粗糙度、晶体结构和界面混合对Co/Pt双层膜THz辐射的影响
THz emission from Co/Pt bilayers with varied roughness, crystal structure, and interface intermixing
摘要：
飞秒激光激发的Co/Pt双层膜的结果在皮秒太赫兹脉冲的有效发射。用于产生THz发射的两种已知机制是通过Co/Pt界面的自旋极化电流，由于逆自旋霍尔效应导致Pt层中的螺旋度无关电流，以及由于逆自旋轨道扭矩效应导致Co/Pt界面处的螺旋度相关电流。在这里，我们探讨如何粗糙度，晶体结构和混合在Co/Pt界面影响太赫兹发射的效率。特别是，我们改变了界面的粗糙度，在0.1-0.4 nm的范围内，通过调整的沉积压力条件在制造的Co/Pt双层。为了控制在Co/Pt界面处的混合，引入具有各种Co和Pt组成的1- 2nm厚的CoxPt 1-x合金间隔层。最后，Co的晶体结构从面心立方转变为六方密排。我们的研究表明，界面的粗糙度是至关重要的螺旋度相关的飞秒激光脉冲诱导的太赫兹辐射的效率。然而，令人困惑的是，混合虽然强烈增强了与螺旋度无关的THz发射，但对与螺旋度相关的THz发射没有影响，该发射被抑制并且类似于光滑界面。
英文摘要：
Femtosecond laser excitation of a Co/Pt bilayer results in the efficient emission of picosecond THz pulses. Two known mechanisms for generating THz emission are spin-polarized currents through a Co/Pt interface, resulting in helicity-independent electric currents in the Pt layer due to the inverse spin-Hall effect and helicity-dependent electric currents at the Co/Pt interface due to the inverse spin-orbit torque effect. Here we explore how roughness, crystal structure and intermixing at the Co/Pt interface affect the efficiency of the THz emission. In particular, we varied the roughness of the interface, in the range of 0.1-0.4 nm, by tuning the deposition pressure conditions during the fabrication of the Co/Pt bilayers. To control the intermixing at the Co/Pt interface a 1-2 nm thick CoxPt1-x alloy spacer layer was introduced with various compositions of Co and Pt. Finally, the crystal structure of Co was varied from face centered cubic to hexagonal close packed. Our study shows that the roughness of the interface is of crucial importance for the efficiency of helicity-dependent THz emission induced by femtosecond laser pulses. However, it is puzzling that intermixing while strongly enhancing the helicity-independent THz emission had no effect on the helicity-dependent THz emission which is suppressed and similar to the smooth interfaces.
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