Current-induced asymmetric magnetoresistance due to energy transfer via quantum spin-flip process

MD09

105

-Current-induced asymmetric magnetoresistance due to

energy transfer via quantum spin-flip process

Kab-Jin Kim1*_{, T. Moriyama}2_{, T. Koyama}3_{, D. Chiba}3_{, S. -W. Lee}4_,

S. -J. Lee5_{, K. -J. Lee}4,5_{, H. -W. Lee}6_{, T. Ono}2

1_{Institute for Chemical Research, Kyoto University, Japan}

2_{Department of Applied Physics, Faculty of Engineering, The University of Tokyo, Japan} 3_{Department of Materials Science and Engineering, Korea University, Republic of Korea}

4_{KU-KIST Graduate School of Converging Science and Technology, Korea University, Republic of Korea} 5_{PCTP and Department of Physics, Pohang University of Science and Technology, Republic of Korea}

Owing to its ability to manipulate the magnetic moment of materials electrically, the spin transfer torque (STT) effect has made magnetic nanodevices realistic candidates for a variety of spintronic applications, such as STT magnetic random access memory (MRAM), racetrack memory, STT oscillator, STT diode, STT memristor and so on1)_{. We note that all these devices operate in GHz, which is limited fundamentally by the physics of STT.}

Physical background of STT has been established based on the spin angular momentum conservation during the exchange interaction between conduction electron spin and magnetization. Another important mechanism in the interaction, i.e., energy transfer based on the energy conservation, has been ignored so far, though it is physically rightful to consider it. Here, we would like to raise a question on this physically important issue.

In this work, we provide experimental evidences of energy transfer. In ferromagnet/heavy metal bilayers, we observe in-plane current-induced asymmetric magnetoresistance depending on the relative direction of current and magnetization. Combined with electron-magnon scattering, current-induced excitation of magnons by the energy transfer can naturally explain all experimental features of the asymmetry, including characteristic time scale, angular dependence and temperature-dependence, whereas the STT cannot. From the thorough theoretical approach and experimental magnetic field dependence of magnetoresistance, we found that magnons in THz range can be excited by the energy transfer mechanism. Hence effects of the energy transfer mechanism are widely separated in frequency from the corresponding effect of the STT mechanism in GHz range. Our results therefore unveil another aspect of current-induced magnetic excitation, and open a channel for the dc-current-induced generation of THz magnons2)_.

References

[1] Locatelli et al. Nat. Mater. 13, 11, (2014) [2] Kab-Jin Kim et al. Arxiv. 1603. 08746 (2016)