Time resolved optical studies of spin and quasiparticle dynamics in ferromagnetic thin films and superconductor

This thesis presents the studies of spin and quasiparticle dynamics in ferromagnetic thin films and iron based superconductors by ferromagnetic resonance (FMR) and time-resolved pump-probe optical techniques.;First, the FMR spectroscopies were applied to study the spin dynamics both in frequency and time domains for the epitaxially grown Fe/GaAs thin films and FeCoB/Cr/FeCoB multilayer structures. In the single layer Fe/GaAs thin films, magnetization precessions were studied to characterize the magnetic dynamical parameters. Our results show that the magnetic crystalline anisotropy is dominative and the magnetic damping is strongly dependent on the in-plane magnetic field orientations. In FeCoB/Cr/FeCoB multilayer films, both the acoustic and the optical spin wave modes were identified in the FMR spectra. We reveal that the adjacent magnetic layers in the trilayer structures are antiferromagnetic coupled with an effective interlayer coupling constant Jeff. The magnetic dynamical parameters can be accurately optimized by the interlayer coupling constant Jeff..;Second, we employed the time-resolved pump-probe magneto-optical Kerr effect (MOKE) spectroscopy to study the spin dynamics in the Fe/GaAs thin films at picosecond time scale. The time-resolved MOKE results were combined with static magnetic hysteresis loops at various time delays to understand the ultrafast demagnetization dynamics. The ultrafast demagnetization process is faster than the time required for the electron-phonon equilibration and therefore the spin-orbital coupling has to be included with the conventional electron thermalization model to understand our results. Moreover, we show that the ultrafast magnetization excitation and reorientation can be coherently controlled by varying the polarization of the pump beam. The magnetization excitation and reorientation are attributed to the laser induced effective magnetic field in the sample.;Third, the quasiparticle relaxation dynamics were studied in electron-doped BaFe1.9Ni0.1As2 and BaFe1.85Co 0.15As2 superconductors by time-resolved pump-probe optical spectroscopy. Two distinct relaxation components observed in the transient reflectivity spectra are attributed to the quasiparticle recombination in the superconducting state and quasiparticle relaxations from the higher excited band due to the multiband excitation. The results show the multi-gap characteristic in BaFe1.9Ni0.1As2 and BaFe1.85Co 0.15As2 superconductors. Moreover, the estimated electron-phonon coupling constant and the Coulomb pseudopotential indicate that the electron-phonon interaction is not large enough to induce the SC transition. A spin mediated pairing mechanism is necessary to understand the SC phase transition in the iron based superconductors.