Large enhancement of ultrafast thermal spin current in metal/ferrimagnetic insulator heterojunction by amorphous YIG intercalation

Ultrafast spin currents have become a pivotal solution for spintronic devices to overcome speed limitations and achieve lower consumption. Nevertheless, a straightforward approach to enhance ultrafast spin currents without introducing undesired magnetic effects remains elusive. Here, we report large enhancements (49%, 130%) in ultrafast spin currents and Terahertz (THz) emission from heavy metal (Au, Pt)/YIG heterojunctions by intercalating the paramagnetic amorphous YIG (a-YIG) layers. Furthermore, we demonstrate for the first time that spin fluctuations in amorphous YIG are the primary mechanism for enhancing ultrafast spin transport. This work provides an efficient strategy, along with theoretical insight, for advancing the development of ultrafast spintronic devices and compact THz emitters.