Spin and Lattice Dynamics in Thin Films: From Femtoseconds to Nanoseconds.

In this dissertation we set out to quantitatively investigate the dynamics of magnetic thin films. Specifically, we studied the spin dynamics in epitaxial metallic ferromagnets and the coupling to other degrees of freedom, such as electron and phonon excitations. Key aspects of the spin dynamics were found to occur across a wide range of temporal scales, from femtoseconds to nanoseconds. Accordingly, new instrumental and experimental tools were developed in order to address the complex behavior of the magnetization under strongly non-equilibrium conditions. A new pump-probe fiber-laser-based magnetometer was built and used to access the time-dependence of the magnetic behavior during spin wave excitation and relaxation. The performance of this instrument offers significant advantages over existing methods, including: an unusually large temporal dynamic range (150 fs-10 ns), high frequency bandwidth (~5 THz), high detection sensitivity that corresponds to a signal to noise ratio of better than 107, and fast data acquisition at kilohertz scanning rates. These instrumental capabilities allowed us to perform unprecedented studies of coherent spin waves propagating through epitaxial Fe films. The femtosecond laser pulse induces coherent magnetization dynamics indirectly via thermal excitation, resulting in magnon-electron and magneto-elastic coupling. The spin wave propagation speeds and attenuation lengths were determined during spin wave propagation and reflection at the film boundaries. Coherent spin waves with frequency less than 24 GHz, propagate at velocities < 1.3 km/s, consistent with their dispersion relation. A not well understood behavior occurs for spin waves with wavevector k~0, which are transmitted super-sonically through films of about one classical skin depth thick (1.5 μm). A major step in this work was to establish all-optical techniques for manipulation and coherent control of the magnetization vector. An optically-induced spin reorientation transition of first-order is observed for the first time, which provides a new route to ultrafast coherent magnetization switching. The switching is found to be a three-step temporal process: a coherent reorientation (~ 100 ps) is followed by a spin precession in a newly created metastable state (~ 300 ps), which evolves into a dual domain state that undergoes relaxation within ~ 2 - 4 ns.PhDApplied PhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/78913/1/vstoica_1.pd