Exchange-driven all-optical magnetic switching in compensated $3d$ ferrimagnets

Understanding how a single laser pulse can toggle magnetization in a compensated $3d$ ferrimagnet is a critical problem in ultrafast magnetism. To resolve it, we test single-shot all-optical switching of magnetization in ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ at different temperatures using femto- to picosecond pulses in the visible to far-infrared spectral ranges. The switching process is found to be independent of photon energy, but strongly dependent on both the pulse duration and sample temperature. Switching is disabled whenever the starting temperature $T_0$ is above the compensation point of ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$, but as $T_0$ is lowered below compensation, increasingly longer pulses become capable of toggling the magnetization. We explain the observations in terms of a switching process driven by exchange relaxation of the angular momenta of the manganese sublattices, and propose a common framework to account for the similarities and differences of all-optical switching in ${\mathrm{Mn}}_2{\mathrm{Ru}}_x\mathrm{Ga}$ and GdFeCo alloys.

Figure 1

Typical background-corrected magneto-optical images taken after exposing ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.75}\mathrm{Ga}$ at room temperature to consecutive optical pulses of photon energy 124 meV and duration $\tau$ as indicated in rows (a) and (b). The images shown in row (a) demonstrate single-shot AOS whereas those shown in row (b) demonstrate demagnetization.

Figure 2

State map recorded for ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.75}\mathrm{Ga}$ indicating whether single-shot AOS, demagnetization or a mixture of the two effects is achieved by an 800-nm pulse with the indicated duration at the starting temperature $T_0$.

Figure 3

Threshold pulse duration ${\tau }_c$ for the two MRG and the GdFeCo films measured as a function of the difference between the compensation temperature and the measurement base temperature $T_0$. The compensation temperatures $T_{\text{comp}}$ of ${\mathrm{Gd}}_{25}$(FeCo)${}_{75}$, ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.8}\mathrm{Ga}$, and ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.75}\mathrm{Ga}$ are 320, 345, and 370 K, respectively.

Figure 4

State map recorded for ${\mathrm{Mn}}_2{\mathrm{Ru}}_{0.75}\mathrm{Ga}$ indicating whether single-shot AOS or demagnetization is achieved by a pulse with the indicated photon energy and pulse duration, measured at room temperature.

Figure 5

(a) Representative equilibrium thermal dependence of angular momentum $S$ in MRG, showing the change in angular momentum of the $4a$ (red) and $4c$ (blue) sublattice as the temperature ($T_0$) varies in equilibrium. Equilibrium changes in temperature $\mathrm{\Delta }T$ lead to nonequal changes of angular momenta $|\mathrm{\Delta }{S}_{4a}|\ne |\mathrm{\Delta }{S}_{4c}|$ via spin-lattice relaxation. (b) The nonequilibrium exchange-driven process of magnetic switching triggered by the 100-fs laser pulse at $T_0<T_{\text{comp}}$. The exchange relaxation results in $|\mathrm{\Delta }{S}_{4a}|=|\mathrm{\Delta }{S}_{4c}|$ when the temperature changes in nonequilibrium. Since $|S_{4a,0}|<|S_{4c,0}|$, $S_{4a}$ crosses zero before $S_{4c}$, giving rise to successful magnetic switching.