Ultrafast Photo-induced Ferromagnetism in III-Mn-V Semiconductors

We report on ultrafast photoenhancement of hole-mediated ferromagnetism, and paramagnetic to ferromagnetic phase transition in III-Mn-V semiconductor GaMnAs via laser excited transient carriers. Our femtosecond UV pump/NIR probe vectorial MOKE spectroscopy reveals sub-picosecond demagnetization, precessional trajectory of the magnetization vector, and establishment of the ferromagnetic order on a 100-ps time scale. The dynamic enhancement of the magnetic ordering, manifesting as the photo-induced magnetization below and above the Curie temperature T$_{c}$, is well explained by a transient increase of T$_{c}$ via a population of photo-generated holes. This constitutes the first evidence for an ultrafast, nonthermal manipulation of the magnetic order in ferromagnetic semiconductors, thereby opening up fascinating opportunities for future high-speed spin-photon-carrier integrated devices, and above GHz magneto-optical recording.