Magnetic Dipolar Interactions and Reentrant Spin-Glass Transition

重入自旋玻璃(RSG)相变是自旋玻璃(SGS)的一种众所周知的现象。然而，负责RSG过渡的机制尚未解决。在这个项目中，我们首次发现了一个再现RSG现象的现实模型，并研究了RSG转变的机制。RSG模型我们考虑了具有反铁磁次近邻相互作用的稀铁磁(FM)海森堡模型AxBi-x(0&lt；x&lt；L)的磁序，其中A是铁磁原子，B是非磁原子。我们用x~0.8对不同尺寸的LXLXL晶格进行了大规模的蒙特卡罗模拟，发现该模型再现了RSG相变。也就是说，随着温度的降低，磁化强度在一定温度以下迅速增加，达到最大值，然后在较低的温度下停止。低温相为铁磁性团簇…表征的SG相更多的人。然后，在对不同的x进行相同的模拟后，我们确定了该模型的“浓度(X)-温度(7)相图”。我们应用该模型研究了EuxSRI-Xs的相图，这是该模型的一个实验实现。RSG跃迁和涨落星团我们研究了稀薄FM模型中的RSG跃迁。我们计算了位置的磁化强度，开发了一种新的模拟算法，去掉了自旋的均匀旋转。结果表明，在FM相中存在自旋起伏的团簇。随着温度的降低，这些团簇中的自旋冻结，FM长程有序断裂为铁磁性团簇。这些结果与实验观测结果是一致的。起伏的团簇出现在自旋浓度较低的区域。因此，我们认为RSG相变来自于磁原子的局域浓度的散射。Heisenberg模型的遗传算法我们在推广了Pal的Ising SG遗传算法的基础上，发展了Heisenberg模型的遗传算法。利用该算法分析了稀化模型的基态性质。较少

Reentrant spin-glass (RSG) transition is a well-known phenomenon of spin glasses (SGs). However, the mechanism responsible for the RSG transition has not yet been resolved. In this project, we found for the first time a realistic model which reproduces the RSG phenomena and studied the mechanism responsible to the RSG transition.1. RSG ModelWe considered the magnetic ordering of a dilute ferromagnetic (FM) Heisenberg model AxBi-x (0<x<l) with antiferromagnetic next-nearest neighbor interactions, where A is a ferromagnetic atom and B a non-magnetic atom. We performed a large scale Monte Carlo simulation of the model with x~0.8 on various sizes of the LXLXL lattice and found that the model reproduces the RSG transition. That is, as the temperature is decreased, the magnetization increases rapidly below a certain temperature, reaches a maximum value, then ceases at some lower temperature. The low temperature phase was suggested to be a SG phase that is characterized by ferromagnetic clust … More ers. Then having performed the same simulation for various x, we determined a "concentration(x)-temperature(7) phase diagram"of the model. We applied the model to study the phase diagram of EuxSri-xS, which is an experimental realization of the model.2. RSG Transition and Fluctuating ClustersWe examined the RSG transition in the dilute FM model. We calculated the site magnetization having developed a novel simulation algorism that removes uniform rotation of the spins. Results showed that clusters of fluctuating spins exist in the FM phase. As the temperature is decreased, the spins in those clusters freeze and the FM long-range order breaks to ferromagnetic clusters. These results are compatible with experimental observations. The fluctuating clusters appear in domains with lower spin concentrations. Hence we suggested that the RSG transition comes from a scattering of local concentration of the magnetic atoms.3. Genetic Algorism for Heisenberg ModelsWe developed a genetic algorism for Heisenberg model having generalized Pal's genetic algorism for the Ising SG. The algorism was used to analyze the ground state properties of the dilute model. Less