Optically-induced ultrafast spin dynamics study on perpendicular magnetic anisotropy materials

Abstract

In this thesis, in order to study ultrafast spin dynamics of promising perpendicular magnetic anisotropy materials, we have accomplished two types of optically-induced time-resolved magneto-optical Kerr experiments based on Ti:sapphire femtosecond laser. One type is the ultrafast magnetic field-induced experiment and the other type is the femtosecond laser-induced experiment. For the field-induced experiment, with the photolithography technique, we have carefully fabricated the $[Co/Pd]_5$ multilayer dot pattern on the photoconductive switch pattern structure which can generate a ultrafast current pulse. Unexpectedly, we could not observe any precessional behavior of Co/Pd sample. Assuming that this overdamped-like motion was attributed to its intrinsic many energy dissipation channels, the Gilbert damping constant, $\alpha$ was estimated to be much larger than one. However, because extrinsic possibilities such as the weak magnetic pulse field and the corrosion of the sample during the lift-off process cannot be excluded, further study needs to be progressed. For the laser-induced study with completeness, two different samples have been prepared. The amorphous $Tb_{35}Fe_{65}$ alloy film was aimed at the fundamental study on the subpicosecond time regime and the $L1_0$ -ordered $Fe_{50}Pt_{50}$ alloy film has been made with the application point of view. In case of $Tb_{35}Fe_{65}$ sample, interestingly enough, steplike demagnetization has been observed even at a subpicosecond when electrons are not in a thermal equilibrium state. With an extended three-temperature model simulation, we have identified that unusual ultrafast demagnetization is attributed to an energy transfer from a nonthermal electron to a spin system. For the application point of view, we have studied spin precessional dynamics of $L1_0$- ordered $Fe_{50}Pt_{50}$ at a few tens of picosecond time scale. Although its extremely tiny precessional signal, we obviously have detected and anlayz...