Dynamics of Spin Relaxation in a Ferromagnet by Ultrafast laser Spectroscopy

Abstract

The study of real time dynamics of electrons in metals has become possible with the advent of ultrashort pulse (i.e., femtosecond) laser techniques. An impulse of photoexcitation (pump) displaces electrons from equilibrium, and their subsequent relaxation is monitored by measuring transient changes in the optical constants with time-delayed probe pulses. In this manner, information about electron-electron scattering, electron-phonon interaction, and hot electron transport has been obtained in simple metals (e.g., Ag. AI, Au). In strong contrast, direct information on spin and magnetization dynamics of such nonequilibrium electrons in a ferromagnetic metal is very scarce so it is appealing to extend ultrafast laser studies to such media for fundamental reasons as well as for practical purposes (e.g., for possible high speed magnetization switching). Recently, some efforts have been made to apply the time-resolved optical techniques to magnetic metallic systems. Vaterlaus er al. I employed time-resolved spin-polarized photoemission from a rapidly laser heated lattice to estimate a spin-lattice relaxation time for ferromagnetic Gd ( - 100psec). close to their experimental resolution. Beaurepaire et al.' studied the relaxation processes of electrons and their spins in ferromagnetic Ni on a picosecond timescale and interpreted their results in terms of an effective temperature model for the coupled spin-electron-lattice system. In the work reported here, we set up an initially spin-polarized photoexcited gas of electrons in a ferromagnetic metal film by interband excitation with circularly polarized femtosecond blue laser pulses (3 eV). As a model system we use the COR* (ordered) alloy which has been recently introduced as a candidate material for magneto-optical recording application^.^ The films possess perpendicular magnetic anisotropy which facilitates such selective excitation, applied at a modest level (typically - IOi8 electrons per cm3) so that the system overall is only weaked perturbed. The transient magneto-optical response is then recorded, specifically in terms of the transient magneto-Kerr ellipticity, as a function of magnetic field. We are able to clearly separate the spin dependent parts of the electron dynamics from the spin independent electron and lattice relaxation processes. Two distinct stages in the transient magnetization changes can be identified. The first of these concems the spin relaxation rate of hot, nonthermal electrons that takes place within about 600 fsec at room temperature; the second process involves equilibration of a thermalized gas of 'warm' spins with the lattice which requires approximately I5 psec at room temperature. Such results show how the experimental technique offers a general method for gaining insight into understanding the spin dynamics in photoexcited ferromagnetic systems.