Partially Disorered Dipole-and Staggered Quadrupole-Ordering in Spin Systems with Competing Interactions

本研究对具有阻挫诱导双线性和/或双二次交换作用的系统中偶极和/或四极序的稳定性和特征类型的物理起源进行了深入系统的研究，主要结果如下：1）对于双线性-双二次交换± J-Ising模型，我们进行了非平衡弛豫MC计算，阐明了随机性减弱了铁磁偶极有序的重入现象，但对交错四极有序没有影响。临界指数依赖于（而不是依赖于）偶极（四极）有序的随机性。2）为了讨论轴向次近邻Ising（ANNNI）模型中的部分无序态，我们提出了两个扩展模型，即（i）层内交换交替模型和（ii）混合自旋模型。通过标准MC和位置依赖分子场计算，我们证实了两种模型的部分无序态的稳定性以及熵贡献对部分无序态出现的重要性。3）我们利用直方图MC模拟方法研究了双二次相互作用对3D堆积三角晶格结构的经典平面旋转体模型的影响。我们得到了该体系完整的磁相图，阐明了出现的各种相态的本质，包括不对称或手征偶极（四极）有序等。

In the present study, we performed through and systematic investigations of the stability and the physical origin of characteristic types of dipole and/or quadrupole orderings in the systems with frustration inducing bilinear and/or biquadratic exchange interactions.Main results are as follows ;1)For bilinear-biquadratic exchange ± J Ising model, we performed the non-equilibrium relaxation MC calculation and clarified that randomness weakens the re-enntrant phenomena of ferromagnetic dipole ordering but not affects on the ataggered quadrupole ordering. The critical exponents depend (not depend) on the randomeness for the dipole (quadrupole) ordering.2)In order to discuss the partially disordered states in the axial next nearest neighbor Ising (ANNNI) model, we proposed two extended models, i.e. (i) intralayer exchang alternating model and (ii) mixed spin model. By using standard MC and site dependent molecular field calculation, for both models we confirmed the stability of partially disordered states and the importance of entropy contribution for occurence of the partially disordered states.3)We have investigated the effects of the biquadratic interactions on 3D classical plane rotator model with stacked triangular lattice geometry by use of histogram MC simulation methods. We obtained complete magnetic phase diagrams for the system, and clarified the nature of appearing various pahases including incommensurate-or chiral dipole (quadrupole) ordering, etc.