Giacomo SALA: Asynchronous current-induced switching of rare-earth and transition-metal sublattices in ferrimagnetic alloys

Abstract:
Rare-earth transition-metal (RE-TM) ferrimagnetic alloys have raised considerable interest because of their  ultrafast laser- and current-induced dynamics. Intense laser pulses toggle the RE-TM magnetization in few ps [1,2], and spin-orbit torques drive ferrimagnetic domain walls at record speeds of the order of km/s [3,4].
However, there remain important questions on the current-induced sublattice dynamics and the interaction of the RE and TM magnetic moments with spin-orbit torques. Here, I will present complementary studies of the switching of the RE- TM magnetization that address these open points.
First, time-resolved Hall measurements reveal that the overall switching speed is limited by the nucleation of a seed domain and that this process is influenced by stochastic thermal fluctuations [5]. The speed and reproducibility of the dynamics can be enhanced by reducing the duration of the electric pulses to the sub-ns regime. Second, element-resolved X-ray measurements disclose a variety of dynamics characterized by a
variable degree of coupling between the RE and TM sublattices [6]. Surprisingly, they can switch asynchronously in time and inhomogeneously in space and form a transient ferromagnetic state as long as 2 ns.
Micromagnetic simulations and atomic-structure measurements show that the asynchronous switching results from the combination of two factors: 1) the weak intersublattice antiferromagnetic coupling, which is strongly dependent on the atomic structure, and 2) the master-agent dynamics determined by the unequal action of spinorbit torques on the two sublattices. Overall, these studies provide a deeper insight into the ferrimagnetic dynamics induced by spin-orbit torques.

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