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Magnon blocking effect and magnonic skin effect

Date:14-09-2021 Print

Spin waves, or magnons, as the elementary excitation of the magnetic system, can transfer spin angular momentum, which provides wide prospects for the Non-volatile, low-energy-consumption, high-speed and small-size microelectronic devices in the post-Moore period. Magnonics, encompassing the generation, transport and manipulation of magnons, has become the latest development direction of spintronics and an emerging discipline of condensed matter physics. In recent years, Prof. X.F. Han's research group in Institute of Physics, Chinese Academy of Sciences, has successfully developed a magnon valve with a core structure of magnetic insulator (MI)/spacer(S)/magnetic insulator (MI) (such as YIG/Au/YIG) [PRL 120 (2018) 097205], a magnon junction (such as YIG/NiO/YIG) [PRB 98 (2018) 134426] and a magneto electric separator which can be used as magnon generator and magnon detector (such as Pt/YIG/Pt) [PRB 93 (2016) 060403(R)], aiming to use pure electrical methods and the change of the magnetic structures to effectively control the generation and transport of magnons, realizing a 100% transmission switch on-off ratio of the magnon currents. Therefore, a further in-depth understanding of the transport properties of incoherent or coherent magnons in a completely electrically insulated magnon junction will become the key physical basis for the development of practical magnonic devices and circuits in the future.

In order to better understand the mechanism of magnon transmission in magnon junction from the microscale, Zhengren Yan (PhD student), Caihua Wan (Associate Professor), and Xiufeng Han (Professor) studied the magnon transmission in the sandwich structure of ferromagnetic insulator (FMI)/antiferromagnetic insulator (AFI)/ferromagnetic insulators (FMI) by atomistic spin-model simulations. It is shown that magnon junction effect (MJE) or magnon valve effect (MVE) can be reproduced in this work, demonstrating the magnetization-dependent magnon transmission. The MJE and MVE are stemmed from the polarization of spin wave. In general, spin-up (spin-down) lattices can only accommodate right- (left-) handed circularly polarized magnons. While only right-handed circularly polarized magnons are favored in FMI with upward magnetization, both left- and right-handed circular polarizations are permitted in AFI owing to two spin-opposite lattices. This selection rule thus makes the total reflection of spin wave occur when magnons try to diffuse into a spin lattice, which does not support their polarization. For instance, when right-handed circular magnons excited in spin-up region are injected into the spin-down region, the selection rule would result in low magnon transmission across the interface. This phenomenon called magnon blocking effect (MBE). This achievement was published in Phys. Rev. Appl. [Z. R. Yan, C. H. Wan and X. F. Han, Magnon Blocking Effect in an Antiferromagnet-Spaced Magnon Junction, Phys. Rev. Appl. 14 (2020) 044053, DOI: 10.1103/PhysRevApplied.14.044053].

MBE shows that the spin-wave polarization plays an important role in magnon transmission. Furthermore, PhD students Zhengren Yan and Yaowen. Xing together with Prof. Xiufeng Han theoretically studied the scattering behavior of spin waves at the interface of an antiferromagnetically coupled heterojunction. It is shown that the SWs passing through the interface are evanescent waves and the incident waves are all reflected back, demonstrating a magnetization-dependent magnon blocking effect in this structure. The theoretical result indicates that with the increase of the spin-wave frequency, the decay length decreases and the evanescent wave are more concentrated at the interface, showing a magnonic skin effect (MSE) which is similar to the skin effect of electromagnetic waves. Furthermore, a positive magnonic Goos-H?nchen shift (MGHS) of the reflected waves is also predicted. It can be understood by an effective reflection interface shift induced by the nonzero decay length of the EWs. This achievement was published in Phys. Rev. B [Z. R. Yan, Y. W. Xing and X. F. Han, Magnonic skin effect and magnon valve effect in an antiferromagnetically coupled heterojunction, Phys. Rev. B 104 (2021) L020413, DOI: 10.1103/PhysRevB.104.L020413].

In summary, the above theoretical research results clearly show that the efficient manipulation of coherent/incoherent magnons by magnon junctions stems from the inherent chirality of magnons in magnetic materials. These discoveries confirm the physical basis of magnon devices to efficiently manipulate magnon transport, and provides a new development direction and technical route for the development of pure magnon-type storage and logic devices.

These works were financially supported by the National Natural Science Foundation of China [NSFC No. 51831012], National Key Research and Development Projects of China [MOST No. 2017YFA0206200, 2016YFA0300802], and the Chinese Academy of Sciences [No. XDB33000000, QYZDJ-SSW-SLH016, 112111KYSB20170090].

Figure 1. Left: Schematic diagram of magnon junction structure and magnon blocking effect (MBE); (a) Spatial distribution of magnon concentration under different magnetic structures; (b) Channel-resolved magnon concentration spatial distribution; Magnon junction in the parallel state (c) and the antiparallel state (d). The magnon transport schematic diagram shows that the magnon blocking effect occurs at the interface with the opposite magnetic moment direction, and magnons can be partially transmitted in the case of the same magnetic moment direction. Right: Schematic diagram of Magnon Skin Effect (MSE). The direction of the magnetic moment of the medium 1 (2) is along the +z (-z) direction, allowing the transmission of right (left) handed circularly polarized spin waves. When the spin waves enter the left-handed medium 2 from the right-handed medium 1, the waves in the medium 2 are evanescent waves, and their decay length is expressed as Ld. (b) When the spin waves enter GdIG from YIG, Ld changes with frequency ω and incident angle θi. (c) When the spin waves enter YIG from GdIG, Ld changes with frequency ω and incident angle θi.

Article link:
https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.14.044053
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.L020413

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

Key word:
Magnon Junction (MJ); Magnon Junction Effect (MJE); Magnon Valve Effect (MVE); Magnon Blocking Effect (MBE); Magnonic Skin Effect (MSE); Magnonics.

Abstract:
Prof. Xiufeng Han's research group shows a magnon blocking effect (MBE) in magnon junction and magnonic skin effect (MSE) in an antiferromagnetically coupled heterojunction. The MBE and MSE can confirm the physical basis of magnon devices to efficiently manipulate magnon transport, and provide a new development direction and technical route for the development of pure magnon-type storage and logic devices.