Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator

📝 Abstract

We report large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet(AF)/yttrium iron garnet(YIG), where a thin AF insulating layer of NiO or CoO can enhance spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the N'eel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM = 3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.

💡 Analysis

We report large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet(AF)/yttrium iron garnet(YIG), where a thin AF insulating layer of NiO or CoO can enhance spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the N'eel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM = 3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.

📄 Content

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Enhancement of Thermally Injected Spin Current through an Antiferromagnetic Insulator
Weiwei Lin,1,* Kai Chen,2 Shufeng Zhang2 and C. L. Chien1,† 1Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland 21218, USA. 2Department of Physics, University of Arizona, Tucson, Arizona 85721, USA

Abstract We report large enhancement of thermally injected spin current in normal metal (NM)/antiferromagnet(AF)/yttrium iron garnet(YIG), where a thin AF insulating layer of NiO or CoO can enhance spin current from YIG to a NM by up to a factor of 10. The spin current enhancement in NM/AF/YIG, with a pronounced maximum near the Néel temperature of the thin AF layer, has been found to scale linearly with the spin-mixing conductance at the NM/YIG interface for NM = 3d, 4d, and 5d metals. Calculations of spin current enhancement and spin mixing conductance are qualitatively consistent with the experimental results.

*wlin@jhu.edu †clchien@jhu.edu

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Pure spin current phenomena and devices are new advents in spin electronics [1,2]. A pure spin current has the unique attribute of delivering spin angular momentum without a net charge current thus with higher energy efficiency. A pure spin current can be generated by several mechanisms, including spin Hall effect [1-3], lateral spin valves [4,5], spin pumping [6,7], and longitudinal spin Seebeck effect (LSSE) [8,9]. The inverse spin Hall effect (ISHE) in a metal can detect a pure spin current by converting it into a charge current with resultant charge accumulation [3,10]. Inevitably a spin current decays as it traverses through a material on the scale of the spin diffusion length SF, which depends on the strength of the intrinsic spin orbit interaction and the quality of the material [5]. The transmission of a spin current across an interface between two materials, such as a ferromagnet and a non-magnetic material, is further limited by the spin-mixing conductance at the interface [7]. The rapidly diminishing spin current has severely hampered its exploitation. It is highly desirable to explore ways to enhance pure spin current. Pure spin current phenomena and devices have employed ferromagnetic (FM) metals [3-5,10], FM insulators [8,9], and normal metals (NM) [3,8-10], where the FM magnetization sets the spin index of the spin current injected from the FM material, light NM (e.g., Cu) and heavy NM (e.g., Pt) respectively transmits and detects the spin current. Very recently spin current exploration involves antiferromagnetic (AF) materials [11-18]. The employment of antiferromagnets in spintronic devices is particularly attractive for terahertz (THz) devices [19]. Recently, spin pumping experiment in Pt/YIG (where YIG = Y3Fe5O12) shows enhanced spin transport through an intervening AF NiO layer between YIG and Pt at room temperature [13,14]. It was suggested that the spin transport through the AF insulators is related to AF magnons and spin fluctuations [13,14], where the AF spins, strongly coupled to the precessing YIG magnetization, transport the spin current [13,14]. However, thus far, spin transport through AF insulators has only employed ferromagnetic resonance measurements (FMR) at the GHz frequency range [11,13-15,18], 3

which is far less than the characteristic frequencies (up to 1 THz) of the AF NiO. The excitation and transmission of spin current, including amplification, through AF is far from clear. Coherent Néel dynamics employed to explain the spin transport and enhancement in such systems at room temperature [16], implies more prevalent spin transport enhancement at T « TN. With the absence of the key experimental results, the mechanism for the large spin current enhancement observed at room temperature remains elusive [14]. The spin current amplification phenomena have thus far been observed in Pt/NiO/YIG and only at FMR frequencies. To unlock the underlying physics, it is essential to employ different spin current injection method, different AF materials, and a variety of metals other than Pt, and perform measurements over a wide temperature range. The comprehensive experimental studies would constrain the theory that accounts for the results. In this Letter, we report enhanced spin current through AF (AF = NiO and CoO) generated by the longitudinal spin Seebeck effect (LSSE) in the layer structure of NM/AF/YIG over a wide temperature range. The pure spin current injected from YIG, transporting through the AF layer, is detected by the ISHE in various 3d, 4d, and 5d NM. In contrast to spin pumping, LSSE is a DC injection method without coherent resonance excitations at high frequencies. We show that the transmitted spin current detected in the NM has a maximum near the TN of the AF layer of a specific thickness, indicating the dominant roles of magnons and spin fluctuation in the AF on the spin transport, rather than the collective AF o

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