Magnetism of Two-Dimensional Ferromagnetic Materials

Abstract

In recent years, the discovery of numerous two-dimensional (2D) magnetic materials exhibiting unique magnetic and spin transport properties has opened up new possibilities for spintronics applications. While long-range ferromagnetic ordering is theoretically unstable in 2D systems due to strong thermal fluctuations, this can be overcome by introducing anisotropy, which breaks continuous symmetry and allows for the emergence of long-range order. But still, the magnetization magnitude in 2D systems depends on both the exchange interaction and the total effective field, even at low temperatures, in contrast to 3D systems in which the magnetization magnitude only depends on the exchange interaction. Thus, two-dimensional magnetization is fundamentally different than three-dimensional magnetization. In this dissertation, we employ a self-consistent Random Phase Approximation to analytically investigate magnetization in 2D systems, accounting for the significant spin fluctuations. We examine the impact of random fields on 2D magnets and reveal a shift from a second-order to a first-order phase transition. Additionally, we develop a two-dimensional quantum Stoner-Wohlfarth model, demonstrating the temperature dependence of hysteresis and astroid diagrams, which differs from the 3D case. Finally, our simulation of the magnetization reversal dynamics in a 2D single-domain magnet shows that the process usually involves magnetization collapse followed by remagnetization in the direction of the external field, resulting in a significantly shorter reversal time.