Laser-induced ultrafast demagnetization, spin reversal and spin-orbit torque

Laser-induced femtosecond demagnetization (femtomagnetism) presents a new opportunity for magnetic storage technology, as it significantly shortens the read/write time, a necessity for large data storage device.  However, how a magnet can lose so much spin moment over only a few hundred femtoseconds has been under intense debate for over a decade. In this talk, I will first review the history of femtomagnetism, with emphasis on the controversy in the time-resolved magneto-optics. It is known that in the static magneto-optical Kerr effect (MOKE), the spin moment change is directly correlated with the Kerr rotation angle and ellipticity, but on a femtosecond time scale, such a correlation becomes ill-defined. A method is presented to overcome this problem. Then, I will discuss the all-opticalhelicity-dependent switching (AOS) in ferrimagnets.  AOS potentially has its broad application in storage devices. Our model predicts that an ultrafast laser pulse in general only induces a spin precession, not switching; but with appropriate parameters, the laser can even reverse spin from one direction to another. This works for both the circularly and linearly polarized light. The spin reversal window is narrow. These unexpected results closely resemble all-optical helicity-dependent magnetic switching found in much more complicated ferrimagnetic rare earth compounds. Our spin-orbit coupled model may find some important applications in spin switching processes, a hot topic in femtomagnetism.