Magnetic Modulation of Terahertz Waves via Spin-Polarized Electron Tunneling Based on Magnetic Tunnel Junctions

Zuanming Jin, Jugeng Li, Wenjie Zhang, Chenyang Guo, Caihua Wan, Xiufeng Han, Zhenxiang Cheng, Chao Zhang, Alexey V. Balakin, Alexander P. Shkurinov, Yan Peng, Guohong Ma, Yiming Zhu, Jianquan Yao, and Songlin Zhuang

Phys. Rev. Applied 14, 014032 – Published 13 July 2020

Abstract

Magnetic tunnel junctions (MTJs) are a key technology in modern spintronics because they are the basis of read-heads of modern hard disk drives, nonvolatile magnetic random access memories, and sensor applications. In this paper, we demonstrate that tunneling magnetoresistance can influence terahertz (THz) wave propagation through a MTJ. In particular, various magnetic configurations between parallel state and antiparallel state of the magnetizations of the ferromagnetic layers in the MTJ have the effect of changing the conductivity, making a functional modulation of the propagating THz electromagnetic fields. Operating in the THz frequency range, a maximal modulation depth of 60% is reached for the parallel state of the MTJ with a thickness of 77.45 nm, using a magnetic field as low as 30 mT. The THz conductivity spectrum of the MTJ is governed by spin-dependent electron tunneling. It is anticipated that the MTJ device and its tunability scheme will have many potential applications in THz magnetic modulators, filtering, and sensing.

Received 22 February 2020
Revised 26 May 2020
Accepted 19 June 2020

DOI:https://doi.org/10.1103/PhysRevApplied.14.014032

Physics Subject Headings (PhySH)

Magnetoresistance Photoconductivity Ultrafast magnetic effects

Magnetic tunnel junctions

Terahertz time-domain spectroscopy

Atomic, Molecular & OpticalCondensed Matter, Materials & Applied Physics

Authors & Affiliations

Zuanming Jin ^1,2,3,*,¶, Jugeng Li ^2,¶, Wenjie Zhang², Chenyang Guo⁴, Caihua Wan^4,†, Xiufeng Han ⁴, Zhenxiang Cheng ⁵, Chao Zhang⁶, Alexey V. Balakin ^1,7, Alexander P. Shkurinov^1,7, Yan Peng^1,3, Guohong Ma^2,8,‡, Yiming Zhu^1,3,§, Jianquan Yao⁹, and Songlin Zhuang^1,3

¹Terahertz Technology Innovation Research Institute, Terahertz Spectrum and Imaging Technology Cooperative Innovation Center, Shanghai Key Lab of Modern Optical System, University of Shanghai for Science and Technology, 516 JunGong Road, Shanghai 200093, China
²Department of Physics, Shanghai University, 99 Shangda Road, Shanghai 200444, China
³Shanghai Institute of Intelligent Science and Technology, Tongji University, Shanghai 200092, China
⁴Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China
⁵Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, New South Wales 2500, Australia
⁶School of Physics, University of Wollongong, Wollongong, New South Wales 2522, Australia
⁷Department of Physics and International Laser Center, Lomonosov Moscow State University, Leninskie Gory 1, Moscow 19991 Russia
⁸STU & SIOM Joint Laboratory for Superintense Lasers and Applications, Shanghai 201210, China
⁹College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin 300110, China

^*physics_jzm@usst.edu.cn
^†wancaihua@iphy.ac.cn
^‡ghma@staff.shu.edu.cn
^§ymzhu@usst.edu.cn
^¶These authors contributed equally to this work.

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Vol. 14, Iss. 1 — July 2020

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Images

Figure 1
(a) Schematic illustration of the THz tunneling magnetoresistance effect and the MTJ sample structure used in this work. The application of an external magnetic field, H, leading to parallel configuration creates a large flow of electrons as compared with antiparallel configuration (H = 0 mT). The statically measured (b) in-plane magnetization curve and (c) TMR ratio of the MTJ stack sample.
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Figure 2
(a) Time-domain THz waveforms E(t) through the MTJ multistack for magnetizations aligned in parallel (blue line) and antiparallel (red circle) states. Inset: the orientation of the magnetic field is along the x axis, which rotates M_free-layer (green arrow) along the x axis. In the case of θ = 90° and φ = 0°, M_free-layer is aligned parallel to M_pinned-layer. The zero time delay corresponds to an arbitrary starting position. (b) Instantaneous THz intensity calculated from the square of the measured electric field.
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Figure 3
Time-domain THz waveforms E(t) through the MTJ multistack measured at H = 0 mT (red circles) and at H = 30 mT applied along the x axis (blue traces) for selected azimuthal angles of (a) θ = 180°, (b) θ = 270°, and (c) θ = 360°. (d) THz intensities $I = \int E^{2} (t) d t$ are calculated from the THz waveforms transmitted through the MTJ versus θ in the configuration of antiparallel state. The solid line shows a cos² θ fit. (e) As M_free-layer is fixed along the x axis, φ is dependent on I. The full fitting (blue line) includes both the AMR (green dotted line) and TMR (red dashed line) contributions.
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Figure 4
Example amplitude spectra of the measured THz time traces under different magnetization configurations. (a) Antiparallel state (θ = 270°, φ = 180°), (b) perpendicular state (θ = 0°, φ = 90°), and (c) parallel state (θ = 90°, φ = 0°). For the Fourier transformation, the THz signal in the time domain −5 ps < t < 10 ps is used, which limits the frequency resolution to be 0.26 meV. (d) The frequency-dependent modulation depth of the THz waveforms $M_{φ} (ω / 2 π)$ calculated using Eq. (2) for φ = 0°, 90°, and 180°. (e) The angle-dependent modulation depth $M_{ω / 2 π} (φ)$ for ω/2π = 0.6 THz and 1.0 THz. (f) The calculated $M (φ, ω / 2 π)$ as functions of frequencies and angles, using the experimental E_P(ω/2π) and E_AP(ω/2π) as input.
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Figure 5
The magnetic-field-induced change in sheet conductivity Δσ of the MTJ multistack for three representative magnetization configurations, calculated from the Fourier transform of the time-domain electric field E_φ through the sample and E_AP for the reference. Real and imaginary parts of Δσ for (a) antiparallel state (θ = 270°, φ = 180°), (b) perpendicular state (θ = 0°, φ = 90°), and (c) parallel state (θ = 90°, φ = 0°).
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