Current induced magnetic domain wall oscillators and static interaction in submicron NiFe linewidth structures
|關鍵字:||自旋轉移力矩;磁區壁;磁性奈米線;spin transfer torque;magnetic domain wall;magnetic nanowire|
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.