镍薄膜超快热化动力学的实验研究及数值模拟
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摘要
目前对计算机数据传输速率的要求不断提高。这就要求数据的读出过程需要较宽的电子传输带宽,写入过程需要较短的反转时间。目前的主要障碍在于写入过程,磁性存储装置中的磁性层转换特性限制了磁性装置速度的提高。为了克服这个困难,需要对磁性层在超快时间尺度内的动力学过程进行研究,得到其发生特性转换的时间,进而改进计算机中现有的磁性存储装置。因此,本文对典型的3d金属镍及其复合膜在飞秒激光下的超快热化过程进行了详细研究。
首先,采用磁控溅射技术制备了不同厚度的Ni薄膜及其复合膜,对部分样品进行了500℃真空退火,利用扫描电子显微镜和原子力显微镜对退火前后的样品进行表征,发现退火后的薄膜结晶性增强了;对于Cu/Ni复合膜,较厚的Cu层使薄膜退火后的结晶性更好,晶粒多而均匀;退火使得复合膜Cu/Ni比Cr/Ni更有利于薄膜表面晶粒的生长。
其次,对已制备好的薄膜样品,采用飞秒激光泵浦-探测技术测试了其在不同参数条件下的瞬态反射率信号,重点研究了泵浦光与探测光功率、散热层和退火对瞬态反射率曲线的影响,获得以下结论:受飞秒激光脉冲激发后,Ni薄膜瞬态反射率在约0.2ps时从稳定值突变至极值,随后有一个较缓慢的恢复过程,并且在15ps,30ps和45ps附近出现了应力回波产生的尖峰;泵浦光功率的升高,使得反射率变化率突变的幅值增大;探测光与泵浦光功率之比的减小可以提高系统的信噪比;不同散热层下薄膜瞬态反射率急剧下降的时间是一致的,但是具有较大电子比热容常数和强电子声子耦合常数的散热层如Cr更能提高瞬态热传导的散射效率。退火后的复合膜相比退火前在突变后的恢复过程进行得更快,Cu层较厚时退火后的瞬态反射率恢复率比Cu层较薄时大;相比Cr(40nm)/Ni(40nm),复合膜Cu(40nm)/Ni(40nm)具有较小的晶格失配率,退火更有利于后者内部电子和声子以扩散方式进行的热传递。
最后,采用有限差分法对三温模型进行了数值求解,并对Ni薄膜和NiFe薄膜的超快热化过程进行了模拟。引进一个相当于自旋比热容常数的系数γ_s来描述自旋比热随自旋温度的变化趋势;结合实验对镍铁合金薄膜受激发后的反射率变化进行了模拟,发现镍薄膜电子温度峰值与激光功率几乎成正比,且修改后的模型相比原始模型得到的模拟结果与实验结果更接近;最后,对不同厚度的镍薄膜在飞秒激光作用下的热化动力学进行了分析,发现当薄膜厚度小于光透深度时,电子、自旋的峰值温度明显高于较厚薄膜,这与实验结果一致。
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.
首先,采用磁控溅射技术制备了不同厚度的Ni薄膜及其复合膜,对部分样品进行了500℃真空退火,利用扫描电子显微镜和原子力显微镜对退火前后的样品进行表征,发现退火后的薄膜结晶性增强了;对于Cu/Ni复合膜,较厚的Cu层使薄膜退火后的结晶性更好,晶粒多而均匀;退火使得复合膜Cu/Ni比Cr/Ni更有利于薄膜表面晶粒的生长。
其次,对已制备好的薄膜样品,采用飞秒激光泵浦-探测技术测试了其在不同参数条件下的瞬态反射率信号,重点研究了泵浦光与探测光功率、散热层和退火对瞬态反射率曲线的影响,获得以下结论:受飞秒激光脉冲激发后,Ni薄膜瞬态反射率在约0.2ps时从稳定值突变至极值,随后有一个较缓慢的恢复过程,并且在15ps,30ps和45ps附近出现了应力回波产生的尖峰;泵浦光功率的升高,使得反射率变化率突变的幅值增大;探测光与泵浦光功率之比的减小可以提高系统的信噪比;不同散热层下薄膜瞬态反射率急剧下降的时间是一致的,但是具有较大电子比热容常数和强电子声子耦合常数的散热层如Cr更能提高瞬态热传导的散射效率。退火后的复合膜相比退火前在突变后的恢复过程进行得更快,Cu层较厚时退火后的瞬态反射率恢复率比Cu层较薄时大;相比Cr(40nm)/Ni(40nm),复合膜Cu(40nm)/Ni(40nm)具有较小的晶格失配率,退火更有利于后者内部电子和声子以扩散方式进行的热传递。
最后,采用有限差分法对三温模型进行了数值求解,并对Ni薄膜和NiFe薄膜的超快热化过程进行了模拟。引进一个相当于自旋比热容常数的系数γ_s来描述自旋比热随自旋温度的变化趋势;结合实验对镍铁合金薄膜受激发后的反射率变化进行了模拟,发现镍薄膜电子温度峰值与激光功率几乎成正比,且修改后的模型相比原始模型得到的模拟结果与实验结果更接近;最后,对不同厚度的镍薄膜在飞秒激光作用下的热化动力学进行了分析,发现当薄膜厚度小于光透深度时,电子、自旋的峰值温度明显高于较厚薄膜,这与实验结果一致。
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.
引文
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