Time-resolved imaging of pulse-induced magnetization reversal with a microwave assist field

The reversal of the magnetization under the influence of a field pulse has been previously predicted to be an incoherent process with several competing phenomena such as domain wall relaxation, spin wave-mediated instability regions, and vortex-core mediated reversal dynamics. However, there has been no study on the direct observation of the switching process with the aid of a microwave signal input. We report a time-resolved imaging study of magnetization reversal in patterned magnetic structures under the influence of a field pulse with microwave assistance. The microwave frequency is varied to demonstrate the effect of resonant microwave-assisted switching. We observe that the switching process is dominated by spin wave dynamics generated as a result of magnetic instabilities in the structures, and identify the frequencies that are most dominant in magnetization reversal

Strain‐modulated ferromagnetism at an intrinsic van der Waals heterojunction

The van der Waals interaction enables atomically thin layers of exfoliated 2D materials to be interfaced in heterostructures with relaxed epitaxy conditions, however, the ability to exfoliate and freely stack layers without any strain or structural modification is by no means ubiquitous. In this work, the piezoelectricity of the exfoliated van der Waals piezoelectric α-In2Se3 is utilized to modify the magnetic properties of exfoliated Fe3GeTe2, a van der Waals ferromagnet, resulting in increased domain wall density, reductions in the transition temperature ranging from 5 to 20 K, and an increase in the magnetic coercivity. Structural modifications at the atomic level are corroborated by a comparison to a graphite/α-In2Se3 heterostructure, for which a decrease in the Tuinstra-Koenig ratio is found. Magnetostrictive ferromagnetic domains are also observed, which may contribute to the enhanced magnetic coercivity. Density functional theory calculations and atomistic spin dynamic simulations show that the Fe3GeTe2 layer is compressively strained by 0.4%, reducing the exchange stiffness and magnetic anisotropy. The incorporation of α-In2Se3 may be a general strategy to electrostatically strain interfaces within the paradigm of hexagonal boron nitride-encapsulated heterostructures, for which the atomic flatness is both an intrinsic property and paramount requirement for 2D van der Waals heterojunctions

Magnetic domain wall dynamics and spin transport in confined geometries

In this work we study the controlled propagation of magnetic domain walls in ferromagnetic nanowires made of Permalloy (Ni80Fe20), including curved geometries, with varying width (asymmetric rings). Two types of motion were studied, firstly field driven domain wall motion via fast rotating magnetic field pulses, and secondly the automotive domain wall propagation in nanoscale spintronic devices. In the first experimental approach, we directly observed domain wall spin structure transformations during motion and quantitatively determined the contribution of the spatially varying potential landscape to its propagation. An angular dependence of the domain wall velocity has been observed and explained by the interplay between the domain wall spin structure and relevant forces that act on the vortex wall. However, in contrast to symmetric ring systems, the interplay between these forces leads to distortion-free domain wall motion. Therefore, using this varying domain wall potential landscape, we are able to control spatially the internal domain wall spin structure transformation and synchronization of the domain wall velocities in ring geometries, even above the Walker breakdown. For the second experimental approach, we report a direct dynamic experimental visualization of spontaneous domain wall propagation in asymmetric ferromagnetic rings, with different widths in the narrowest part. Surprisingly, we observed domain wall automotion with an average velocity of about ~ 60 m/s, which is a significant speed for spintronics devices. We show that the domain wall inertia and the stored energy allow the walls to overcome both the local extrinsic pinning and the topological repulsion between domain walls. Our observation can be explained based on the minimization of the magnetostatic and exchange energies. In order to provide more device functionality we went beyond the propagation of one or two walls and managed to achieve a major breakthrough in the development of methods of information processing in spintronics, by demonstrating a scheme to induce synchronous motion of multiple in-plane domain walls in ferromagnetic nanowires using perpendicular field pulses. This paradigm shifting achievement provides the required functionality for nonvolatile domain wall-based shift register devices. The direct visualization of the domain wall spin structure in all experiments was performed employing time resolved scanning transmission X-ray microscopy, which combines the requisite temporal and lateral resolution needed in our measurements. Finally in order to investigate the influence of miniaturization for ultra-small devices we studied magnetic nanocontacts in order to understand the interaction between spin polarized charge carriers and magnetization on the nanoscale. In particular we studied the evolution of the domain wall magneto-resistance in electromigrated ferromagnetic nanocontact fabricated in ultra-high vacuum conditions. We find that the domain wall pinning strength increases on decreasing the contact cross section. Moreover, we measured the depinning field’s angular dependence and symmetry in order to determine the complete domain wall pinning potential in a device with a narrow constriction. The work presented here paves the way for the development of a new generation of non-volatile spintronic components, which could be implemented in a wide range of applications for logic, sensing as well as data storage devices based on the reliable manipulation of domain walls

Strain tuning of Néel temperature in YCrO₃ epitaxial thin films

International audienceEpitaxial strain is a useful handle to engineer the physical properties of perovskite oxide materials. Here, we apply it to orthorhombic chromites that are a family of antiferromagnets showing fruitful functionalities as well as strong spin-lattice coupling via antisymmetric exchange interaction along Cr-O-Cr bonds. Using pulsed laser deposition, we grow YCrO 3 thin films on various substrates imposing strain levels in the range from −1.8% to +0.3%. The films are stoichiometric with a 3+ valence for Cr both within the films and at their surface. They display an antiferromagnetic spin order below their Néel temperature, which we show can be strongly tuned by epitaxial strain with a slope of −8.54 K/%. A dimensionless figure of merit (defined as the slope normalized by the Néel temperature of bulk) is determined to be 6.1, which is larger than that of other perovskites, such as manganites (5.5), ferrites (2.3), or nickelates (4.6). Density functional theory simulations bring insight into the role of Cr-O bond lengths and oxygen octahedral rotations on the observed behavior. Our results shed light on orthorhombic chromites that may offer an energy-efficient piezo-spintronic operation

Correlation between spin structure oscillations and domain wall velocities

Magnetic sensing and logic devices based on the motion of magnetic domain walls rely on the precise and deterministic control of the position and the velocity of individual magnetic domain walls in curved nanowires. Varying domain wall velocities have been predicted to result from intrinsic effects such as oscillating domain wall spin structure transformations and extrinsic pinning due to imperfections. Here we use direct dynamic imaging of the nanoscale spin structure that allows us for the first time to directly check these predictions. We find a new regime of oscillating domain wall motion even below the Walker breakdown correlated with periodic spin structure changes. We show that the extrinsic pinning from imperfections in the nanowire only affects slow domain walls and we identify the magnetostatic energy, which scales with the domain wall velocity, as the energy reservoir for the domain wall to overcome the local pinning potential landscape.publishe

Strain-induced Shape Anisotropy in Antiferromagnetic Structures

We demonstrate how shape-induced strain can be used to control antiferromagnetic order in NiO/Pt thin films. For rectangular elements patterned along the easy and hard magnetocrystalline anisotropy axes of our film, we observe different domain structures and we identify magnetoelastic interactions that are distinct for different domain configurations. We reproduce the experimental observations by modeling the magnetoelastic interactions, considering spontaneous strain induced by the domain configuration, as well as elastic strain due to the substrate and the shape of the patterns. This allows us to demonstrate and explain how the variation of the aspect ratio of rectangular elements can be used to control the antiferromagnetic ground state domain configuration. Shape-dependent strain does not only need to be considered in the design of antiferromagnetic devices, but can potentially be used to tailor their properties, providing an additional handle to control antiferromagnets

Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets

Magnetic skyrmions are topologically protected spin textures that exhibit fascinating physical behaviours and large potential in highly energy-efficient spintronic device applications. The main obstacles so far are that skyrmions have been observed in only a few exotic materials and at low temperatures, and fast current-driven motion of individual skyrmions has not yet been achieved. Here, we report the observation of stable magnetic skyrmions at room temperature in ultrathin transition metal ferromagnets with magnetic transmission soft X-ray microscopy. We demonstrate the ability to generate stable skyrmion lattices and drive trains of individual skyrmions by short current pulses along a magnetic racetrack at speeds exceeding 100 m s-1as required for applications. Our findings provide experimental evidence of recent predictions and open the door to room-temperature skyrmion spintronics in robust thin-film heterostructures.United States. Department of Energy. Office of Basic Energy Sciences (Award DE-SC0012371