Microwave amplification in a magnetic tunnel junction induced by heat-to-spin conversion at the nanoscale

Abstract

Heat-driven engines are hard to realize in nanoscale machines because of efficient heat dissipation1. However, in the realm of spintronics, heat has been employed successfully—for example, heat current has been converted into a spin current in a NiFe|Pt bilayer system2, and Joule heating has enabled selective writing in magnetic memory arrays3. Here, we use Joule heating in nanoscale magnetic tunnel junctions to create a giant spin torque due to a magnetic anisotropy change. Efficient conversion from heat dynamics to spin dynamics is obtained because of a large interfacial thermal resistance at an FeB|MgO interface. The heat-driven spin torque is equivalent to a voltage-controlled magnetic anisotropy4,5 of approximately 300 fJ V−1 m−1, which is more than twice the value reported in a (Co)FeB|MgO system6,7. We demonstrate an electric microwave amplification gain of 20% in a d.c. biased magnetic tunnel junction as a result of this spin torque. While electric d.c. power amplification in spintronic devices has been realized previously8, the microwave amplification was limited to relatively small amplification gains (G = radiofrequency output voltage/radiofrequency input voltage) and has never exceeded 1 (refs 9,10,11,12,13). A magnetic tunnel junction driven by radiofrequency spin transfer torque using ferromagnetic resonance enabled a relatively large gain of G ≈ 0.55 (ref. 12). Furthermore, radiofrequency spin waves were tuned by the spin transfer effect14,15. The heat-driven giant spin torque in the FeB|MgO16,17 magnetic tunnel junction, which shows a large magnetization precession and resistance oscillation under a d.c. bias, overcomes the above limitations and provides a gain larger than 1.

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Fig. 1: Schematic of the MTJ and measurement circuit.
Fig. 2: Magnetic field dependence of microwave reflectivity S11.
Fig. 3: Resonance frequency and magnetic anisotropy.
Fig. 4: Magnetic anisotropy and temperature.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

This research was supported by Bilateral Programs (MEXT), MIC, the ImPACT programme of the Council for Science, Technology, and Innovation (Cabinet Office, Government of Japan) and JSPS KAKENHI (grant no. JP16H03850).

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M.G., Y.W. and U.K.O. performed the experiments and analysis. N.S. and B.D. performed the numerical simulation of the thermal distribution. H.K., K.Y., A.F. and S.Y. prepared the sample. The experiments and analysis were supervised by S.M. and Y.S. Y.S. conceived and designed the experiments and conducted the theoretical study.

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Correspondence to Yoshishige Suzuki.

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Supplementary Figures 1–4, Supplementary Tables 1–3

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Goto, M., Wakatake, Y., Oji, U.K. et al. Microwave amplification in a magnetic tunnel junction induced by heat-to-spin conversion at the nanoscale. Nature Nanotech 14, 40–43 (2019). https://doi.org/10.1038/s41565-018-0306-9

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