Parametric spin wave control using device structure and Oersted field

Abstract

Spin wave is the collective dynamical motion in magnetically ordered medium. The research on spin waves has attracted great attention because of their interesting physics as well as applicable characteristics for various devices. Especially, the realization of logical operation is one of the main issue in magnonics, a research field utilizing spin waves as information carriers. Carrying out the logical operation is used the wave property of spin waves such as amplitude and phase. Depending on the phase of spin waves propagated from several input signal lines, the constructive or destructive interference occur on the gathering area. Accordingly, Studies have been conducted about a lot of efficient spin wave excitation methods to propagate to the region of interference and the manipulation method to control of the phase. In this thesis, we focused on the spin wave excitation method called parametric pumping using a source as microwave RF current. One of the nonlinear excitation method, parametric pumping have the threshold characteristics, that is utilized for inducing a spin wave mode selectivity to magnetic system. And the intensity of spin waves increases exponentially as the pumping energy fully compensates the dynamic damping constant of the spin wave mode. Therefore, it is useful for spin wave excitation and amplification in magnonic systems. The first chapter shows the theoretical background used in our experiments. The basic theories related to spin waves including the various effective energy in LLG (Landau-Liftshitz-Gilbert) equation, the dispersion relation in nanoscale magnetic element, and the parametric pumping of spin waves in wire-type magnetic structure are explained briefly. Also, we refer to the optical measurement, μ-BLS (micro-Brillouin Light Scattering) spectroscopy which is the key equipment in this research, and its usage for confined magnetic system are described in detail in chapter 2. Chapter 2 shows the how adequate to use the μ-BLS technique for measuring the spin wave in confined magnetic system. The confined magnetic system has a triangular shape of micron scale. We obtained the 2-dimensional spatial profile using spatial resolved BLS technique. Comparing experimental profile with simulated one, the information of localized spin wave mode could be analyzed. It clearly demonstrates the strength of μ-BLS for spin wave measurement in the nano or micro scale system. Chapter 3 is the research for parametric spin waves control using device geometry, especially change of aspect ratio. The characteristics of parametric spin-wave pumping are investigated in three rectangular permalloy blocks with in-plane aspect ratios of 12, 6, and 4. μ-BLS spectroscopy is used to detect the intrinsic properties of excited spin waves under various excitation conditions of an external magnetic field and microwave power. Based on the theoretical dispersion relation and BLS intensity and its spatial profile, the parametric spin wave with a frequency of f_sw=5.6 GHz is found to be excited in fundamental mode in a block of largest aspect ratio while the other block have higher-order or edge-localized modes along the width direction. The data shows that the nature of spin waves is quite sensitive on the conditions of sample geometry, applied field and microwave power. Chapter 4 shows the parametric spin waves control using dc Oersted field gernarated by DC current flowing copper antenna. For parametric excitation, a magnetic nanowire system that has a built-in dc current line to produce an Oersted field is designed, A spin wave with a frequency of fsw = 5.6 GHz is observed when a pumping microwave with a frequency of fmw = 11.2 GHz is applied. The wave is found to be of the n = 1 width mode (n is the antinode number), and its mode changes to an edge-localized (or possibly n > 1) mode when the Oersted field (or current) varies. Joule heating effects are not observed in the pumping process. Thus, spin wave mode control by the built-in current would be a convenient and useful method to enhance the efficiency and compatibility in applications of spin-based electronics. The final chapter summarizes the results of each study on parametric spin wave control.