Spin pumping, spin transfer, and spin Hall effects in magnetic insulator-normal metal systems

Abstract

This thesis presents four research papers on two topics within the field of spintronics. Three of the papers present theoretical models concerned with the topics of spin wave mode-dependent spin pumping, spin transfer, and spin Hall effects in ferromagnetic insulator--normal metal layered structures. The fourth paper details results from experiments investigating the training and recovery of the exchange bias effect in a metallic spin valve structure.

Paper I addresses the enhancement of the Gilbert damping that occurs due to the spin pumping effect from a precessing magnetization. We studied a ferromagnetic insulator in contact with a normal metal that is assumed to act as a perfect spin sink. In this paper we showed for the first time that the higher excited spin wave modes in the insulator film has twice as strong renormalization of the Gilbert damping as the uniform mode. We also show that the Gilbert damping renormalization for an easy-axis surface anisotropy-induced surfacelocalized spin wave mode can be an order of magnitude stronger than for the uniform mode.

Paper II extends the formalism of Paper I to a system consisting of a ferromagnetic insulator--normal metal--ferromagnetic insulator stack. We compute the Gilbert damping renormalization spectrum for both symmetric and antisymmetric film thickness configurations, and show that the modes are either acoustically or optically coupled across the metallic spacer layer. The acoustic and optical modes experience a different renormalization of the Gilbert damping, depending on the thickness ratio of the two insulator films, the nonlocal dipole--dipole interactions through the in-plane wave number, as well as the spin relaxation properties and conductivity of the metallic spacer layer. We also discuss how an easy-axis surface anisotropy can induce surface modes that are robust to thickness mismatches of the two insulator films.

Paper III is a sequel to Paper I, and presents a more accurate model of the eigenmodes and their Gilbert damping renormalization in the bilayer system. In this paper we take the effect of spin back flow from the normal metal into account. The different permutations of the field configuration and the surfaceanisotropy types are adressed, and measures for the alternating and direct inverse spin Hall effect are presented.

Paper IV presents findings related to the training effect in an exchange-biased metallic spin valve structure. High rate field sweeps were executed while performing magnetoresistive measurements on the valve. The measurements show reduced coercive field as a function of sweep iteration number. Upon pausing the field sweeps, the coercive field was shown to recover towards its initial state at a speed depending on the sweep rate, sweep number and the rest interval length in a logarithmic fashion.