Hermann Stoll

Magnetic vortex core reversal by excitation of spin waves

Micron-sized magnetic platelets in the flux-closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations, which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin-wave excitation the concept of a critical velocity as the switching condition has to be modified.

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.

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.

Dynamic domain wall chirality rectification by rotating magnetic fields

We report on the observation of magnetic vortex domain wall chirality reversal in ferromagnetic rings that is controlled by the sense of rotation of a magnetic field. We use time-resolved X-ray microscopy to dynamically image the chirality-switching process and perform micromagnetic simulations to deduce the switching details from time-resolved snapshots. We find experimentally that the switching occurs within less than 4 ns and is observed in all samples with ring widths ranging from 0.5 μm to 2 μm, ring diameters between 2 μm and 5 μm, and a thickness of 30 nm, where a vortex domain wall is present in the magnetic onion state of the ring. From the magnetic contrast in the time-resolved images, we can identify effects of thermal activation, which plays a role for the switching process. Moreover, we find that the process is highly reproducible so that the domain wall chirality can be set with high fidelity.

Spin wave mediated magnetic vortex core reversal

The magnetic vortex is the simplest, non-trivial ground state configuration of micron and sub-micron sized soft magnetic thin film platelets and therefore an interesting subject for the study of micro magnetism. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling was discovered by excitation of the gyrotropic eigenmode at sub-GHz frequencies. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. We demonstrated, experimentally and by micromagnetic simulations, that the unidirectional vortex core reversal process also occurs when azimuthal spin wave modes are excited in the multi-GHz frequency range. This finding highlights the importance of spin wave – vortex interaction and boosts vortex core reversal to much higher frequencies, which may offer new routes for GHz spintronics applications.

Investigation of Electromigration in Copper Interconnects by Noise Measurements

Electromigration in sub-micron conductors of Cu and CuAl was studied by 1/f noise measurements for the first time. 1/f noise can serve as a very sensitive indicator for electromigration damage: The 1/f noise level is increased by up to two orders of magnitude whereas the resistance of the damaged interconnects is enhanced by less than a factor of two only. The most striking advantage of the 1/f noise measurement technique compared to the methods frequently used at present for electromigration studies (e.g., the Median Time of Failure, MTF technique) is that it is possible to determine the distribution of the activation energies of the processes involved from a single sample at progressive electromigration damaging. In Cu interconnects a strong increase in the number of mobile defects is observed during electromigration damaging whereas the shape of the distribution of the activation energies (maximum between 0.8 and 0.95 eV) does not change much, except shortly before the failure of the interconnect lines where a shift to higher activation energies (maximum: 1.05 eV) is measured. Significantly higher activation energies observed in undamaged and electromigration damaged CuAl0.5wt% interconnects indicate an advanced resistance of CuAl alloys to electromigration when compared to pure Cu lines.