Experimental Investigation On Ultrafast Optical And Electron Dynamics Properties Of Tb_0.27Dy_0.73Fe₂ Thin Films

Femtosecond laser unique characters such as ultrashort pulse width, ultrahigh peak power and the electric field intensity has decided it will having broad application in many fields. Femtosecond laser technology was used to investigate ultrafast phenomenon in the interaction process between laser and different material, especially the ultrafast process occurred in picosecond and femtosecond time domain. Such as the magnetization and demagnetization process of ultrahigh speed memory and read-write system and the giant magnetostrictive material applying to the micro-electromechanical systems (MEMS). Transient reflectivity and electron dynamics of magnetic thin film is the key issue in femtosecond ultrafast measurement domain and it will provide various magnetic storage devices and high frequency micro-magnetic systems with more design basis.TbDyFe thin films with different thickness were prepared by magnetron sputter and they were annealed under different temperature and atmospheres (vacuum, nitrogen). The component,surface topography and crystallinity property of TbDyFe thin films were investigated by scanning electronic microscope (SEM),X-ray diffraction (XRD) and atom force microscope (AFM) methods. The surface crystal gain diameters D of thin film were calculated by Scherrer formula and it has a good agreement with SEM observation result.Optical transient reflectivity change of TbDyFe thin films were measured by femtosecond laser pump-probe measurement technique in our laboratory in order to investigate the influence of thin film thickness,pump power and annealing process. Combine with Fresnel reflect formula and K-K transformation, real and imaginary part of the complex dielectric constant was derived from reflectivity relative change curves. It reveals that the film surface properties have rapid change after laser irradiation, at the same time, inside of thin film have more absorption to laser pulses. After 1ps, the thin film surface properties become steady but imaginary part of dielectric constant increasing gradually and the reflection and absorption of laser pulses in film recover to initial state.We described the electron dynamics process in TbDyFe thin film after femtosecond laser excitation combine with two-temperature and three-temperature model and use the three-temperature model equation to obtain the electron,spin and lattice system temperature variation curves. Extremum time of reflectivity shorter than former research results and we may ascribe this phenomenon to the local 4f electrons in rare earth ions that doping in thin film and improvement of the temporal resolution. The wavelike phenomenons are observed in equilibration process of the reflectivity change, particularly in 400nm and 600nm thin film. Thickness of the thin film determines the echo time and echo wave in 800 nm thin film is not obvious and regular. Last, experiment results were compared with GaAs crystal and the cause bring about difference was analyzed.