Projekte

We numerically investigate the interaction between propagating spin waves and a transverse domain wall in a nanowire by using micromagnetic simulations. In order to understand the mechanisms that lead to domain wall motions, we calculate domain wall velocity in a defect-free nanowire and the depinning fields for a pinned domain wall that is depinned in and against the direction of the spin-wave propagation. We find that the physical origin of the spin-wave-induced domain wall motion strongly depends on the propagating spin-wave frequency. At certain spin-wave frequencies, transverse domain wall vibrations lead to transverse wall displacements by the spin waves, while at other frequencies, large spin-wave reflection drives domain wall motion. By analyzing the depinning field calculations, the different underlying physical mechanisms are distinguished.

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.

In this article, an analytical model for current-induced vortex core displacement is developed. By using this model, one can solve the equations of motion analytically to determine the effects of the adiabatic and nonadiabatic spin-torque terms. The final displacement direction of the vortex core due to the two torque terms mirrors their relative strengths. The resulting vortex core displacement direction combined with the amplitude of the displacement is thus a measure for both torque terms.

We review the details of domain wall (DW) propagation due to spin-polarized currents that could potentially be used in magnetic data storage devices based on domains and DWs. We discuss briefly the basics of the underlying spin torque effect and show how the two torques arising from the interaction between the spin-polarized charge carriers and the magnetization lead to complex dynamics of a spin texture such as a DW. By direct imaging we show how confined DWs in nanowires can be displaced using currents in in-plane soft-magnetic materials, and that when using short pulses, fast velocities can be attained. For high-anisotropy out-of-plane magnetized wires with narrow DWs we present approaches to deducing the torque terms and show that in these materials potentially more efficient domain wall motion could be achieved.

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.

Studying the interaction of spin-polarized currents with the magnetization configuration is of high interest due to the possible applications and the novel physics involved. High-resolution magnetic imaging is one of the key techniques necessary for a better understanding of these effects. Here, we present an extension to a magnetic microscope that allows for in situ current injection into the structure investigated, and furthermore for the study of current induced magnetization changes during pulsed current injection. The developed setup is highly flexible and can be used for a wide range of investigations. Examples of current-induced domain wall motion and vortex core displacements measured using this setup are presented.

We use a pump-probe photoemission electron microscopy technique to image the displacement of vortex cores in Permalloy discs due to the spin-torque effect during current pulse injection. Exploiting the distinctly different symmetries of the spin torques and the Oersted-field torque with respect to the vortex spin structure we determine the torques unambiguously, and we quantify the amplitude of the strongly debated nonadiabatic spin torque. The nonadiabaticity parameter is found to be β=0.15±0.07, which is more than an order of magnitude larger than the damping constant α, pointing to strong nonadiabatic transport across the high magnetization gradient vortex spin structures.

Field-induced domain wall (DW) motion in Permalloy microwires under the influence of thermal gradients is studied using magneto-optical Kerr-microscopy. The DW is found to be pinned at two positions in a laser spot focused onto the wire. By tuning the laser power, the pinning strength can be engineered. On increasing the laser power, the depinning field for the first pinning site in the first half of the laser spot is found to decrease, while for the second pinning site in the spot region located beyond the centre of the spot it increases. Below a threshold power WT, the pinning strength can be tuned reversibly while above WT irreversible increases in the pinning strength due to structural changes occur. This effect can be employed for flexible pinning sites for DW devices.

We study the evolution of the magnetoresistance (MR) in Permalloy nanocontacts prepared by controlled low-temperature UHV electromigration in nanoring segment structures with constrictions. The ring geometry allows for the controlled and reproducible positioning of a domain wall in the nanocontacts. We observe three different resistance levels, corresponding to distinct domain-wall positions. A change in the sign of the MR difference, between a domain wall at the constriction and a domain wall next to the constriction, occurs with decreasing constriction width. This is in line with our micromagnetic simulations, where the MR is calculated based on the anisotropic MR (AMR) effect.

We employ the Landau-Lifshitz-Bloch (LLB) equation to investigate current-induced domain wall motion at finite temperatures by numerical micromagnetic simulations. We extend the LLB equation with spin torque terms that account for the effect of spin-polarized currents and we find that the velocities depend strongly on the interplay between adiabatic and nonadiabatic spin torque terms. As a function of temperature, we find nonmonotonous behavior, which might be useful to determine the relative strengths of the spin torque terms experimentally.