Orbital-resolved spin model for thermal magnetization switching in rare-earth-based ferrimagnets

(Received 10 December 2012; revised manuscript received 10 June 2013; published 24 July 2013) The switching of rare-earth-based ferrimagnets triggered by thermal excitation is investigated on the basis of an atomistic spin model beyond the rigid-spin approximation, distinguishing magnetic moments due to electrons in d and f orbitals of the rare earth. It is shown that after excitation of the conduction electrons a transient ferromagneticlike state follows from a dissipationless spin dynamics where energy and angular momentum are distributed between the two sublattices. The final relaxation can then lead to a new state with the magnetization switched with respect to the initial state. The time scale of the switching event is to a large extent determined by the exchange interaction between the two sublattices. The quest for ever increasing speed of data procession has its bottleneck in data storage with current hard disk writing events being on the time scale of nanoseconds. Much quicker writing schemes have been demonstrated based on all-optical magnetization reversal mechanisms using circularly polarized laser light, with the helicity of the light determining the direction of magnetization in the written area. 1‐5 Most surprisingly, it has been demonstrated that even linearly polarized light can trigger a thermally driven switching in ferrimagnetic GdFeCo compounds 6,7 via a so-called “ferromagneticlike state,” where the rare-earth (RE) and transition-metal (TM) sublattice magnetizations are aligned parallel on a picosecond time scale. With these experiments the theoretical understanding of magnetization dynamics in terms of the macroscopic LandauLifshitz-Gilbert (LLG) equation of motion has reached its limits. The short time scale of the laser pulse in connection with the high electron temperatures following the excitation lead to nonequilibrium processes where longitudinal magnetization dynamics becomes pronounced. 2,8‐13 Af ull theoretical explanation of the thermally driven switching process in ferrimagnets and, particularly of the transient ferromagneticlike state, is still missing, though first attempts of a description of longitudinal magnetization dynamics in two-sublattice systems have been proposed recently. 10,14