AI 中文总结
本讲义综述超薄铁磁薄膜的基本原理，分析其二维特性、磁序相变及拓扑激发，解释相关物理现象并指出部分相变机制待明确。
AI 中文摘要
本讲义综述了由3d过渡金属覆盖层构成的超薄薄膜铁磁性研究中形成的一些基本原理。这类薄膜通常以逐层方式生长，该生长模式会沿垂直方向形成量子阱，深刻影响材料的任何物理性质。此外，垂直限域形成了自旋系综，其沿面内方向延伸至宏观距离，沿垂直（法向）方向有限，即呈二维特征。因此，它们展现出源于二维性的基态性质，如“死”磁层或“增强磁矩”、振荡层间磁耦合，以及异常的垂直相对于面内磁各向异性，后者在某些特定情形下会导致自旋的垂直集体取向。在有限温度下，铁磁序持续存在，采用重正化群对超薄薄膜磁序进行分析，为解释这一现象提供了合适框架。铁磁序在相变处消失，该相变紧密遵循二维伊辛普适类，这由对临界点附近数据的精确分析所证实。常观察到垂直自旋取向通过重取向相变转为面内取向，该相变也可由重正化群论证恰当描述。最后，垂直自旋取向会引入铁磁序的拓扑激发，即由反向垂直自旋方向的条纹构成，条纹序向顺磁态的相变尚未完全明晰。
英文摘要
In these Lecture Notes we review some of the fundamental principles that have emerged from research on the ferromagnetism of ultrathin films consisting of 3d transition-metal overlayers. Their growth is often layer-by-layer. This growth mode produces quantum wells along the vertical direction that profoundly impact any physical property of the materials. In addition, the vertical confinement establishes spin ensembles that extend to macroscopic distances along the in-plane directions and are finite along the vertical (perpendicular) direction, i.e. they are two-dimensional. Accordingly, they display ground state properties that originate from the two-dimensionality, such as ``dead'' magnetic layers or ``enhanced magnetic moments'', an oscillatory interlayer magnetic coupling and an anomalous perpendicular versus in-plane magnetic anisotropy that produces, in some specific situations, a perpendicular collective orientation of the spins. At finite temperatures, ferromagnetic order is observed to persist and an analysis of the magnetic order of ultrathin films in terms of the renormalization group provides a suitable framework for explaining this observation. The ferromagnetic order is lost at a phase transition which follows closely the two-dimensional Ising universality class, as shown by an accurate analysis of data in the vicinity of the critical point. The perpendicular spin orientation is often observed to turn in-plane by a reorientation phase transition which is also properly described by a renormalization group argument. Finally, the perpendicular spin orientation introduces topological excitations of the ferromagnetic order, consisting of stripes of reversed perpendicular spin direction. The stripe order undergoes a phase transition to the paramagnetic state that is not yet completely understood.
