| At present, data transfer rate of computer keep rising. To improve the rate of data transferring, a wide bandwidth of electronic transmission is required for read-out process, and a short inversion time is needed for writing process. At present the main obstacle is the writing process, in which the conversion characteristics of magnetic layers limits the speed of magnetic installation. In order to overcome this difficulty, the dynamic process in the ultrafast time scale must be studied, for obtaining the occurrence time of the characteristics conversion and improving existing magnetic device in computer. In this paper, the ultrafast thermalization process of the typical 3d metal Ni films and its composite films have been studied in detail under femtosecond laser.At first. Ni films and its composite films with different thickness were prepared by Magnetron sputtering, and some samples were annealed at 500℃in vacuum, and examined of the surface features using scanning electron microscopy and atomic force microscopy, and found that annealing enhanced the crystalline of the thin films. Cu / Ni composite film with thicker Cu layer after annealing has better crystalline, with more homogeneous grains; Annealing is more favorable to grain growth of sample Cu/Ni than Cr/Ni composite film, as Cu has a closer lattice parameters with Ni.Using femtosecond laser pump-probe technique, the author have tested transient reflectivity signal of Ni film sample prepared under different parameters, focusing on the effect of pump and probe optical power, cooling layer and annealing on the transient reflectivity curves, and from the experimental results we conferred that: After excitation by femtosecond laser pulses, transient reflectivity values of Ni films went up to the maximum at about 0.2ps, and then experienced a slow recovery process, and generated 3 peaks nearl5ps, 30ps and 45ps brought by stress waves; Pump power increasing makes the reflection magnitude flop increase approximately proportionally; Decreasing the ratio of probe power and pump power can improve the SNR of the system to obtain more accurate measurements; For Ni films with different cooling layers, the time points of transient reflectivity change are the same, but cooling layers with a larger electronic specific heat constant and the strong electron-phonon coupling constant, such as Cr can enhance the scattering efficiency of transient heat conduction, so that the transient reflectivity rate of the recovery process was faster. The composite film sample after annealing samples compared to unannealed samples had faster recovery process of transient reflectivity; Composite sample with thicker cooling layer after annealing had higher recovery rate of the transient reflectivity; Compared to Cr (40nm) / Ni (40nm), annealing was more conducive to Cu (40nm) / Ni (40nm) composite film as the lattice mismatch rate between Cu and Ni is smaller, heat transfer rate in the way of diffusion of electrons and phonons improved, making the film surface temperature decrease.Finally, the author carried out numerical computation based on three-temperature (3T) equations with finite- difference method, then modeled and analyzed the ultrafast thermalization process of the Ni film and NiFe thin films induced by femtosecond laser. A coefficient had been introduced to characterize temperature dependence of the specific heat of spin system, to optimize spin temperature. The author engaged in analytical research of reflectivity change of Ni films and found that the modified model matched ultimately the experiment results of Ni films, and studied the laser power dependence of peak value of reflectivity change. Also, the author investigated the ultrafast thermalization dynamics of Ni films with different thickness, it has been found that when the thickness of thin film is less than the depth of penetration, temperature of electron, spin system grow up obviously, compared with thicker films, which is agreeable with the experimental results. |
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