Exploring generation and transport of ps-long spin currents with terahertz probes

Abstract

Ultrashort-laser-induced terahertz (THz) emission is a contact-less probe for charge current generation, which has been recently highlighted in electronics by exploring ultrafast charge dynamics. Here we show it is also applicable in magnetics by exploring spin dynamics. In particular, the generation and transport of an ultrashort spin current, preferably reaching a frequency up to THz, is the core of modern spintronics. First, we demonstrate a picosecond-long spin current generation in a Weyl semimetal Td-WTe2 thin film. The anisotropic dependence of the circular photogalvanic effect in the THz emission indicates that the spin current is generated along the crystalline low-symmetric a-axis, which can be further modulated by the helicity of the laser excitation. Moreover, the generated spins are polarized along the surface normal, which potentially provides an efficient means to manipulate magnetic devices with perpendicular magnetic anisotropy. Since the prior studies have been limited to the static measurements with in-plane spin orientation, our work provides an insight into dynamic feature of the anisotropic spin current and shows a potential for the THz-spintronic devices with Weyl semimetals. Second, we further show the transport of an ultrashort spin current using THz emission. In a magnetic multiplayer Bi2Se3/NiO/NiFe, we show that a spin current generated in the ferromagnetic NiFe layer is transferred to the Bi2Se3 layer through the antiferromagnetic insulator NiO layer. The magnetic and insulating features of NiO enables magnons, i.e., the quanta of spin waves, to carry spin currents without moving charges, which therefore can avoid substantial energy dissipation caused by Joule heating and subsequently provides an efficient means to manipulate the magnetization. Our works, which are relevant to the high-speed and energy-efficient control of spintronic devices, will invigorate magnon-based memory and logic devices.