为了研究铁磁薄膜材料在飞秒激光作用下的热化动力学过程,采用有限差分法对三温(3T)模型进行了数值求解,并结合飞秒抽运探测实验进行了分析。引进一个相当于自旋比热容常数的系数来描述自旋比热随自旋温度的变化趋势,对其合理性进行了详细研究。首先对镍铁合金薄膜在抽运激光下反射率变化进行了测量和模拟,修正后的模型模拟结果和实验结果吻合得较好,进一步研究了反射率变化峰值随激光功率的变化情况;其次对不同厚度的镍薄膜的超快热化动力学进行了分析,当薄膜厚度小于光透深度时,电子、自旋的峰值温度以及系统达到平衡时的温度显著升高,当薄膜厚度大于光透深度时,薄膜厚度的增大对热化动力学过程影响不大。

To study the thermalization dynamics process of ferromagnetic thin films excited by femtosecond laser,numerical computation based on three-temperature (3T) equations with finite difference method was carried out,and the analysis of this process combining with the results of the femtosecond laser pump-probe experiment was given. A coefficient was introduced to characterize temperature dependence of the specific heat of spin system,and the rationality was analyzed in details. First,analytical research of reflectivity change of NiFe films was carried out and the modified model matched ultimately the experiment results of NiFe films. The laser power dependence of peak value of reflectivity change was studied. Also,the ultrafast thermalization dynamics of nickel (Ni) films with different thicknesses were investigated associated with the experimental results. When the thickness of thin film is less than the depth of penetration,electron,spin system and the balance value of the 3T system grow up obviously,and when the thin film is thicker,the thickness of ferromagnetic thin film hardly affects the thermalization dynamics.