Physics of 2D magnets and magnetic thin films: Surface structure and surface phase transition, criticality and skyrmions

Abstract: Recently, there is an increasing renewed interest in 2D magnetism such as Van der Waals magnets. The physics of 2D magnetism and ultra-thin magnetic films has a long history. This chapter is a review devoted to some fundamental theoretical properties of 2D magnets and and magnetic thin films including frustrated systems and topological spin textures. These properties allow to understand macroscopic behaviors experimentally observed in thin films and superlattices where the surface and the interface play a crucial role. The chapter begins with a review on 2D magnets, their spin structures and phase transitions. Next, the case of thin films is considered. The theory of surface spin waves is discussed in various situations with and without surface reconstruction of spin ordering. Various interactions are taken into account: surface interaction different from the bulk one, competing interactions, Dzyaloshinskii-Moriya interaction. Surface phase transitions are shown in some particularly striking cases. Finally, some cases of topological spin textures called "skyrmions" are reviewed. All the results shown in this chapter have been published in various research papers cited in the text. Therefore, we will discuss some important results but avoid to enter complicated methods. Instead, the reader is referred to original papers for detailed demonstrations.