An attempt to simulate laser-induced all-optical spin switching in a crystalline ferrimagnet

Abstract

Interest in all-optical spin switching (AOS) is growing rapidly. The recent discovery of AOS in Mn2RuGa provides a much needed clean case of crystalline ferrimagnets for theoretical simulations. Here, we attempt to simulate it using the state-of-the-art first-principles method combined with the Heisenberg exchange model. We first compute the spin moments at two inequivalent manganese sites and then feed them into our model Hamiltonian. We employ an ultrafast laser pulse to switch the spins. We find that there is a similar optimal laser field amplitude to switch spins. However, we find that the exchange interaction has a significant effect on the system switchability. Weakening the exchange interaction could make the system unswitchable. This provides a crucial insight into the switching mechanism in ferrimagnets.

First-principles insights into all-optical spin switching in the half-metallic Heusler ferrimagnet Mn$_2$RuGa

“…In GdFeCo 2 Gd is SAS while Fe is OAS. Switching starts with OAS 52 ; because the ferrimagnetic coupling between SAS and OAS is frustrated and has a lower potential barrier to overcome if the spin moment is smaller 60 , SAS is dragged into the opposite direction by OAS through the spin torque JS i × S j 38,41 , to realize all-optical spin switching. Because the Heusler compounds have excellent tunability 13,17,19,22,29,33 , the future research can investigate the effect of the spin moments at SAS and OAS on the spin switchability 8 .…”

“…Just as found in GdFeCo 2,11 , for every consecutive pulse, the spin direction is switched. This discovery 37 demonstrates the extraordinary tunability of Heusler compounds, which now changes the trajectory of AOS research [38][39][40] , with Mn 2 RuGa as a crystalline model system where the first-principles investigation is now possible.…”

“…Table II summarizes these results. In general, the orbital moment on Mn 1 is small, around 0.025µ B , and that on Mn 2 is slightly larger, reaching -0.046µ B , which is beneficial to the spin-orbit torque 38,43 , important for AOS 37 .…”

All-optical spin switching on an ultrafast time scale

“…According to Yang et al [76] and Galanakis et al [77], Ru takes the (4b) position and Ga takes (4d), but according to Zic et al [78] and Fowley et al [79], it is the other way around. First-principles calculations with the WIEN2k code [80,81], yield the same total energy but with the spins on the Mn atoms swapped, so they are equivalent [82]. Mn(4a) has a spin moment of 3.17 232 µ B Table 3.…”

“…The first theoretical simulation [82] adopted a simple cubic lattice with each site occupied by Mn(4c) and Mn(4d) atoms, The z component of the spins at the Mn 1 and Mn 2 sites as a function of time at the optimal laser field amplitude. Here, the laser amplitude is 0.017 V Å −1 , and the pulse duration is 60 fs.…”