| Summary: | 博士 === 國立臺灣大學 === 物理學研究所 === 89 === In this thesis, we investigate the spin configurations and magnetization processes of Permalloy thin films thoroughly by means of numerical micromagnetic simulation.
A complete description of stable spin configurations is obtained including the critical sizes for stability of one-vortex and quasiuniform state. The magnetization processes of the quasiuniform state of elliptical elements resembles that of a single-domain ellipsoidal particle of similar shape. Nevertheless, the calculated hysteresis loops show a considerable reduction of the switching fields in comparison with the Stoner-Wohlfarth model due to the development of a new mode of instability.
Multivortex states show interesting magnetization processes due to shifting of the vortices perpendicular to the field. The magnetization curves show significant dependence on the direction of external field and always experience abrupt jump whenever vortices are nucleated or annihilated.
The existence of the new bending state explains naturally the evolution from the instability of quasiuniform state to vortex state with increasing of the particle’s diameter and this evolution depends considerably on the particle thickness.
For some elements with special geometries, the magnetization patterns of soft type network are also investigated. A classification of different patterns based on the total magnetic charge accumulated near the junction is suggested. Besides, the magnetization reversal and magnetoresistance of Permalloy ring are investigated both by simulation and experiment. We find that three stable states can exist under different external field. The transition between these states will lead to many interesting phenomena of magnetoresistance.
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