Intrinsic vs. Extrinsic Contributions to Antiferromagnetic Spin Seebeck Effect

Spin Seebeck effect (SSE) offers an effective way to thermally generate spin current in antiferromagnetic insulators (AFMI). Although the antiferromagnetic magnon SSE picture has been established, a complete understanding of the experimental results is still lacking. A pivotal question to address is whether extrinsic factors other than antiferromagnetic magnons significantly affect the SSE signals and how to extract the intrinsic signals from the SSE measurements. Our systematic experimental SSE results in bulk Cr₂O₃ crystals have revealed that when a magnetic field is applied along the easy axis, the SSE signal below the spin-flop transition highly depends on how the Cr₂O₃ surface is prepared. To understand the origin of this signal, we further measure the SSE with the field perpendicular to the easy axis. A main component of this hard-axis response can be described by the Brillouin function, indicative of a paramagnetic behavior due to independent spins. The hard-axis paramagnetic SSE component varies in differently prepared samples. Because of the isotropic nature of the paramagnetic signal, it modifies the characteristic easy-axis SSE signals, in particular the magnitude below the spin-flop field. We show that by removing the paramagnetic background, we can reveal the intrinsic antiferromagnetic SSE. This analysis provides a good understanding of the intrinsic SSE responses in Cr₂O₃ thin films where a relatively stronger paramagnetic contribution exists.