Temperature Dependence of Laser-Induced Demagnetization in Ni: A Key for Identifying the Underlying Mechanism
Temperature Dependence of Laser-Induced Demagnetization in Ni: A Key for Identifying the Underlying Mechanism
复制标题
DOI：
10.1103/physrevx.2.021006
复制
发表时间：
2012-05-16
影响因子：
12.5
通讯作者：
Aeschlimann, M.
中科院分区：
文献类型：
--
作者：
Roth, T.;Schellekens, A. J.;Aeschlimann, M.
光激发引起铁磁金属中磁有序超快丧失的微观机制仍在争论中。其中一个持续存在的争议是关于超快退磁的热起源。虽然不同的理论研究支持热源的主要驱动机制，但对于激光与自旋系统之间的相干相互作用或超扩散自旋输运提出了不同的描述。另一个重要的争论来自于对4f铁磁体Gd退磁动力学的两个时间尺度的实验观察。在这里，我们仍然不清楚是否有必要援引两种不同的微观机制来解释这种行为，或者只有一种机制就足够了。为了揭示这两个悬而未决的问题背后的物理原因，我们通过对不同激光强度和环境温度的考察，探索了镍中超快退磁动力学的相关性。利用时间分辨的磁光克尔效应在很大范围内测量了这些外部参数，并显示了最大磁化损失和退磁轨迹的时间分布的显著变化。最引人注目的观察是，同一材料体系(镍)可以表现出从一步(一个时标)到两步(两个时标)的退磁转变，随着环境温度的升高而发生。我们发现，假设只有一个散射机制将自旋系统耦合到电子系统的温度，超快退磁的通量和温度依赖关系--包括从一步动力学到两步动力学的转变--在理论上是重现的。这一发现意味着超快退磁的起源是热的，并且只有一个微观通道足以描述3D铁磁体在所有时间尺度上的磁化动力学。
The microscopic mechanisms responsible for the ultrafast loss of magnetic order triggered in ferromagnetic metals by optical excitation are still under debate. One of the ongoing controversies is about the thermal origin of ultrafast demagnetization. Although different theoretical investigations support a main driving mechanism of thermal origin, alternative descriptions in terms of coherent interaction between the laser and the spin system or superdiffusive spin transport have been proposed. Another important matter of debate originates from the experimental observation of two time scales in the demagnetization dynamics of the 4f ferromagnet gadolinium. Here, it is still unclear whether it is necessary to invoke two distinct microscopic mechanisms to explain such behavior, or if one single mechanism is indeed sufficient. To uncover the physics behind these two unsolved issues, we explore the dependence of ultrafast-demagnetization dynamics in nickel through a survey of different laser intensities and ambient temperatures. Measurements in a large range of these external parameters are performed by means of the time-resolved magneto-optical Kerr effect and display a pronounced change in the maximum loss of magnetization and in the temporal profile of the demagnetization traces. The most striking observation is that the same material system (nickel) can show a transition from a one-step (one time scale) to a two-step (two time scales) demagnetization, occurring on increasing the ambient temperature. We find that the fluence and the temperature dependence of ultrafast demagnetization-including the transition from one-step to two-step dynamics-are reproduced theoretically assuming only a single scattering mechanism coupling the spin system to the temperature of the electronic system. This finding means that the origin of ultrafast demagnetization is thermal and that only a single microscopic channel is sufficient to describe magnetization dynamics in the 3d ferromagnets on all time scales.