believe that the principles shown here could be used to produce more effective spin current emitters. Our results also corroborate the work of others showing that the magnetic phase transition begins on a very fast picosecond timescale, but this timescale is often hidden by measurements which are confounded by the slower domain dynamics.

The accurate measurement of the Dzyaloshinskii-Moriya interaction (DMI) in ultrathin ferromagnetic/heavy metal heterostructures was recently shown to require extreme experimental care. In this presentation the current state-of-the-art of measuring interfacial DMI is reviewed, and two of the most popular techniques - non-reciprocity of thermally activated magnetostatic spin wave dispersion and asymmetric bubble domain expansion - are discussed for their reliability and applicability. Furthermore, the large discrepancies observed recently between the two techniques are analyzed, and possible origins are discussed. The presentation aims to achieve a deeper comprehension of the reasons behind the discrepancies and to assess the applicability of various techniques to different heterostructures.

The current-driven motion of magnetic domain walls (DWs) is the working principle of magnetic racetrack memories. In this type of spintronic technology, high current densities are used to propel DW motion in magnetic nanowires, causing significant wire heating. Synthetic antiferromagnets are known to show very fast DW motion at high current densities, but lower current densities around onset of motion have received less attention. Here we use scanning transmission x-ray microscopy to study the response of DWs in a SAF multilayer to currents. We observe that the DWs depin at $\sim 3 \times 10^{11}$~A/m$^2$ and move more quickly in response to 5~ns duration current pulses than in comparable conventional multilayers. The results suggest that DWs in SAF structures are superior to conventional Néel DWs for low energy consumption racetrack technologies.

We report on domain pattern transfer from a ferroelectric BaTiO$_{\mathrm{3}}$ substrate with a $(111)$-orientation of the surface to an epitaxial Co film grown on a Pd buffer layer. Spatially modulated interfacial strain transfer from ferroelectric/ferroelastic domains and inverse magnetostriction in the ferromagnetic film induce stripe regions with a modulation of the in-plane uniaxial magnetic anisotropy direction. Using spin-polarized low energy electron microscopy, we observe the formation of two distinct anisotropy configurations between stripe regions. Moreover, through application of a magnetic field parallel or perpendicular to these stripes, head-to-head or head-to-tail magnetization configurations are initialized. This results in four distinct magnetic domain wall types associated with different energies and widths, which in turn affects whether domain pattern transfer can be achieved.

We use spin polarized low energy electron microscopy to investigate domain pattern transfer in a multiferroic heterostructure consisting of a $(111)$-oriented BaTiO$_{\mathrm{3}}$ substrate and an epitaxial Ni film. After in-situ thin film deposition and annealing through the ferroelectric phase transition, interfacial strain transfer from ferroelastic domains in the substrate and inverse magnetostriction in the magnetic thin film introduce a uniaxial in-plane magnetic anisotropy that rotates by $60^{\circ}$ between alternating stripe regions. We show that two types of magnetic domain wall can be initialized in principle. Combining experimental results with micromagnetic simulations we show that a competition between the exchange and magnetostatic energies in these domain walls have a strong influence on the magnetic domain configuration.

The Dzyaloshinskii-Moriya interaction (DMI), being one of the origins of chiral magnetism, is currently attracting considerable attention in the research community focusing on applied magnetism and spintronics. For future applications, an accurate measurement of its strength is indispensable. Here we present a review of the state-of-the-art of measuring the coefficient of the Dzyaloshinskii-Moriya interaction, the DMI constant D, focusing on systems where the interaction arises from the interface between two materials (i.e. interfacial DMI). We give an overview of the experimental techniques as well as their theoretical background and models for the quantification of the DMI constant. The measurement techniques are divided into three categories: a) domain wall-based measurements, b) spin wave-based measurements and c) spin-orbit torque-based measurements. We analyze the advantages and disadvantages of each method and compare D values at different interfaces. The review aims to obtain a better understanding of the applicability of the different techniques to various stacks and of the origin of apparent disagreements among literature values.

Thulium iron garnet, Tm3Fe5O12 with perpendicular magnetic anisotropy is fabricated using an on-axis sputtering technique followed by annealing, whereas previous reports have used unusual off-axis geometries. Stoichiometric Tm3Fe5O12 is obtained after the modification of the deposition conditions involving the position of the substrate relative to the cathode, which affects both the chemical and structural properties. The effective perpendicular magnetic anisotropy of 8.6 kJ/m3 is well in line with the results of previous studies using pulse laser deposition and off-axis sputtering. A maze domain pattern is observed, and the domain-wall energy is evaluated as 0.69 mJ/m2.

Synthetic antiferromagnets have great potential as skyrmion carriers in which new properties are expected for these spin textures, owing to changed magnetostatics and the absence of net topological charge. Here we numerically simulate the static and dynamic behaviour of skyrmions in these systems and clearly highlight the benefits compared to ferromagnetic single layers. In particular, our results show a reduction of the skyrmion radius, an increase of their velocity under current, and a vanishing of their topological deflection. We also provide a robust and straightforward analytical model that captures the physics of such skyrmions. Finally, by extending the model to the case of an unbalanced SAF, we show some conditions for the system that optimise the properties of the skyrmion for potential spintronic devices.

The motion of a skyrmion driven by propagating spin waves in a thin film track with interfacial perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction was investigated by numerical micromagnetic simulations. At long simulation times, we find that the skyrmion always moves in the direction of the spin-wave propagation. At shorter times, rather than observing skyrmion motion in a single direction, the skyrmion shows transient retrograde motion when the track width is wider than 50 nm (for Gilbert damping a = 0.02) or a < 0.06 (for a track width of 100 nm). A simple treatment of spin-wave-driven skyrmion motion based on the Thiele equation predicts that the skyrmion will be pulled back towards the source of the spin waves because of the gyrotropic and dissipative effects. The forward motion away from the spin wave source can be explained by also considering the effect of a repulsive force from the edges of the track when the skyrmion is in close contact with it. We discuss these effects quantitatively in terms of the spin-wave transmission and reflection coefficients of the skyrmion.

Skyrmions are small topologically protected magnetic structures that hold promise for applications from data storage to neuromorphic computing and they have been shown to possess internal microwave frequency excitations. Skyrmions in a synthetic antiferromagnet have been predicted to be smaller and faster than their ferromagnetic equivalents and also shown to possess more internal modes. In this work, we consider the breathing modes of skyrmions in a four repetition synthetic antiferromagnetic multilayer by means of micromagnetic simulations and examine the further splitting of the modes into different arrangements of out-of-phase, in-phase, and modes with more complex phase relationships. This results in a lowering of frequencies, which is promising for skyrmion sensing applications in a synthetic antiferromagnet.