The Role of Angular Momentum in Ultrafast Magnetization Dynamics

Abstract

Many aspects of magnetization dynamics are related to the fact that a certain amount of angular momentum is associated with magnetic moments. Here the dynamics of angular momentum is considered in ferrimagnetic rare-earth–transition-metal alloys that consist of two antiferromagnetically coupled sublattices, where both magnetization and angular momenta are temperature dependent. For certain compositions, such ferrimagnets can exhibit a magnetization compensation temperature T _M where the magnetizations of the sublattices cancel each other, and similarly, an angular momentum compensation temperature T _A where the net angular momentum vanishes. At the latter point, the frequency of the homogeneous spin precession diverges. As a consequence, ultrafast heating of a ferrimagnet across its compensation points may result in subpicosecond magnetization reversal. Moreover, the magnetization can be manipulated and even reversed by a single 40 femtosecond laser pulse, without any applied magnetic field. This optically induced ultrafast magnetization reversal is the combined result of laser heating of the magnetic system and the exchange interaction between the sublattices. This novel reversal pathway is shown to crucially depend on the net angular momentum, reflecting the balance of the two oppositely magnetized sublattices.