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Pure spin current phenomena and devices are recent advents. A pure spin current has the attribute of delivering spin angular momentum with a minimal of charge carriers in a metal and no carriers in an insulator. Spin pumping and thermal injection are often used with a ferromagnetic (FM) material as a spin injector. However, FM metals are plagued by numerous parasitic effects, whereas more favorable conditions are afforded in FM insulators, such as Y₃Fe₅O₁₂ (YIG), with which spin injection and detection can be unequivocally revealed. It has been recently reported that spin current generated in YIG can be enhanced by one order with a 1 nm antiferromagnetic (AF) NiO layer inserted between YIG and Pt [1, 2]. The spin current enhancement shows a maximum near the Néel temperature of the NiO layer, and is proportional to the spin mixing conductance at the normal metal/YIG interface [2]. Spin backflow from the AF insulator/YIG interface has been detected using the spin Hall magnetoresistance, showing a strong temperature dependence dominated by spin conductance [3]. These results demonstrate spin transport in AF insulators is dominated by incoherent thermal magnons and spin fluctuations. Temperature dependence of spin injection and other aspects will also be discussed.
[1] H. L. Wang, C. H. Du, P. C. Hammel, and F. Y. Yang, Phys. Rev. Lett. 113, 097202 (2014).
[2] W. Lin, K. Chen, S. Zhang, and C. L. Chien. Phys. Rev. Lett. 116, 186601 (2016).
[3] W. Lin and C. L. Chien. Phys. Rev. Lett. 118, 067202 (2017).