HMCSD Simulation on Metaric Spin-Glass Alloys

1.计算机模拟算法的发展对于研究复杂系统，特别是自旋玻璃模型，我们最近发展了两种有效的模拟方法：(1)海森堡模型的混合蒙特卡罗自旋动力学(HMCSD)方法；(2)伊辛模型的团簇热浴(CHB)方法。Rudermann-Kittel-Kasuya-Yoshida(RKKY)模型的模拟用RKKY模型描述了金属自旋玻璃合金。(1)我们对RKKY模型进行了详细的模拟，确定了RKKY模型的相图。(2)我们研究了偶极相互作用对RKKY模型的影响，发现随着温度的降低，磁化强度在居里温度以下一度增大，然后随着温度的进一步降低而减小。我们认为这是重入自旋玻璃现象的一个关键点。铁磁和反铁磁原子的混合物两个磁性原子的混合物是另一种复杂的系统，预计会出现自旋玻璃。我们对Ising模型和Heisenberg模型进行了计算机模拟，得到了以下结果。(1)在伊辛模型中，我们发现了即使在二维情况下也存在自旋玻璃相的证据，这与以前的推测相反。(2)在海森堡模型中，我们发现了一个纵向铁磁性和横向反铁磁性共存的新型磁相。其他受挫模型我们也研究了各种模型的磁结构，但也有一些受挫。它们是三角形晶格上的量子自旋系统，具有竞争积分作用的类伊辛模型，以及具有偶极相互作用的铁磁海森堡模型。

1. Development of computer simulation algorithmWe have newly developed two simulation methods which are efficient for studying complex systems, especially for spin-glass models : (1) a hybrid Monte-Carlo spin-dynamics (HMCSD) method for the Heisenberg models, and (2) a cluster heat bath (CHB) method for the Ising models.2. Simulation of the Rudermann-Kittel-Kasuya-Yoshida (RKKY) modelMetallic spin-glass alloys are described by using the RKKY model. (1) We have made detailed simulation of the RKKY model and determined the phase diagram of it.(2)We have studied effects of the dipolar interaction on the RKKY model just above its critical consentration and found that, as the temperature is decreased, the magnetization once increases below the Curie temperature then decreases as the temperature is further decreased. We believe this is a key point for reentrant spin-glass phenomena.3. Mixtures of ferromagnetic and antiferromagnetic atomsMixtures of two magnetic atoms are another kind of complex systems for which a spin-glass is expected to occur. We have made computer simulations of both the Ising and Heisenberg models and obtained the following results. (1) In the Ising model, we have found an evidence of the occurrence of the spin-glass phase even for two-dimensions, in contrast with a previous speculation. (2) In the Heisenberg model, we have found a novel magnetic phase in which a longitudinal ferromagnetism and a transverse antiferromagnetism coexist.4. Other frustrated modelsWe have also investigated magnetic structures of various models with some frustrations. Those are a quantum spin systems on the triangular lattice, Ising-like models with competing intaractions, and a ferromagnetic Heisenberg model with dipolar interactions.