薄膜磁化翻转的理论研究
Theoretical studies on switching of magnetisation in thin film
AI总结：
本研究采用 Monte-Carlo 模拟考察 Ising 与 Blume-Capel 模型的磁化翻转，揭示随机场、表面相互作用、各向异性及自旋态数对亚稳态寿命和成核规律的影响。
摘要：
在本章中，我们重点研究磁化翻转，即铁磁系统的亚稳态寿命。在这方面，特别是 Ising 模型和 Blume-Capel 模型，已通过基于 Metropolis 算法的 Monte-Carlo 模拟技术，在存在外加磁场的情况下进行了模拟。研究发现，在无序存在时磁化翻转更快，这里的无序以淬随机场建模。观察到随机场强度发挥着与温度相似的作用。经典成核的 Becker-Döring 理论（最初为自旋 1/2 的 Ising 系统提出）已在随机场 Ising 模型中得到验证。然而，更强的随机场会影响成核区域。在立方 Ising 晶格中，发现表面反转时间不同于体相反转时间。本文通过引入相对界面相互作用强度（$R$）研究了表面与体相相反的这种独特行为。根据 $R$、温度和外加场的不同，观察到表面和体相磁化的竞争性翻转。研究了各向异性（$D$）对亚稳态寿命的影响。我们报告了平均宏观反转时间与适当定义的微观反转时间之间存在线性依赖关系。反转后的饱和磁化强度 $M_f$ 被观察到强烈依赖于 $D$。发现 $M_f$、$D$ 和 $h$（场）遵循一个提出的标度关系。最后，Becker-Döring 理论以及 Avrami 定律在自旋 $s$ Ising 模型和 Blume-Capel 模型中得到验证。翻转时间取决于可访问自旋态的数量。
英文摘要：
In the present chapter, we focus on the switching of magnetisation, or the metastable lifetime of a ferromagnetic system. In this regard, particularly the Ising model and the Blume-Capel model, have been simulated in the presence of an externally applied magnetic field by the Monte-Carlo simulation technique based on the Metropolis algorithm. Magnetisation switching is found to be faster in the presence of disorder, modelled here by a quenched random field. The strength of the random field is observed to play a similar role to that played by temperature. Becker-Döring theory of classical nucleation (originally proposed for the spin-1/2 Ising system) has been verified in the random field Ising model. However, a stronger random field affects the nucleation regime. In a cubic Ising lattice, surface reversal time is found to be different from the bulk reversal time. That distinct behaviour of the surface in contrast to the bulk has been studied here by introducing a relative interfacial interaction strength ($R$). Depending on $R$, temperature, and applied field, a competitive switching of magnetisation of surface and bulk is noticed. The effect of anisotropy ($D$) on the metastable lifetime has been investigated. We report a linear dependency of the mean macroscopic reversal time on a suitably defined microscopic reversal time. The saturated magnetisation $M_f$, after the reversal, is noticed to be strongly dependent on $D$. $M_f$, $D$, and $h$ (field) are found to follow a proposed scaling relation. Finally, Becker-Döring theory as well as Avrami's law are verified in spin-$s$ Ising and Blume-Capel models. The switching time depends on the number of accessible spin states.
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