電流驅動鎳鐵次微米線之磁壁振盪與靜磁交互作用

Abstract

本論文探討磁區壁受自旋轉移力矩作用下的動力學及靜磁學反應。自旋轉移力矩效應是來自於自旋電子角動量變化使磁矩由於角動量守恆的緣故而發生翻轉或是進動。我們利用此效應為基礎，以實驗證明由自旋轉移力矩對磁區壁所產生的運動特性。 首先，我們用實驗來探討以高頻交流電來驅動磁區壁在直線型鎳鐵自旋閥之高頻振盪行為。在外加橫向的磁場下，被對稱突出形磁區壁陷阱所固定住的磁區壁會與高頻交流電產生共振效應，頻率可高達至2.92GHz。而藉由改變稱突出形磁區壁陷阱的寬度，磁區壁共振頻率會隨寬度的增加而減小。 第二部份我們將研究磁區壁在突出形與凹陷形的磁區壁陷阱中的靜磁作用。由實驗數據我們發現突出形磁區壁陷阱有較強的固定磁區壁的效應，並且在脫離磁區壁陷阱的過程中，磁區壁能維持原有的結構。第三部分將討論橢圓環中的磁區壁與鄰近橢圓環中磁區壁的靜磁交互作用。我們發現交錯排列的橢圓環渦旋磁矩結構能有較穩定的現象。最後我們以實驗探討直流電驅使圓柱形自旋閥產生渦旋態的現象，並研究外加磁場與電流及樣品直徑對渦旋形磁區的影響。

This dissertation presents our investigation of the effects of spin transfer torque on a magnetic domain wall. Spin transfer torque is generated by the transfer of angular momentum from spin polarized electrons to a ferromagnet. This torque provides an efficient means to manipulate the dynamic motion of the magnetization of a nanomagnet, and can be strong enough to induce magnetization reversal or steady-state precession. We have developed new techniques to characterize such dynamics induced by spin transfer torque. In the first study, we experimentally demonstrate domain wall (DW) oscillators excited by in-plane ac current through Permalloy based pseudo-spin valve wires which contain one pair of artificial protrusions. By measuring the spin-transfer-torque induced resonance of a pinned transverse DW, under transverse external fields, we show that the transverse DW oscillates with a resonance frequency as high as 2.92 GHz, depending on the widths of protrusions. For DW oscillations induced by injection of dc currents, the observed peaks in dV/dI associated with the reversible change of magnetoresistance are attributed to the reversible motions of the DW. In the second study, we report on static interaction of domain wall in submicron structures. The DW pinning and depinning behavior using square notch and protrusions in NiFe nanowires has been experimented. Pinning strengths as well as distribution of depinning field were measured. Domain wall traps with protrusions were found to be more effective than notches, and the domain wall kept its structure during the depinning processes. In the next study, magnetic interactions of DW between permalloy elliptical rings in different arrangements of two-dimensional arrays were investigated. Formation of domain walls during magnetization reversals results in magnetostatic coupling between adjacent units. When two neighboring rows were shifted one half pitch along the short axis to form a shift-type array, large stable field range for vortex states was obtained. In the last study, we investigate the characteristic of vortex states induced by a dc current in trilayer circular magnetic nanopillars experimentally. The relative chiralities between the two layers’ vortex configurations as functions of external field and current are studied with various diameters. The current induced magnetization behaviors due to the spin transfer torque effect and the additional Oersted field are clearly identified on the resistance behavior.