First Femtosecond Coherent Spin Reorientation in Magnetic Nanostructures
An exciting question in modern magnetism and materials science is whether one can detect, manipulate, and understand collective spins in their highly nonequilibrium, non-thermal states at femtosecond (10-15 s) time scales. Such processes are at least 1,000 times faster than those of the traditional thermal-magnetic processes, which set the upper limit of the magnetic switching time in modern magneto-optical recording technology. First anticipated theoretically in 2006, the first experimental demonstration for light-induced magnetization rotation in the femtosecond regime (∼200 femtoseconds) in magnetic semiconductors occurred in 2009, using GaMnAs/ GaAs heterostructures of 73 nm thickness. The initial 80-femtosecond laser pulse at 3.1eV induces spin rotation. This rotation angle is measured by a second femtosecond, polarized pulse using the optical Kerr effect (θK). These results provide a strong experimental demonstration of how a laser field can modify the collective spin rotation via photoexcited coherences at femtosecond time scales, much faster than those from thermal fluctuations. In summary, dynamic magneto-optical spectroscopy, exploiting femtosecond laser pulses, was used to demonstrate and explain (via simulations) photo-induced coherent magnetization rotation in a ferromagnetic semiconductor during 100 femtoseconds.