メソスコピック薄膜磁性体のスピン構造のモンテカルロシミュレーション

概要

論文の詳細を見る
An efficient Monte-Carlo simulation method for a microscopic magnetic model with both exchange interactions and magnetic dipolar interactions was developed. The method reduces the computer CPU time from of the order N^2 to Nlog_2 N, where N is the number of the spins. Using the method, we studied the spin structure of the model on square lattices of various sizes. As a result, we found that the size of the surface area is an important parameter for determining the magnetic properties of the model. For the isotropic model, as the size increases, the spin structure changes from a ferromagnetic one to a circular (vortical) one. On the other hand, for a model with strong uniaxial anisotropy, we found three lattice size ranges, A, B, and C. In range A (small lattices), the model exhibits properties of the usual Ising model. In range B (mediumsized lattices), the ferromagnetic phase at low temperatures changes to the paramagnetic phase at high temperatures through an intermediate incommensurate pahse. In range C (large lattices), a similar change in the magnetic structure occurs, but the low-temperature phase has striped domains. For a model with moderate strength of the uniaxial anisotropy, we found a magnetic reorientation from an out-of-plane oriented striped phase at low temperature to an in-plane oriented circular phase at high temperature. The spin structures of models of stacked layers are also studied.
2000-03-01