Licentiate thesis defense: Ultrafast spin dynamics and relaxation in metallic ferromagnets

We performed ultrafast demagnetization measurements on epitaxial hcp-cobalt thin film with the easy axis in the plane of the film by pumping the sample with 400 nm radiation and probing the magneto-optic effects in the longitudinal magneto optic Kerr effect geometry using 800 nm radiation. The in-plane magnetic anisotropy of the sample enabled us to measure the demagnetization dynamics along two different crystalline directions, namely along the easy axis and at 45 degrees between the easy and hard axes. The spin relaxation is found to be systematically faster in the 45 degrees axis direction compared to the easy axis of magnetization. The observed effect can be related to the anisotropic electron-phonon coupling in cobalt. Our results emphasize the importance of the lattice symmetry in ultrafast demagnetization dynamics.

We also performed transient reflectivity measurements on platinum and silver with THz/400 nm pump and 800 nm probe. The optically induced reflectivity is usually explained using the phenomenological two temperature model (2TM). We could see that the reflectivity induced by 400 nm could be explained in terms of the two temperature model whereas the THz induced reflectivity could not be explained with it since the microscopic mechanism of pump excitation is not dealt within the 2TM. We could qualitatively explain the deviation from the 2TM by considering the effect of pump on the band structure of the materials under investigation.