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Magnon Valve Effect Between Two Magnetic Insulators

Spin-based electronic devices such as magnetic memory and spin logic rely on spin information transport. In last decades, the spin-valve based structures such as giant magnetoresistance (GMR in 1988) effect and tunneling magnetoresistance (TMR in 1995) effect have been widely used in hard disk drives (HDD), magnetic random-access memory (MRAM), and magnetic sensors. Owing to the contribution to the discovery of GMR effect, Profs. A. Fert and P. Grünberg have been awarded Noble Prize in 2007. As the quasi-particles of collective excited spin wave, compared to conduction electrons, magnons have some advantages such as (1) long-distance spin information propagation, Joule-heating free, (2) manipulation of phase degree of freedom besides of magnitude and (3) emerging of new phenomena based on magnons, superfluid, superconductor, Bose-Einstein Condensation, Josephson Junctions and so on. As we know, spin valves play the key role in spintronic devices, therefore, for the magnon spintronics, we need a basic building block – magnon valve to accomplish functional information processing and data storage.

During 2012-2016, M02 group headed by Prof. Xiufeng Han, State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences experimentally fabricated a new spin valve type structure where a thin magnetic insulator is sandwiched by two heavy metallic layers - Pt/YIG/Pt (A kind of heavy metal/magnetic insulator/heavy metal sandwich structure, HM/MI/HM). And they observed the magnon drag effect in this structure which is theoretically proposed by Prof. Shufeng Zhang at University of Arizona: due to the magnon excitation and propagation in YIG, the spin/charge current in one Pt layer drags another opposite spin/charge current in another Pt layer. This work demonstrates that the spin information could be carried by magnons in magnetic insulators [H. Wu and X. F. Han, et al., Phys. Rev. B 93, 060403(R) (2016)].

Recently, they overcome the fabrication challenges by creatively making high-quality magnetic insulator based magnon valve structure - YIG/Au/YIG (a type of magnetic insulator/nonmagnetic metal/magnetic insulator sandwich structure, MI/NM/MI) and report for the first time the magnon valve effect: the magnon current transmission coefficient across the magnon valve could be controlled by the relative magnetic orientation of two YIG layers. They use the spin Seebeck effect (SSE) to excite the magnon current in YIG, and inverse spin Hall Effect (ISHE) to detect the magnon current across the magnon valve. They interpret the magnon valve effect by the angular momentum conversion and propagation between magnons in two YIG layers and conduction electrons in the Au layer. The temperature dependence of magnon valve ratio (Magnon Valve Ratio, MVR) shows approximately a power law, supporting the above magnon-electron spin conversion mechanism. By fitting the Au thickness dependence of MVR, a 15.1 nm spin diffusion length of Au is obtained, which is close to the value by spin pumping measurement. [ Hao Wu and X. F. Han et al., Magnon Valve Effect Between Two Magnetic Insulators, Phys. Rev. Lett. 120 (2018) 097205, DOI: https://doi.org/10.1103/PhysRevLett.120.097205, Editors’ suggestion & Featured in Physics ].

This work conceptually proves the possibility of using magnon valve structures to manipulate the magnon current in magnetic insulators, which will be a basic building block for future magnon spintronics and can be potentially applied in magnon based curcuit, logic, memory, diode, transistors, magnon waveguide, and on-off switching devices, and so on.

This study entitled “Magnon Valve Effect Between Two Magnetic Insulators” has been published on Phys. Rev. Lett. and is also highlighted as Editors’ Suggestion and Featured in Physics.

The journal Editor of Physics, America Physics Scociety, also follow-up comments this related work and points out that “Three new transistors (i.e. magnon valve) for spin-based currents may lead to a new type of circuitry that is faster and more efficient than traditional electronics.” [Physics 11 (2018) 23].

Article link:https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.120.097205;

The study was supported by the Ministry of Science and Technology of China, the National Science Foundation, and the Chinese Academy of Sciences.

Fig. 1 (a) Illustration of the magnon valve effect. (b) The cross-sectional scanning transmission electron microscopy (STEM) of GGG/YIG interface. (c) The cross-sectional high-resolution transmission electron microscopy (HRTEM) of YIG/Au/YIG region.
Fig. 2 (a)-(d) Thickness, temperature, magnetization direction, and heating current dependences of the magnon valve effect.

Han Xiufeng, Institute of Physics, Chinese Academy of Sciences, Email:xfhan@iphy.ac.cn

Key word:
Magnon valve effect (MVE), Mganon valve ratio (MVR), magnetic insulators (MI), spin Seebeck effect (SSE), inverse spin Hall effect (ISHE)

The key physics of the spin valve involves spin-polarized conduction electrons propagating between two magnetic layers such that the device conductance is controlled by the relative magnetization orientation of two magnetic layers. Here, we report the effect of a magnon valve which is made of two ferromagnetic insulators (YIG) separated by a nonmagnetic spacer layer (Au). When a thermal gradient is applied perpendicular to the layers, the inverse spin Hall voltage output detected by a Pt bar placed on top of the magnon valve depends on the relative orientation of the magnetization of two YIG layers, indicating the magnon current induced by spin Seebeck effect at one layer affects the magnon current in the other layer separated by Au. We interpret the magnon valve effect by the angular momentum conversion and propagation between magnons in two YIG layers and conduction electrons in the Au layer. The temperature dependence of magnon valve ratio shows approximately a power law, supporting the above magnon-electron spin conversion mechanism. This work opens a new class of valve structures beyond the conventional spin valves.

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