Domain Wall Dynamics in Magnetic Nanostructures

Abstract

We have investigated the pinning of laterally confined head-to-head domain walls at constrictions. The experiments provide insight into the static properties of pinned domain walls. We found that position, extent, and pinning strength can be reliably determined by transport measurements. The position of the domain walls inside the pinning potential has been determined from the angular dependence of the depinning field, and we found that transverse walls are pinned symmetrically inside the notch whereas vortex walls are pinned on either side, but repelled from the centre of the notch. Both domain wall types can be stable in different regions of the same sample, if one tries to lift the vortex wall over the barrier separating the two individual potential wells, the wall is eventually transformed into a transverse wall which stays pinned inside the notch [KVW+04]. Depinning measurements constitute a solid base for understanding the potential landscape around a constriction and are sufficiently understood to be used as a tool to gain further insight into dynamic processes.The results of our combined current- and field- induced domain wall propagation experiments at different temperatures provide insight into the interplay between the adiabatic and non-adiabatic spin-transfer torque terms and thermal effects. We could demonstrate the importance of thermal activations for the current- and field-induced case: while for purely field-induced domain wall propagation the field threshold is always lowered for increasing temperatures, the current threshold shows a non-monotonous dependence on the temperature.Even though we could confirm purely current driven domain wall propagation, we found an intrinsic dependency of the spin-torque efficiency on the temperature which has the opposite trend as expected from the simple theory, pointing to the need of further theoretical studies. Having determined width and depth of the pinning potential and the interaction with currents, we have performed domain wall excitation experiments with AC currents and showed that domain walls can be described as quasiparticles with an effective mass, oscillating in the pinning potential. We have presented a novel physical effect, the homodyne rectification of AC currents by oscillating domain walls, which we explained by micromagnetic simulations and a harmonic oscillator model. The results obtained using homodyne rectification have been confirmed by a second independent method, the depinning spectroscopy. It was found that the homodyne detection is much faster and additionally offers a higher frequency resolution than the depinning spectroscopy.The homodyne detection method has the additional advantage that the parameters power, temperature and external field can be varied and thus their influence on the domain wall resonance and on the underlying potential well shape can be studied. We presented a fast and reliable method based on the homodyne detection to determine the polarity of vortex cores, demonstrating an inversion of the vortex core polarity using out-of-plane magnetic fields and resonant microwave pulses. The homodyne detection technique was used to directly measure the local profile of a symmetric pinning potential, and an experiment has been proposed to completely chart an asymmetric pinning potential.