Elimination of two-magnon scattering for evaluation of intrinsic spin decay length in antiferromagnetic insulator*

We present an experimental evaluation of the intrinsic spin decay length in an antiferromagnetic insulator (AFI). A frequently-used sample configuration to study the spin-current propagation in AFIs is a ferromagnet/AFI/heavy-metal trilayer structures, where the ferromagnet and heavy-metal act as spin-current injectors and detectors, respectively. We found that at the ferromagnet/AFI interface, a spin current generated by spin pumping is strongly suppressed by two-magnon scattering enhanced by position-dependent fluctuation of microscopic exchange bias, which is inevitable at ferromagnet/antiferromagnet junctions. By eliminating the two-magnon contribution from the spin transmission signal, we discovered that the characteristic length of spin decay in polycrystalline NiO, a prototypical AFI, was around 100 nm, which is an order of magnitude longer than what was previously believed. Our results provide a new perspective in the emerging field of antiferromagnetic spintronics, especially for the fundamental understanding of spin transport physics in insulators.