Qualitative Identification of the Spin-to-Orbital Conversion Mechanism Modulated by Rare-Earth Nd, Gd, and Ho Metals via Terahertz Emission Measurements

Abstract

It is crucial to study the materials that could effectively facilitate the inter-conversion between charge, spin, and orbital degrees of freedom. In this work, the conversion among these three types of degrees of freedom in Pt/CoFeB/rare-earth (RE, represents Nd, Gd, and Ho)/Ti multilayers is manipulated. Through terahertz (THz) emission measurements, it is found that the spin current induced by a femtosecond laser is converted into an orbital current via the spin-orbit coupling of the RE layer. Notably, the light RE (Nd) and heavy RE (Gd and Ho) induce the orbital current with opposite polarization directions, ultimately leading to a weakening or enhancement of the THz emission intensity, respectively. Moreover, the fast Fourier transform reveals that Gd exerts the most significant influence on increasing the whole THz spectrum within the Pt/CoFeB/RE/Ti structure. The findings of RE-modulated spin-to-orbital conversion provide valuable insights into the fundamental transport mechanism of the orbital current.