稀磁半导体(Ga,Mn)As薄膜激光诱导超快磁化动力学过程拟合方法探究

Analysis of fitting methods for laser-triggered ultrafast magnetization dynamics in diluted magnetic semiocnductor(Ga, Mn)As film

本文对稀磁半导体(Ga,Mn)As薄膜中超快激光诱导磁化动力学响应信号的不同拟合方法进行了对比分析.通过Landau-Lifshitz-Gilbert(LLG)方程的数值拟合发现,由于薄膜平面内和平面外磁光响应强度不同,磁矢量三维进动的叠加可以导致多个频率振动模式的假象.当使用高于(Ga,Mn)As带边的能量激发时,磁化进动的磁光响应信号中叠加着来自光极化载流子的响应,此时单纯利用LLG方程对薄膜整体磁化动力学过程拟合应谨慎使用.本工作为正确分析和理解脉冲激光对(Ga,Mn)As铁磁性的超快调控提供了拟合方法上的指导.

Laser-triggered magnetization dynamics for diluted magnetic semiconductor （Ga, Mn）As has drawn great attention in recent years, aiming at studying the ultrafast manipulation of collective spin excitations towards spintronic information processing. In this work, different fitting methods for time-resolved magneto-optical Kerr effect （TR-MOKE） study of the laser-triggered magnetization dynamics in a diluted magnetic semiconductor （Ga, Mn）As are analyzed and compared. It is known that the exponentially damped cosine harmonic function and the numerical simulation based on Landau-Lifshitz-Gilbert （LLG） equation are usually applied to fit the laser-induced magnetization dynamics from TR-MOKE measurements. Under the specified experimental conditions, it is sometimes hard to fit the TR-MOKE response well with single-mode uniform precession by using the exponentially damped cosine harmonic function. Although the fitting with multiple precession frequencies may usually show much better fitting results, the numerical simulation based on LLG equation reveals that the multi-frequency precessional modes are caused by the superposition of three-dimensional trajectories of magnetization precession with different contributions from the in-plane and out-of-plane magneto-optical response in （Ga, Mn）As. Thus, the multi-frequency precessional modes obtained by adopting the fitting method with exponentially damped cosine harmonic function could be the fake ones. Meanwhile, it is important to note that though the LLG equation can be used to fit the macroscopic magnetization precession well with single frequency, the contribution of pulse-like background response from photo-generated polarized carriers at the above-bandgap excitation is strongly superimposed on the magnetization precession response, and the pulse-like background response cannot be described by LLG equation. Thus one should be cautious of applying LLG equation only to fit the entire TR-MOKE signal, especially when the excitation energy is above the band gap of （Ga, Mn）As. One may combine both fitting methods, namely, fitting with the exponentially damped cosine harmonic function and the LLG simulation by considering both the in-plane and out-of-plane magneto-optical response of （Ga, Mn）As film in order to properly fit the laser-triggered magnetization dynamic response from TR-MOKE measurements. The proper handling of fitting methods helps to extract the dynamic magnetic parameters correctly and to further understand the physical mechanisms for triggering the ultrafast manipulation of collective spin dynamics. This is fundamentally important for developing novel spintronics based on diluted magnetic semiconductor （Ga, Mn）As.