Micromagnetics of thermally assisted switching of magnetization in magnetic recording system

Abstract

As the current magnetic recording technology approaches to recording density of 40 Gbit/$inch^2$ and frequency range up to 1 GHz, time dependent phenomena of magnetization become important issues. Generally, there are two limitations related to the time dependent magnetization reversal process in recent magnetic recording technology. One is the thermal stability of recorded informations and the other is the gyromagnetic limit on medium switching time. In this thesis, dynamics of magnetization reversal including thermal effect have been studied. To study the thermal effect effectively, the Monte Carlo and Langevin methods are included in micromagnetics model. In the first part, the gyromagnetic limit of magnetic switching time have been studied. It was found that the thermal effect should be considered in studying the switching time since the switching time is significantly reduced by thermal fluctuation. Dependency of the switching time on α/($1+α^2$) in single domain particle is certified when the magnetization reversal of the single domain particle follows the Stoner-Wohlfarth coherent rotation. The Arrhenius-Neel law which is generally used in studying a magnetic after-effect is not valid when the energy barrier is smaller than $k_BT$ or is comparable to $k_BT$ . For longitudinal magnetic recording media, magnetic interactions and microstructures such as an exchange, magnetostatic interaction, grain size and orientation of easy axis can strongly affect the remanent coercivity in the range of nano second field duration. Significant degradation of coercivity will occurr if the effective exchange constant $A^{*}$ would not be reduced as grain size decreases. The "bicrystal" structure shows more sensitive coercivity variations in nano second field duration range than that of non-bicrystal structure, which may be a crucial demerit for high density recording applications. In the second part, the reason of linear dependency on ln(t) of magnetization decay and effec...