ABSTRACT: The recent discovery of carrier-induced ferromagnetism in magnetic III-V semiconductors such as InMnAs and GaMnAs has not only opened up new device opportunities but also provided a novel material system in which to study the physics of itinerant carriers interacting with localized spins. This talk will describe our recent magneto-optical studies of ferromagnetic InMnAs/GaSb heterostructures to investigate carrier states, dynamics and interactions. We used femtosecond mid-infrared pulses to create transient carriers in the InMnAs layer and then monitored the time evolution of ferromagnetic order by ultrafast magneto-optical Kerr spectroscopy (MOKE). We observed ultrafast (~2 ps) photo-induced changes in the MOKE signal induced by the large density of transient carriers. Our data clearly demonstrate ferromagnetic order is affected and controlled in the picosecond time scale.