Magnetism of Site Random Model - Coexistence of Ferromagnetism and Antiferromagnetism -

1.自旋玻璃的低温相我们提出了一种计算有限晶格上自旋玻璃模型缺陷能的新方法。利用该方法，我们重新考察了两个模型的自旋玻璃相的稳定性，即二维的伊辛模型和三维的海森堡模型。对于这两种模型，缺陷能随着晶格尺寸的增大而增加，这意味着发生了有限温度相变。这些结果驳斥了以前认为只有在三维伊辛模型中才能实现自旋玻璃相的观点。新的模拟算法我们发展了两种模拟方法，一种是团簇热浴(CHB)方法，另一种是离散更新蒙特卡罗方法。前者是一种解析法和随机法的混合方法，对研究复杂系统非常有效。后者是一种针对长程相互作用体系的方法，用于研究介观体系的自旋结构。介观超薄磁性薄膜的自旋结构我们用离散更新蒙特卡罗方法研究了具有单轴各向异性的微米和亚微米级超薄铁磁薄膜的磁性。我们发现，不同晶格尺寸的薄膜表现出不同的磁性。在小的晶格中，薄膜表现出与通常的铁磁体相似的性质。在中等晶格中，比热呈现双峰，磁化强度在较低的峰值温度附近急剧增加。在大晶格中，磁化强度没有增加，但比热仍呈现双峰。

1. Low Temperature Phase of Spin GlassesWe have proposed a new method for evaluating the defect energy of spin glass models on finite lattices. Using the method, we have reexamined the stability of the spin glass phase of two models, the Ising model in two dimensions and the Heisenberg model in three dimensions. For both the models, the defect energy increases with the size of the lattice, suggesting the occurrence of a finite temperature phase transition. These results disprove the previous idea that the spin glass phase is realized only in the Ising model in three dimensions.2. New Simulation AlgorithmWe have developed two simulation methods, a cluster heat bath (CHB) method and a discrete update Monte Carlo method. The former method is a hybridized one of an analytic method and a stochastic method, and very effective for studying complex systems. The latter method is one for systems with long-range interactions and used for studying the spin structure of mesoscopic systems.3. Spin Structure of Mesoscopic Ultrathin Magnetic FilmsWe have studied magnetic properties of micron- and submicron-scale ultrathin ferromagnetic films with uniaxial anisotropy using a discrete update Monte Carlo method. We have found that those films exhibits different magnetic properties depending on their lattice size. In small lattices, the films exhibit properties similar to those of a usual ferromagnet. In moderate lattices, the specific heat exhibits a double peak and the magnetization increases abruptly around the lower peak temperature. In large lattices, the magnetization does not increase but the specific heat still exhibits a double peak.