Magnetic switching dynamics due to ultrafast exchange scattering: A model study

We study the heat-induced magnetization dynamics in a toy model of a ferrimagnetic alloy, which includes localized spins antiferromagnetically coupled to an itinerant carrier system with a Stoner gap. We determine the one-particle spin-density matrix including exchange scattering between localized and itinerant bands as well as scattering with phonons. While a transient ferromagneticlike state can always be achieved by a sufficiently strong excitation, this transient ferromagneticlike state only leads to magnetization switching for model parameters that also yield a compensation point in the equilibrium $M\left(T\right)$ curve.

Figure 1

Computed results for a demagnetization scenario with $J=100$ meV, $U=400\text{meV}$, and $T_{\text{e}}^{\left(0\right)}=1500\text{K}$. The spin-flip times are (a) ${\tau }_{\text{sf}}=200\text{fs}$ and (b) ${\tau }_{\text{sf}}\to \infty$. Top: Dynamics of the averaged localized spin $S_{\parallel }$ (solid curve) and itinerant spin $s_{\parallel }$ (dashed curve). Middle: Quasiequilibrium temperatures and $T_{\text{C}}$ (thin line). Bottom: Quasichemical potentials for the two itinerant bands.

Figure 2

Top: Computed dynamics of the average localized spin $S_{\parallel }$ (solid curve) and the average itinerant spin $s_{\parallel }$ (dashed curve) for a TFS scenario with (a) $T_{\text{e}}^{\left(0\right)}=2000\text{K}$ and $U=400\text{meV}$ as well as a switching scenario with (b) $T_{\text{e}}^{\left(0\right)}=2500\text{K}$ and $U=500\text{meV}$. Bottom: Electronic quasichemical potentials. $J=100$ meV is unchanged.

Figure 3

Computed dynamics of average localized spin $S_{\parallel }$ (red, starting from 1) and average itinerant spin (blue, starting from $-0.5)$ after excitation with $T_{\text{e}}^{\left(0\right)}=1500$ K (solid lines), $T_{\text{e}}^{\left(0\right)}=2000$ K (dashed lines), and $T_{\text{e}}^{\left(0\right)}=2500$ K (dashed-dotted lines) for (a) $U=400$ meV, (b) $U=500$ meV, and $J=100$ meV.

Figure 4

Color-coded dynamics: Demagnetization (black), transient ferromagnetic-like state (gray), and switching (white) as a function of Stoner parameter $U$ and excitation temperature $T_{\text{e}}^{\left(0\right)}$. The exchange coupling constant is $J=100$ meV. The arrow marks the “critical value” of $U=445$ K, above which a compensation point in the equilibrium $M\left(T\right)$ relation occurs.