Ultrafast Spin Dynamics and Critical Behavior in Half-Metallic Ferromagnet:Sr2FeMoO6

Ultrafast spin dynamics in ferromagnetic half-metallic compound ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$ is investigated by pump-probe measurements of the magneto-optical Kerr effect. The half-metallic nature of this material gives rise to anomalous thermal insulation between spins and electrons and allows us to pursue the spin dynamics from a few to several hundred picoseconds after the optical excitation. The optically detected magnetization dynamics clearly shows the crossover from microscopic photoinduced demagnetization to macroscopic critical behavior with universal power law divergence of relaxation time for a wide dynamical critical region.