Diep Hung T
Laboratoire de Physique Théorique et Modélisation, Université de Cergy-Pontoise, CNRS, UMR 80892, Avenue Adolphe Chauvin, CEDEX, 95302 Cergy-Pontoise, France.
Entropy (Basel). 2019 Feb 13;21(2):175. doi: 10.3390/e21020175.
In this review, we outline some principal theoretical knowledge of the properties of frustrated spin systems and magnetic thin films. The two points we would like to emphasize: (i) the physics in low dimensions where exact solutions can be obtained; (ii) the physics at phase boundaries where interesting phenomena can occur due to competing interactions of the two phases around the boundary. This competition causes a frustration. We will concentrate our attention on magnetic thin films and phenomena occurring near the boundary of two phases of different symmetries. Two-dimensional (2D) systems are in fact the limiting case of thin films with a monolayer. Naturally, we will treat this case at the beginning. We begin by defining the frustration and giving examples of frustrated 2D Ising systems that we can exactly solve by transforming them into vertex models. We will show that these simple systems already contain most of the striking features of frustrated systems such as the high degeneracy of the ground state (GS), many phases in the GS phase diagram in the space of interaction parameters, the reentrance occurring near the boundaries of these phases, the disorder lines in the paramagnetic phase, and the partial disorder coexisting with the order at equilibrium. Thin films are then presented with different aspects: surface elementary excitations (surface spin waves), surface phase transition, and criticality. Several examples are shown and discussed. New results on skyrmions in thin films and superlattices are also displayed. By the examples presented in this review we show that the frustration when combined with the surface effect in low dimensions gives rise to striking phenomena observed in particular near the phase boundaries.
在本综述中，我们概述了受挫自旋系统和磁性薄膜性质的一些主要理论知识。我们想要强调两点：（i）低维物理，在其中可以获得精确解；（ii）相边界处的物理，在那里由于边界周围两相的竞争相互作用会出现有趣的现象。这种竞争导致了受挫。我们将把注意力集中在磁性薄膜以及不同对称性的两相边界附近出现的现象上。二维（2D）系统实际上是具有单层的薄膜的极限情况。自然而然地，我们将首先处理这种情况。我们从定义受挫开始，并给出受挫二维伊辛系统的例子，我们可以通过将它们转化为顶点模型来精确求解。我们将表明，这些简单系统已经包含了受挫系统的大多数显著特征，例如基态（GS）的高度简并性、相互作用参数空间中GS相图中的多个相、这些相边界附近出现的再入现象、顺磁相中的无序线以及平衡时与有序共存的部分无序。然后从不同方面介绍薄膜：表面元激发（表面自旋波）、表面相变和临界性。展示并讨论了几个例子。还展示了关于薄膜和超晶格中斯格明子的新结果。通过本综述中给出的例子，我们表明，受挫与低维表面效应相结合会产生特别是在相边界附近观察到的显著现象。
