Ultrafast dynamical path for the switching of a ferrimagnet after femtosecond heating

Ultrafast laser-induced magnetic switching in rare earth-transition metal ferrimagnetic alloys has recently been reported to occur by ultrafast heating alone. Using atomistic simulations and a ferrimagnetic Landau-Lifshitz-Bloch formalism, we demonstrate that for switching to occur it is necessary that angular momentum is transferred from the longitudinal to transverse magnetization components in the transition metal. This dynamical path leads to the transfer of the angular momentum to the rare earth metal and magnetization switching with subsequent ultrafast precession caused by the intersublattice exchange field on the atomic scale.

Figure 1

Atomistic computer simulations of laser induced magnetization reversal in a $40\times 40\times 5$ nm${}^3$ size GdFeCo ferrimagnetic dot. The $z$ component of each sublattice shows that the reversal takes place, while the $x$ component shows that a magnetic precession is developed.

Figure 2

Magnetization dynamics for a range of pulse temperatures, $T_{\text{max}}$. Panels (a) and (b) show the $z$ component, and magnitude of the transverse component respectively. Panel (c) shows the peak value of the transverse component as a function of the pulse temperature, points represent the data and the line is a guide to the eye. The schematic shows what we observe from the model, namely, a mutual precessional motion due to the exchange field exerted by the opposite sublattice with high frequency precessional motion towards the direction of the other sublattice.

Figure 3

Numerical integration of the switching behavior for the nonstochastic LLB with a small angle (${15}^{\circ }$) between sublattices (solid lines). Without the angle (dashed lines) switching does not occur, as predicted. The time $t=0$ corresponds to the end of the square shaped laser pulse with $T_{\text{max}}=1500$ K. For the integration at temperatures above $T_C$ we use the paramagnetic version of the ferrimagnetic LLB equation with MFA.[10]

Figure 4

Longitudinal relaxation rates as a function of temperature in the LLB equation, evaluated for GdFeCo parameters. The dashed line shows the TM-RE relaxation rate and the solid line is that of the TM-TM interaction. At low temperatures ${\Gamma }_{\text{TT}}\gg {\Gamma }_{\text{TR}}$, due to the small value of the susceptibility ${\tilde{\chi }}_{\text{T},\parallel }$, therefore the relaxation of the TM magnetization is always to its own equilibrium. However, at temperatures close to $T_c$, ${\Gamma }_{\text{TT}}<{\Gamma }_{\text{TR}}$, thus the TM prefers to relax towards the RE magnetization in this regime.