Deutsch J M, Mai Trieu
Department of Physics, University of California, Santa Cruz, California 95064, USA.
Phys Rev E Stat Nonlin Soft Matter Phys. 2005 Jul;72(1 Pt 2):016115. doi: 10.1103/PhysRevE.72.016115. Epub 2005 Jul 18.
Magnetic thin films exhibit a strong variation in properties depending on their degree of disorder. Recent coherent x-ray speckle experiments on magnetic films have measured the loss of correlation between configurations at opposite fields and at the same field, upon repeated field cycling. We perform finite temperature numerical simulations on these systems that provide a comprehensive explanation for the experimental results. The simulations demonstrate, in accordance with experiments, that the memory of configurations increases with film disorder. We find that nontrivial microscopic differences exist between the zero field spin configuration obtained by starting from a large positive field and the zero field configuration starting at a large negative field. This seemingly paradoxical behavior is due to the nature of the vector spin dynamics and is also seen in the experiments. For low disorder, there is an instability which causes the spontaneous growth of linelike domains at a critical field, also in accord with experiments. It is this unstable growth, which is highly sensitive to thermal noise, that is responsible for the small correlation between patterns under repeated cycling. The domain patterns, hysteresis loops, and memory properties of our simulated systems match remarkably well with the real experimental systems.
磁性薄膜的性质会因其无序程度而呈现出强烈变化。近期对磁性薄膜进行的相干X射线散斑实验测量了在反复的磁场循环过程中，相反磁场和相同磁场下构型之间相关性的丧失。我们对这些系统进行了有限温度数值模拟，为实验结果提供了全面解释。模拟结果表明，与实验一致，构型的记忆随着薄膜无序度的增加而增强。我们发现，从大的正磁场开始得到的零场自旋构型与从大的负磁场开始的零场构型之间存在非平凡的微观差异。这种看似矛盾的行为是由于矢量自旋动力学的性质导致的，在实验中也能观察到。对于低无序度情况，存在一种不稳定性，它会在临界磁场下导致线状畴的自发增长，这也与实验相符。正是这种对热噪声高度敏感的不稳定增长，导致了反复循环下图案之间的小相关性。我们模拟系统的畴图案、磁滞回线和记忆特性与实际实验系统非常匹配。
