Optically driven spin angular momentum transport in paramagnetic insulator NiO

In antiferromagnetic insulators, spin angular momentum can be transferred even above the N\'eel temperature, where the material becomes a paramagnetic insulator. In this regime, spin transport takes the form of short-range spin fluctuations (paramagnons) rather than long-range spin waves. In this study, we demonstrate optically driven spin transport in a Pt/NiO/Co trilayer structure, where Pt is the spin generation layer, NiO is the spin transporting layer, and Co is the spin absorbing layer. By quantitatively comparing optically and electrically driven spin transport, we determine the interfacial coupling efficiency of 0.1 (optical) and 0.3 (electrical) and the relaxation length of 2.2 nm (optical) and 10.3 nm (electrical) of paramagnons of NiO. We attribute this discrepancy to the energy difference between the nonthermal and thermal processes.