General Features of Photoinduced Spin Dynamics in Ferromagnetic and Ferrimagnetic Compounds

Ultrafast photoinduced spin dynamics has been investigated by time-resolved magneto-optical Kerr spectroscopy for various ferromagnetic and ferrimagnetic compounds: ${\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{r}}_2{\mathrm{S}}_4$, ${\mathrm{C}\mathrm{o}\mathrm{C}\mathrm{r}}_2{\mathrm{S}}_4$, ${\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{r}}_2{\mathrm{S}\mathrm{e}}_4$, ${\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{r}}_2{\mathrm{S}\mathrm{e}}_4$, ${\mathrm{L}\mathrm{a}}_{0.6}{\mathrm{S}\mathrm{r}}_{0.4}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$, and ${\mathrm{S}\mathrm{r}\mathrm{R}\mathrm{u}\mathrm{O}}_3$. The temporal demagnetization process, which is observed commonly for all the compounds, essentially consists of two components: One is an instantaneous change which originates perhaps from multiple emissions of magnetic excitations during nonradiative decay of photoexcited carriers, and the other is a delayed response due to thermalization of the spin system. The time constant of the delayed change depends strongly on materials and is scaled with the magnetocrystalline anisotropy, indicating that spin-orbit coupling is a dominant interaction for this process.

Figure 1

Photoinduced changes in magneto-optical Kerr rotation or ellipticity. (a) ${\mathrm{F}\mathrm{e}\mathrm{C}\mathrm{r}}_2{\mathrm{S}}_4$ (pump photon energy: 3.1 eV; probe photon energy: 1.9 eV; pump intensity: $85\mu \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$), (b) ${\mathrm{C}\mathrm{o}\mathrm{C}\mathrm{r}}_2{\mathrm{S}}_4$ (3.1 eV, 0.83 eV, $350\mu \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$), (c) ${\mathrm{C}\mathrm{u}\mathrm{C}\mathrm{r}}_2{\mathrm{S}\mathrm{e}}_4$ (3.1 eV, 1.9 eV, $340\mu \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$), (d) ${\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{r}}_2{\mathrm{S}\mathrm{e}}_4$ (3.1 eV, 1.9 eV, $220\mu \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$), (e) ${\mathrm{L}\mathrm{a}}_{0.6}{\mathrm{S}\mathrm{r}}_{0.4}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ (1.5 eV, 3.1 eV, $500\mu \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$), and (f) ${\mathrm{S}\mathrm{r}\mathrm{R}\mathrm{u}\mathrm{O}}_3$ (1.55 eV, 3.1 eV, $30\mu \mathrm{J}/{\mathrm{c}\mathrm{m}}^2$). (g) Five characteristic components of time-resolved magneto-optical Kerr effect signal as exemplified for ${\mathrm{C}\mathrm{o}\mathrm{C}\mathrm{r}}_2{\mathrm{S}}_4$: spikelike (1), steplike (2), oscillation (3), delayed (4), and decay (5) components, respectively.

Figure 2

Normalized temperature ($T/T_{\mathrm{C}}$) dependence of (a) the steplike component in the temporal response of the photoinduced demagnetization, apart from ${\mathrm{L}\mathrm{a}}_{0.6}{\mathrm{S}\mathrm{r}}_{0.4}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ whose component is too small to discriminate from other components, (b) the magnetization $M$ measured at 0.2 T, (c) the maximum of the photoinduced change in magneto-optical Kerr rotation ($\theta$) or ellipticity ($\eta$) excluding the initial spikelike component and oscillation, and (d) thermally induced change of magnetization, $\Delta M/M_{\mathrm{S}}=\frac{M(T+\Delta T)-M(T)}{M_{\mathrm{S}}}$ with $\Delta T=0.1T_{\mathrm{C}}$, where $M_{\mathrm{S}}$ is the lowest-temperature saturation moment.

Figure 3

(a) Normalized temperature ($T/T_C$) dependence of time constant ($\tau$) of the growing delayed component. (b) The inverse of the relaxation time constant ($1/\tau$) is plotted for the minimum and the maximum values against magnetocrystalline anisotropy constant ($K_1$). Dashed lines are the guides to the eyes.

Figure 4

Schematic diagram of the demagnetization process after photoexcitation.