Relaxation times of charges and spins of La$_{0.7}$Ca$_{0.3}$MnO$_3$

The time-resolved photoreflectance and MOKE measurements are used to measure the charge and spin dynamics of La$_{0.7}$Ca$_{0.3}$MnO$_{3}$, a typical colossal magnetoresistance material (CMR). We have directly observed the transient behaviors of charges and spins at temperatures above and below its ferromagnetic transition temperature T$_{C}$ (about 250 K) and in the presence of an external magnetic field ranging from 0 to 6 T. It is surprising that the relaxation time of spins created by a circularly polarized laser pulse is extremely fast at T $>$ T$_{C}$. It is about 130 fs and much faster than the charge carrier relaxation times inferred from the transient reflectance measurements. At T $<$ T$_{C}$, relaxation of spins becomes more complicated. The relaxation starts with a fast time similar to that of above T$_{C}$, then follows with a much slower relaxation time of 64 ps towards a level lower than its original level. The original equilibrium is finally reached about 5 ns later. This complicated relaxation processes of the spins can be explained by a strong spin-spin interactions and the supercooling effect induced by the circularly polarized pump laser.