Spin fluctuation induced spin Seebeck effect near the Neel temperature in antiferromagnetic thin films

It is believed that spin fluctuations in antiferromagnetic (AFM) films can enhance the transmission of spin current originating from ferrimagnetic (FM) insulators in the FM/AFM heterostructures. On the other hand, spin fluctuations alone are a source of spin current around the Neel temperature T_N as demonstrated in FeF₂. To further investigate this effect, we have studied the spin Seebeck effect (SSE) responses of YIG(110)/Cr₂O₃ and Cr₂O₃ devices epitaxially grown by pulsed laser deposition. In the former type of devices, we observe clear SSE peaks with peak temperatures relying on the Cr₂O₃ thicknesses, which could be interpreted by the enhanced spin current from YIG through Cr₂O₃. However, in the latter type, we also find an SSE peak at ~305 K in 80 nm thick Cr₂O₃ film, close to the T_N of bulk Cr₂O₃ crystals. The peak broadens and shifts towards lower temperatures as the magnetic field increases. These results indicate that the observed SSE peaks in both types of devices are likely from the critical fluctuations in the vicinity of T_N. Due to the challenge to measure AFM responses in thin films, our results not only reveal a possible mechanism of enhanced SSE signals in FM/AFM heterostructures, but also provide a novel way of probing spin fluctuations and determining T_N.