Terahertz emission from Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>(YIG)/Pt heterostructures via ultrafast spin Seebeck effect
Authors: Song Bang-Ju , Jin Zuan-Ming , Guo Chen-Yang , Ruan Shun-Yi , Li Ju-Geng , Wan Cai-Hua , Han Xiu-Feng , Ma Guo-Hong , Yao Jian-Quan ,   Department of Physics, Shanghai University, Shanghai 200444, China,   Terahertz Technology Innovation Research Institute, Shanghai Key Lab of Modern Optical System, Engineering Research Center of Optical Instrument and System (Ministry of Education), Terahertz Spectrum and Imaging Cooperative Innovation Center, University of Shanghai for Science and Technology, Shanghai 200093, China,   Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China,   STU & SIOM Joint Laboratory for Superintense Lasers and the Applications, Shanghai 201210, China,   Shanghai Institute of Intelligent Science and Technology, Tongji University, Shanghai 200092, China,   College of Precision Instrument and Opto-electronics Engineering, Tianjin University, Tianjin 300110, China
Published: November 2020

Recently, ferromagnetic/non-magnetic heterostructures have been widely studied for the generation of terahertz (THz) emitter based on spin-to-charge conversion. Actually, thermal spintronics effectively combines thermal transport with magnetism for creating and detecting non-equilibrium spin transport. A spin current or voltage can be induced by a temperature bias applied to a ferromagnetic material, which is called spin Seebeck effect (SSE). In this paper, we present a SSE based THz emission by using the heterostructures made of insulating ferrimagnet yttrium iron garnet (Y<sub>3</sub>Fe<sub>5</sub>O<sub>12</sub>, YIG) and platinum (Pt) with large spin orbit coupling. Upon exciting the Pt layer with a femtosecond laser pulse, a spin Seebeck current arises, applying a temperature gradient to the interface. Based on the inverse spin Hall effect, the spin Seebeck current is converted into a transient charge current and then yields the THz transients, which are detected by electrooptic sampling through using a ZnTe crystal at room temperature. The polarity of the THz pulses is flipped by 180° when the direction of the external magnetic field is reversed. By changing the direction of the pump beam excitation geometry to vary the sign of the temperature gradient at the YIG/Pt interface, the polarity of the THz signal is reversed. Fast Fourier transformation of the THz signals yields the amplitude spectra centered near 0.6 THz with a bandwidth in a range of 0.1–2.5 THz. We systematically investigate the influence of annealing effect on the THz emission from different YIG/Pt heterostructures. It can be found that the THz radiation is achieved to increase ten times in the YIG/Pt grown on a Gd<sub>3</sub>Ga<sub>5</sub>O<sub>12 </sub>(GGG) substrate through high-temperature annealing. The mechanism of annealing effect can be the increase of the spin mixing conductance of the interface between YIG and Pt. Finally, we investigate the pump fluence dependent THz peak-to-peak values for the annealed YIG/Pt grown on the Si substrate. Due to the spin accumulation effect at the interface of the YIG/Pt heterostructure, the THz radiation intensity gradually becomes saturated with the increase of pump fluence. Our results conclude that annealing optimization is of importance for increasing the THz amplitude, and open a new avenue to the future applications of spintronic THz emitters based on ultrafast SSE.