Ultrafast Laser-induced Magnetisation Dynamics: Gilbert damping of Metal and Half-metal

In this thesis, the magnetic damping in different metals and half-metals is studied using an all-optical pump-probe method: the time-resolved magneto-optic Kerr effect (TR-MOKE), combined with several advanced characterization techniques, such as angle-resolved photoemission spectroscopy (APRES) and X-ray magnetic circular dichroism (XMCD).
In Fe/Cr/GaAs heterostructures, the uniaxial magnetic anisotropy (UMA) and the magnetic damping have been studied as a function of the thickness of Cr interlayer. The UMA is attributed to the Fe-GaAs chemical bonding at the interface via increasing the orbit moments of Fe atoms. The Cr interlayer with increasing thickness blocks the Fe-GaAs bonding and the UMA gradually. The magnetic damping shows a dramatic drop when Cr interlayer is deposited, even with only 0.5 ML. The results have indicated that the UMA and the damping originate from different mechanisms although the Fe/GaAs interface plays an important role in both phenomena.
The correlation between magnetic damping and the electronic structures have been investigated in Co2FeAl Heusler alloy. By varying the growth temperature, both the magnetic damping and the electronic structure have been found affected significantly. It is experimentally demonstrated that the low damping constant originates from a low density of state (DOS) at Fermi level. A “shoulder-like” peak in the vicinity of Fermi level in the energy distribution curve would enhance the DOS and the damping.
A pair of half-metallic Fe3O4 films, with and without structural defects, have been compared in order to investigate the effect of defects in magnetic damping. The enhancement of the intrinsic damping 0.063 in defect-free Fe3O4 sample is attributed to the emergence of the perpendicular standing spin wave (PSSW), while the magnetic damping in the as-grown sample 0.039 is lower because the defects would scatter the spin wave or magnon and the energy transfer channel from the uniform precession to the PSSW does not exist.