ISBN 978-3-8439-0088-1

978-3-8439-0088-1, Reihe Physik

Lars Bocklage
Current-Induced Magnetization Dynamics of Ferromagnetic Nanostructures

140 Seiten, Dissertation Universität Hamburg (2010), Softcover, A5

Zusammenfassung / Abstract

New concepts of magnetic memories use electrical currents to control the magnetization in sub-micron magnetic elements. This thesis deals with the interaction of spin-polarized currents with the local magnetization. Ferromagnetic micro- and nanostructures are fabricated and are investigated by electrical measurements and X-ray microscopy. The work focusses on the dynamics of magnetic domain walls and vortices. Quasi-static magnetoresistance measurements and magnetic transmission X-ray microscopy are used to study the propagation of domain walls in nanowires by fields and currents. The experimental findings show that the domain wall structure determines the pinning and depinning at pinning sites. The depinning probability of domain walls exhibits oscillations for excitations with nanosecond long current pulses of varying length due to resonant stimulus of domain walls. Experiments reveal a dependence of domain-wall motion on the rise time of current pulses. The force on domain walls mediated by the spin-torque depends on the strength of the current and on its temporal change. The force due to the temporal change of the current scales with the intrinsic damping time of the domain wall. The latter is in the order of a few nanoseconds in the investigated material permalloy (Ni80Fe20). The generation of domain walls in straight wires by the magnetic field of a stripline is imaged. The localized field of the stripline switches a part of the magnetization of the nanowire and generates two domain walls. The dynamics of domain walls and vortices are studied by time-resolved X-ray microscopy. The results show that domain walls and vortices can be treated as quasi-particles and that their dynamics are not only determined by the spin torque but also by magnetic fields accompanying the current. A phase sensitive inductive measurement scheme is presented. It is capable to detect the vortex dynamics of single elements.