Skyrmions have been proposed as new information carriers in racetrack memory devices. To realize such devices, a small size, high speed of propagation, and minimal skyrmion Hall angle are required. Synthetic antiferromagnets (SAFs) present the ideal materials systems to realize 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 skyrmion 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 the 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 behavior is investigated over a range of experimentally realistic antiferromagnetic interlayer exchange coupling strengths, and the results are compared to previous works. Through this we demonstrate the true effect of varying the exchange coupling strength and gain greater insight into the mechanism of skyrmion motion. In an attempt to more accurately reproduce experimental SAF samples, we investigate the effect of changing the ratio between the layer magnetizations in the SAF, and demonstrate that maximum velocity can be achieved when there is 100% compensation. We also investigate the differences between excitation mechanisms, using both electric fields and spin transfer torques to excite skyrmion motion. These results will help to inform the design of future novel computing architectures based on the dynamics of skyrmions in synthetic antiferromagnets.

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.

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.

Magnetic skyrmions are topological spin structures that arise in chiral magnetic systems which exhibit broken inversion symmetry and high spin-orbit coupling resulting in a sizable Dzyaloshinskii-Moriya inter-action. Understanding the local spin texture of skyrmions is a vital metrological step in the development of skyrmionic technologies required for novel logic or storage devices in addition to providing fundamental insight into the nanoscale chiral interactions inherent to these systems. Here, we propose that there exists a radially dependent stray field signature that emanates from magnetic skyrmions. We employ quantitative magnetic force microscopy to experimentally explore this stray field signature. To corroborate the experimental observations a semianalytical model is developed which is validated against micromagnetic simulations. This unique approach provides a route to understand the unique radially dependent field signature from skyrmions, which allows an understanding of the underlying local magnetization profile to be obtained. From a practical standpoint, our results provide a rapid approach to validate outputs from numerical or micromagnetic simulations. This approach could be employed to optimize the complex matrix of magnetic parameters required for fabricating and modeling skyrmionic systems, in turn accelerating the technology readiness level of skyrmionic based devices.

Abstract Magnetic textures known as skyrmions have captured the attention of the scientific community working on magnetism for a large part of the last decade due to their potential application in novel neuromorphic devices to carry information and perform computational operations[1, 2]. This is due to the ease of manipulation with electrical currents, and the stability of individual skyrmions against interactions with defects or other skyrmions. Now, while there are plenty of reports on skyrmion manipulation using electrical currents, there are very few studies of skyrmion-to-skyrmion interactions (i.e. skyrmion collisions). While there have recently been studies on skyrmion lattices to extract the interaction potential[3, 4], a more direct approach is to manipulate individual skyrmions directly, which however has been challenging so far. The reason for that is that very few tools allow for manipulating skyrmions individually and with enough precision to cause collisions in a controllable and repeatable way. In here, we show how using magnetic force microscopy (MFM), and the field gradient produced by MFM probes, it is possible to visualize skyrmions, manipulate them, and create skyrmion collisions to study skyrmion-to-skyrmion interactions or investigate the role of pinning at material defects.

Spatially resolved thermoelectric detection of magnetic systems provides a unique platform for the investigation of spintronic and spin caloritronic effects. Hitherto, these investigations have been resolution-limited, confining analysis of the thermoelectric response to regions where the magnetization is uniform or collinear at length scales comparable to the domain size. Here, we investigate the thermoelectric response from a single trapped domain wall using a heated scanning probe. Following this approach, we unambiguously resolve the domain wall due to its local thermoelectric response. Combining analytical and thermal micromagnetic modeling, we conclude that the measured thermoelectric signature is unique to that of a domain wall with a Néel-like character. Our approach is highly sensitive to the plane of domain wall rotation, which permits the distinct identification of Bloch or Néel walls at the nanoscale and could pave the way for the identification and characterization of a range of noncollinear spin textures through their thermoelectric signatures.

We report on the depth-sensitive, temperature-dependent exchange coupling in an FePt/FeRh thin-film exchange-spring structure. The depth-dependent in-plane magnetization is measured as a function of applied magnetic field and sample temperature using polarized neutron reflectometry (PNR). The magnetization profiles are interpreted in terms of the competition between anisotropy, exchange coupling, and dipolar coupling as the FeRh undergoes the magnetic phase transition from antiferromagnetic to ferromagnetic ordering. The PNR data are combined with bulk magnetometry and x-ray characterization, allowing us to determine characteristic length scales over which the exchange-spring mechanism is effective at ambient and elevated temperatures.

Cylindrical magnetic nanowires are promising materials that have the potential to be used in a wide range of applications. The versatility of these nanostructures is based on the tunability of their magnetic properties, which is achieved by appropriately selecting their composition and morphology. In addition, stochastic behavior has attracted attention in the development of neuromorphic devices relying on probabilistic magnetization switching. Here, we present a study of the magnetization reversal process in multisegmented CoNi/Cu nanowires. Nonstandard 2D magnetic maps, recorded under an in-plane magnetic field, produce datasets that correlate with magnetoresistance measurements and micromagnetic simulations. From this process, the contribution of the individual segments to the demagnetization process can be distinguished. The results show that the magnetization reversal in these nanowires does not occur through a single Barkhausen jump, but rather by multistep switching, as individual CoNi segments in the NW undergo a magnetization reversal. The existence of vortex states is confirmed by their footprint in the magnetoresistance and 2D MFM maps. In addition, the stochasticity of the magnetization reversal is analysed. On the one hand, we observe different switching fields among the segments due to a slight variation in geometrical parameters or magnetic anisotropy. On the other hand, the stochasticity is observed in a series of repetitions of the magnetization reversal processes for the same NW under the same conditions.

We report studies of spin textures including domain walls and skyrmions in synthetic antiferromagnetic multilayers. We have observed: phase coexistence of uniform antiferromagnetic regions with textured ferromagnetic regions during magnetisation; current-driven domain wall motion at lower current densities than in comparable ferromagnetic multilayers; and current-driven nucleation of a synthetic antiferromagnetic skyrmion.

We experimentally study the thermoelectrical signature of individual skyrmions in chiral Pt/Co/Ru multilayers. Using a combination of controlled nucleation, single skyrmion annihilation, and magnetic field dependent measurements the thermoelectric signature of individual skyrmions is characterized. The observed signature is explained by the anomalous Nernst effect of the skyrmion's spin structure. Possible topological contributions to the observed thermoelectrical signature are discussed. Such thermoelectrical characterization allows for noninvasive detection and counting of skyrmions and enables fundamental studies of topological thermoelectric effects on the nanoscale.

Magnetic force microscopy (MFM) can be considered as a standard tool for nano-scale investigation of magnetic domain structures by probing the local stray magnetic field landscape of the measured sample. However, this generally provides only qualitative data. To quantify the stray magnetic fields, the MFM system must be calibrated. To that end, a transfer function (TF) approach was proposed, that, unlike point probe models, fully considers the finite extent of the MFM tip. However, albeit being comprehensive, the TF approach is not yet well established, mainly due to the ambiguities concerning the input parameters and the measurement procedure. Additionally, the calibration process represents an ill-posed problem which requires a regularization that introduces further parameters. In this paper we propose a guideline for quantitative stray field measurements by standard MFM tools in ambient conditions. All steps of the measurement and calibration procedure are detailed, including reference sample and sample under test (SUT) measurements and the data analysis. The suitability of the reference sample used in the present work for calibrated measurements on a sub-micron scale is discussed. A specific regularization approach based on a Pseudo-Wiener Filter is applied and combined with criteria for the numerical determination of a unique regularization parameter. To demonstrate the robustness of such a defined approach, a round robin comparison of magnetic field measurements was conducted by four laboratories. The guideline, the reference sample and the results of the round robin are discussed.

The equiatomic alloy FeRh is of great scientific and technological interest due its highly unusual first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition. Here we report an exploration of the interplay between topography and phase evolution with a comprehensive magnetic force microscopy study of nominal 50 nm thick FeRh thin films and subtractively patterned wires of width 0.2 µm–2 µm. In continuous films where the surface morphology had not been optimised for smoothness, the topographical variation was observed to dominate the distribution of the magnetic transition temperatures and dictates the nucleation and growth of the magnetic phases. This observation was repeated for patterned elements, where the effects of surface morphology were more significant than those of spatial confinement. These results have clear implications for future studies of low-dimensional FeRh films, as surface topography must be considered when analysing and comparing the transition behaviour of FeRh thin films.

In this work we present the results of an experiment to locally resolve the spin Seebeck effect in a high-quality Pt/YIG sample. We achieve this by employing a locally heated scanning thermal probe to generate a highly local non-equilibrium spin current. To support our experimental results, we also present a model based on the non-equilibrium thermodynamic approach which is in a good agreement with experimental findings. To further corroborate our results, we index the locally resolved spin Seebeck effect with that of the local magnetisation texture by MFM and correlate corresponding regions. We hypothesise that this technique allows imaging of magnetisation textures within the magnon diffusion length and hence characterisation of spin caloritronic materials at the nanoscale.

The future of consumer electronics depends on the capability to reliably fabricate nanostructures with given physical properties. Therefore, techniques to characterize materials and devices with nanoscale resolution are crucial. Among these is magnetic force microscopy (MFM), which transduces the magnetic force between the sample and a magnetic oscillating probe into a phase shift, enabling the locally resolved study of magnetic field patterns down to 10 nm. Here, the progress done toward making quantitative MFM a common tool in nanocharacterization laboratories is shown. The reliability and ease of use of the calibration method based on a magnetic reference sample, with a calculable stray field, and a deconvolution algorithm is demonstrated. This is achieved by comparing two calibration approaches combined with numerical modeling as a quantitative link: measuring the probe's effect on the voltage signal when scanning above a nanosized graphene Hall sensor, and recording the MFM phase shift signal when the probe scans across magnetic fields produced by metallic microcoils. Furthermore, in the case of the deconvolution algorithm, it is shown how it can be applied using the open-source software package Gwyddion. The estimated magnetic dipole approximation for the most common probes currently in the market is also reported.

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

The first order antiferromagnetic to ferromagnetic metamagnetic phase transition of equiatomic FeRh offers new opportunities for novel antiferromagnetic memories and spintronic devices with the caveat that it can be utilized in thin film structures (<50 nm). Here, we report a polarized neutron reflectivity (PNR) study for three representative film thicknesses (5, 20, and 50 nm) aimed at determining the physical and magnetic structure of FeRh at room temperature and partway through the transition. The PNR results are analyzed with reference to X-ray diffraction, X-ray reflectivity, and atomic force microscopy data which together provide a consistent description of the magnetic and physical state of the FeRh thin films. The data demonstrate that the nucleation of the ferromagnetic phase initiates at the MgO substrate, and results from structural and magnetic measurements demonstrate that the magnetic behavior and strain properties of FeRh correlate with the evolving topography of the three films investigated.

Magnetic nanoparticles have many applications in biomedicine and other technical areas. Despite their huge economic impact, there are no standardised procedures available to measure their basic magnetic properties. The International Organization for Standardization is working on a series of documents on the definition of characteristics of magnetic nanomaterials. We review previous and ongoing European research projects on characteristics of magnetic nanoparticles and present results of an online survey among European researchers.

Magnetic skyrmions are topologically protected nanoscale spin textures exhibiting fascinating physical behaviors. Recent observations of room temperature skyrmions in sputtered multilayer films are an important step towards their use in ultra-low power devices. Such practical applications prefer skyrmions to be stable at zero magnetic fields and room temperature. Here, we report the creation of skyrmion lattices in Pt/Co/Ta multilayers by a scanning local field using magnetic force microscopy tips. We also show that those newly created skyrmion lattices are stable at both room temperature and zero fields. Lorentz transmission electron microscopy measurements reveal that the skyrmions in our films are of Néel-type. To gain a deeper understanding of the mechanism behind the creation of a skyrmion lattice by the scanning of local fields, we perform micromagnetic simulations and find the experimental results to be in agreement with our simulation data. This study opens another avenue for the creation of skyrmion lattices in thin films.