Transport and Magnetisation Dynamics in Ferromagnetic Nanostructures

Abstract

We study transport and magnetisation dynamics in nanoscale ferromagnets and ferromagnetic heterostructures. Motivated by its novel physics and significant technological and commercial potential, we devote our attention to how the magnetisation in ferromagnets is affected by spin-flip scattering and applied currents. We also study the opposite effect, namely how the precessing magnetisation generates a spin current in a ferromagnet-superconductor heterostructure.

Excitations of the magnetisation vector are generated by e.g. external magnetic fields or spin currents traversing the ferromagnet. Spin dephasing mechanisms, such as scattering from magnetic impurities or the spin-orbit interaction, imply a loss of angular momentum. Unless the magnetic excitation is sustained by an external source, the magnetisation vector undergoes damped precessional motion towards a stable fixed point. We contribute to the understanding of magnetic dissipation by studying how the mentioned spin dephasing mechanisms affect the time evolution of the magnetisation vector in a uniform ferromagnet.

In inhomogeneous ferromagnets, such as domain walls, several intriguing effects caused by the interaction between current and magnetisation vector can be observed. Since the magnetisation varies in space, electrons flowing through the ferromagnet need to constantly adapt their spin direction to the varying magnetic configuration. Spin dephasing results in a mismatch between itinerant spins and the magnetisation. Mistracking generates current-driven magnetisation dynamics, which affects the magnetisation shape and position. We report analytical calculations of currentdriven dynamics in itinerant ferromagnets and analyse numerical results obtained for ferromagnetic semiconductors.

The spin pumping effect, where a time-dependent magnetisation generates spin currents in neighbouring materials, has been studied extensively in normal metalferromagnet nanostructures. Using sophisticated fabrication techniques, heterostructures with both superconducting and ferromagnetic elements can now be manufactured. Inspired by spin pumping theory and recent interest in the superconducting proximity effect, we calculate charge and spin currents pumped by the magnetisation vector in a normal metal-ferromagnet-superconductor junction. We expand upon previous studies by considering pumping in the presence of applied bias voltage, and obtain results that coincide with expectations based on the spin valve pumping theory. The findings agree qualitatively with previous results in the absence of applied voltage.