The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization $M_S$ and magnetic anisotropy parameters $K$ recover within a nanosecond, and the ratio between $K$ and $M_S$ satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of $M_S$ and $K$ to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale.

中文翻译：

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磁各向异性弛豫对加尔芬醇薄膜中激光诱导磁化进动的影响

在用强和短激光脉冲激发后，磁介质弛豫到其平衡的速率和途径是超快自旋光学控制的关键因素。在这里，我们通过实验研究了铁磁金属加尔酚 (galfenol) 薄膜的磁化强度和磁各向异性的演变。${铁}_{0.81}{嘎}_{0.19}$) 由飞秒激光脉冲激发产生。从磁滞回线的时间演变我们推断出磁化强度${米}_{秒}$ 和磁各向异性参数 $钾$ 在纳秒内恢复，以及之间的比率 $钾$ 和 ${米}_{秒}$在从几皮秒到 3 纳秒的整个时间范围内满足热平衡幂定律。我们进一步使用实验获得的弛豫时间${米}_{秒}$ 和 $钾$ 分析激光引起的进动，并展示它们如何在纳秒时间尺度上促进其频率演化。