| Summary: | 碩士 === 國立彰化師範大學 === 物理學系 === 92 === In this study, a novel technique involving simultaneous measurements of magnetic force microscopy and magnetoresistance in the microstructured permalloy ellipses has been developed. The correlation between magnetic microstructures to the magnetoresistance was then investigated for the understanding of switching mechanisms. The permalloy element with nonmagnetic current/voltage leads were made using electron beam lithography through a lift-off technique. A magnetic force microscope equipped with a home-made horizontal electromagnet capable of applying magnetic field up to 500 Oe was used for the micromagnetic images. Special care was taken for the in-situ measurements of magnetoresistance while the magnetic force microscope images were being taken on the element.
A variety of magnetization configurations were observed and the corresponding magnetoresistance behavior, measured with the external magnetic field applied along the long axis, was identified. Throughout the magnetization reversal two-vortex, three-vortex, cross-tie walls, and mixed state, respectively, in ellipses in accordance with the different aspect ratio and thickness have been successfully imaged. In most cases, the magnetization configurations during switching processes are reversible, but irreversible switching processes were also found in a few cases. The influence of the irreversible switching behavior makes asymmetric magnetoresistance curve, associated with the scattering of conduction carriers from the different domain walls.
In addition, having the external magnetic field applied along the short axis the number of vortices in element can be controlled, decrease after cycling the magnetic fields. Moreover, permalloy ellipse under two orthogonal fields, hard-axis field provided by Oersted field and easy axis field provided by a home-made electromagnet, was investigated through magnetoresistance measurements, with which the switching asteroid was graphed.
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