Coherent acoustic phonon inmagnetic thin films excited by femtosecond laser
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摘要: 飞秒激光脉冲和磁性薄膜中的MnIr层相互作用激发了初始相位为90°，在薄膜中传播速度为4 300 m/s的相干声学声子.该声学声子的振动频率与激光能量密度无关，与该磁性薄膜总厚度成反比.相干声学声子的产生机理可能为飞秒激光激发磁性薄膜的电子，使电子温度急剧上升，随后由于激光吸收而强度减弱形成了一个瞬间的随深度增加而减小的电子温度梯度，使晶格在深度方向相干振荡，即激发了声学声子.另外，外磁场的变化对声学声子的频率的影响在实验误差范围内，说明磁相互作用和晶格中的电相互作用相比是非常弱的.Abstract: The interaction between the femtosecond laser pulse and the MnIr layer in the magnetic thin film excites the coherent acoustic phonon with an initial phase of 90° and a propagation velocity of 4 300 m/s. The vibration frequency of the acoustic phonon is independent of the laser energy density and is inversely proportional to the total thickness of the magnetic thin film. And the acoustic phono can propagate in the adjacent metal layers due to the high lattice matching. The electron temperature in the magnetic thin film increases sharply absorbing by femtosecond laser pulse, then an decreasing electron temperature gradient with increasing depth is generated instantaneously by the absorption of the laser which lead to the lattice oscillate coherently in the depth direction, that is, the acoustic phonon. In addition, the frequency of the acoustic phonon changes caused by the applied magnetic field is within the experimental error range, indicating that the magnetic interaction is very weak compared to the electrical interaction in the lattice.
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Key words:
- femtosecond laser /
- coherent acoustic phonon /
- pump-probe /
- magnetic film
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图 2 不同泵浦光能量下反射率变化量随延迟时间的变化
Fig. 2 Time-resolved reflectivity changes observed in two pump fluence
图 3 不同MnIr厚度的样品反射率变化量随延迟时间的变化
Fig. 3 Time-resolved reflectivity changes observed in different thickness of MnIr layer
图 4 声子的频率随MnIr厚度的变化图
Fig. 4 The frequency of phonon as a fuction of the thickness of MnIr
图 5 不同磁场下样品反射率变化量随延迟时间的变化
Fig. 5 Time-resolved reflectivity changes observed in different applied magnetic fields
图 6 声学声子频率随外磁场的变化曲线
Fig. 6 The frequency of phonon as a fuction of theapplied magnetic fields
表 1 溅射条件
Tab. 1 Sputtering conditions
材料 直流功率/W 溅射气压/mTorr 溅射速率/(Å$\cdot $s$^{-1})$ Ta 60 8.0 0.519 Co 60 5.0 0.284 CoFeB 80 5.0 0.5 Pt 40 5.0 0.5 MnIr 80 6.0 0.6 
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