Ultrafast Photoinduced Insulator-Ferromagnet Transition in the Perovskite Manganite ${\mathrm{Gd}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_3$

We have investigated the ultrafast spin and charge dynamics in the course of a photoinduced phase transition from an insulator with short-range charge order and orbital order (OO) to a ferromagnetic metal in perovskite-type ${\mathrm{Gd}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_3$. Transient reflectivity changes suggest that the metallic state is formed just after the photoirradiation and decays within $\sim 1\mathrm{ps}$. The magnetization, however, increases with the time constant of 0.5 ps and decays in $\sim 10\mathrm{ps}$. The relatively slow increase of the magnetization is attributable to the magnetic-field-induced alignment of ferromagnetic domains in the initially produced metallic state and its slow decay to the partial recovery of the OO.

Figure 1

(a) The phase diagram consisted of mainly $R_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_3$ series [14]. (b) Reflectivity spectra of ${\mathrm{Gd}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_3$ and ${\mathrm{Eu}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{MnO}}_3$. (c) The differential reflectivity spectrum $\Delta R/R$ just after photoexcitation ($\sim 0\mathrm{ps}$, circles) and the numerical one $[R(\mathrm{Eu})-R(\mathrm{Gd})]/R(\mathrm{Gd})$ (solid line) at 10 K.

Figure 2

Time evolutions of $\Delta R/R$ at 0.12 and 3.1 eV (a),(b) and $\Delta {\theta }_{\mathrm{Kerr}}$ at 3.1 eV at 10 K (c),(d). The excitation density is $0.42\mathrm{mJ}/{\mathrm{cm}}^2$ in common. In (a), the solid, dotted, and broken lines show the result of fitting (see text). The schematic of TRMOKE is shown in the inset of (d).

Figure 3

Time evolutions of $\Delta {\theta }_{\mathrm{Kerr}}$ at 10 K for different excitation densities (a),(b) and for $I_0=0.42\mathrm{mJ}/{\mathrm{cm}}^2$ at different temperatures (c),(d).

Figure 4

(a) Temperature dependence of magnetization measured with zero-field-cooled (ZFC) and field-cooled (FC) runs under 0.25 T. (b),(c) Temperature dependence of (b) fast and (c) slow components of $\Delta {\theta }_{\mathrm{Kerr}}$ at 100 ps and 1 ns for $0.42\mathrm{mJ}/{\mathrm{cm}}^2$. The maximum value of the fast component ($\Delta {\theta }_{\mathrm{Kerr}}\sim 1.7\mathrm{mdeg}$) corresponds to about 1% of the full moment. The thermally induced change of magnetization, $\Delta M=M(T+\Delta T)-M(T)$ with $\Delta T=50\mathrm{K}$, is shown by the solid line in (c). (d) The schematics of ultrafast ferromagnetism and subsequent demagnetization processes.