Ultrafast Measurement of Critical Slowing Down of Hole-Spin Relaxation in Ferromagnetic GaMnAs

We have studied ultrafast photoinduced hole spin relaxation in GaMnAs via degenerate ultrafast magneto-optical Kerr spectroscopy. Near-infrared pump pulses strongly excite the sample, and probe pulses at the same photon energy reveal subpicosecond demagnetization accompanied by energy and spin relaxation of holes manifesting themselves as a fast ($\sim $200fs) and a slow (ps) recovery of transient MOKE signals. By carefully analyzing the temporal profiles at different temperatures, we are able to isolate femtosecond hole spin relaxation processes, which are subject to a critical slowing down near the critical temperature of 77K. These results demonstrate a new spectroscopy tool to study the highly elusive hole spin relaxation processes in heavily-doped, correlated spin systems, and have important implications for future applications of these materials in spintronics and magnetic-photonic-electronic multifunctional devices.