Spin Structure of Domain Walls and Their Behaviour in Applied Fields and Currents

Abstract

In this thesis, the spin structure of domain walls in confined magnetic elements was determined and the behaviour of domain walls on the application of external magnetic fields and current pulse injection was observed. For this, magnetic elements of various shapes, materials, and substrates were prepared which were each dedicated to the purpose of the experiment. Different fabrication techniques were described, including several patterning processes and contacting of magnetic elements for current-pulse experiments. The spin structure of transverse walls in constrictions down to 30nm in permalloy wavy lines was measured using electron holography. It is known that the domain wall spin structure influences both the magnetoresistance and spin transfer torque effects. For the transverse walls, three different types were determined and the domain wall width was found to decrease faster than linearly with decreasing constriction width. The magnetic fields needed to depin the domain walls from such constrictions were measured using magnetoresistance measurements and were directly related to the domain wall spin structure. While an approach was made to image the spin structure of patterned thin films of the halfmetal CrO2 coupled to a permalloy thin film using photoemission electron microscopy (PEEM), a complicated coupling mechanism between the two layers was observed. The spin structure of an array of crossed wires corresponding to an array of holes (antidots) in a cobalt thin film was investigated to understand the switching behaviour on the application of an external magnetic field. It was found that switching occurs by the nucleation and propagation of domain chains, and the moving chain ends can be pinned when they meet the ends of perpendicular domain chains or are blocked due to the formation of a 360° domain wall when they approach a perpendicular domain chain. The current-induced domain wall motion in contacted permalloy wires on membrane samples was studied using electron holography. Effects due to the spin torque were separated from heating effects on the domain wall due to the current. A variety of effects such as domain wall transformations, domain wall jumping between two pinning sites, or structural changes of the magnetic material crystallites due to the combined influence of current pulses and heating was observed. A set of indicators was derived to distinguish current-induced domain wall motion due to the spin torque from the heating effects.