André Bisig

Spin configurations in Co2FeAl0.4Si0.6 Heusler alloy thin film elements

We determine experimentally the spin structure of half-metallic Co2FeAl0.4Si0.6 Heusler alloy elements using magnetic microscopy. Following magnetic saturation, the dominant magnetic states consist of quasi-uniform configurations, where a strong influence from the magnetocrystalline anisotropy is visible. Heating experiments show the stability of the spin configuration of domain walls in confined geometries up to 800 K. The switching temperature for the transition from transverse to vortex walls in ring elements is found to increase with ring width, an effect attributed to structural changes and consequent changes in magnetic anisotropy, which start to occur in the narrower elements at lower temperatures.

Tunable steady-state domain wall oscillator with perpendicular magnetic anisotropy

We theoretically study domain wall oscillations upon the injection of a dc current through a geometrically constrained wire with perpendicular magnetic anisotropy. The frequency spectrum of the oscillation can be tuned by the injected current density and additionally by the application of an external magnetic field. Our analytical calculations are supported by micromagnetic simulations based on the Landau–Lifshitz–Gilbert equation. The simple concept of our localized steady-state oscillator might prove useful as a nanoscale microwave generator with possible applications in telecommunications or for rf-assisted writing in magnetic hard drives.

Synchronous precessional motion of multiple domain walls in a ferromagnetic nanowire by perpendicular field pulses

Magnetic storage and logic devices based on magnetic domain wall motion rely on the precise and synchronous displacement of multiple domain walls. The conventional approach using magnetic fields does not allow for the synchronous motion of multiple domains. As an alternative method, synchronous current-induced domain wall motion was studied, but the required high-current densities prevent widespread use in devices. Here we demonstrate a radically different approach: we use out-of-plane magnetic field pulses to move in-plane domains, thus combining field-induced magnetization dynamics with the ability to move neighbouring domain walls in the same direction. Micromagnetic simulations suggest that synchronous permanent displacement of multiple magnetic walls can be achieved by using transverse domain walls with identical chirality combined with regular pinning sites and an asymmetric pulse. By performing scanning transmission X-ray microscopy, we are able to experimentally demonstrate in-plane magnetized domain wall motion due to out-of-plane magnetic field pulses.

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.

Control of spin configuration in half-metallic La0.7Sr0.3MnO3 nano-structures

We investigate the interplay between the governing magnetic energy terms in patterned La0.7Sr0.3MnO3 (LSMO) elements by direct high-resolution x-ray magnetic microscopy as a function of temperature and geometrical parameters. We show that the magnetic configurations evolve from multidomain to flux-closure states (favored by the shape anisotropy) with decreasing element size, with a thickness-dependent crossover at the micrometer scale. The flux-closure states are stable against thermal excitations up to near the Curie temperature. Our results demonstrate control of the spin state in LSMO elements by judicious choice of the geometry, which is key for spintronics applications requiring high spin-polarizations and robust magnetic states.

Direct observation of high velocity current induced domain wall motion

We study fast vortex wall propagation in Permalloy wires induced by 3 ns short current pulses with sub 100 ps rise time using high resolution magnetic imaging at zero field. We find a constant domain wall displacement after each current pulse as well as current induced domain wall structure changes, even at these very short timescales. The domain wall velocities are found to be above 100 m/s and independent of the domain wall spin structure. Comparison to experiments with longer pulses points to the pulse shape as the origin of the high velocities.

Dipolar energy states in clusters of perpendicular magnetic nanoislands

We investigated the energy states in compact clusters of ferromagnetic islands with perpendicular anisotropy arranged on a triangular lattice. Due to their finite nature, we were able to determine the energies of all possible cluster states using dipolar energy calculations. We employed photoemission electron microscopy to observe the magnetic states in arrays of clusters of monodomain Co/Pt multilayer islands and following demagnetization, we observed a shift in the energy distribution to lower energies as the dipolar coupling increased. These multistate island clusters not only provide model arrangements of frustrated Ising-type nanomagnets but are also interesting for data storage applications.

Enhanced Nonadiabaticity in Vortex Cores due to the Emergent Hall Effect

We present a combined theoretical and experimental study, investigating the origin of the enhanced nonadiabaticity of magnetic vortex cores. Scanning transmission x-ray microscopy is used to image the vortex core gyration dynamically to measure the nonadiabaticity with high precision, including a high confidence upper bound. We show theoretically, that the large nonadiabaticity parameter observed experimentally can be explained by the presence of local spin currents arising from a texture induced emergent Hall effect. This study demonstrates that the magnetic damping α and nonadiabaticity parameter β are very sensitive to the topology of the magnetic textures, resulting in an enhanced ratio (β/α>1) in magnetic vortex cores or Skyrmions.

Direct imaging of current induced magnetic vortex gyration in an asymmetric potential well

Employing time-resolved x-ray microscopy, we investigate the dynamics of a pinned magnetic vortex domain wall in a magnetic nanowire. The gyrotropic motion of the vortex core is imaged in response to an exciting ac current. The elliptical vortex core trajectory at resonance reveals asymmetries in the local potential well that are correlated with the pinning geometry. Using the analytical model of a two-dimensional harmonic oscillator, we determine the resonance frequency of the vortex core gyration and, from the eccentricity of the vortex core trajectory at resonance, we can deduce the stiffness of the local potential well.

Vortex domain wall chirality rectification due to the interaction with end domain spin structures in permalloy nanowires

The interaction of vortex domain walls with the end domain spin structure present at the rectangular end of a ferromagnetic nanowire is investigated using Lorentz transmission electron microscopy. When vortex walls are moved with short field pulses towards the wire end an end vortex is formed, whose chirality is independent of the original vortex wall chirality but is determined by the spin configuration of the end domain. This acts as a domain wall chirality “rectifier,” which could be useful for applications based on domain walls. The observed chirality transformations are reproduced by micromagnetic simulations showing a complex reversal mechanism.