| Summary: | 博士 === 國立臺灣大學 === 物理研究所 === 98 === The behavior of magnetization reversal in confined geometry and the degree of spin polarization of magnetic materials are fundamental questions in magnetism and important for a wide range of magnetoresistive device application. Therefore we investigate these two topics experimentally.
First, we studied the behavior of magnetization reversal on pseudo spin valve sub micron structures. The research includes not only geometric shape effect but also dynamic measurements.
We studied nanoscale elliptical ring shaped NiFe/Cu/NiFe trilayer pseudo spin valve structures. The magnetization reversal processes showed simultaneous-reversal single-step transition or double-step transition involving flux closure states. For various aspect ratios (short axis to long axis) and line widths, transition between single-step and double-step magnetization reversals was measured to form a phase diagram. Simulations of the magnetization reversal behavior agreed qualitatively with our results.
We also studied edge roughness effect on the magnetization reversal of spin valve submicron wires. We prepared wires designed with periodic ''spikes'' as artificial roughness. The height and the pitch of the spikes were varied systematically. No obvious dependence was found between the roughness and the domain wall velocity when the spikes were smaller than a threshold of 30 nm for NiFe (the widths were fixed at 400 nm), 40nm for CoFeB. The average velocity was slowed down when the height of the spikes were larger than the threshold. We also studied the current-induced magnetization switching. In-plane transverse magnetic fields help to reduce the critical current density for current induced domain-wall motion. Our results could be attributed to the space modulation of the local magnetization.
In the second topic, the point-contact Andreev reflection technique is employed to determine spin polarization of transport electrons or holes of various ferromagnetic materials. The observed conductance versus voltage spectra exhibited behaviors which were described by the Modified Blonder-Tinkham-Klapwijk (MBTK) model but with significant broadening. We propose a model by introducing a spreading resistance and compare with an existing model which included effective temperature as a parameter. We also established the new three-dimension BTK model to fit some anomalous data. The model considered three-dimension contact and incident current from all angles. The incident current from the ferromagnet was restricted under some conditions which made it decay. The electron would accumulate at the ferromaget/superconductor interface. Not only ideal data but also anomalous spectra are fitted well in this model.
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