Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures

Yang Cheng, Junyu Tang, Justin J. Michel, Su Kong Chong, Fengyuan Yang, Ran Cheng, and Kang L. Wang

Phys. Rev. Lett. 130, 086703 – Published 22 February 2023

Abstract

Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in the heavy metal/ferromagnet bilayer systems. We observe the USMR in $Pt / α − {Fe}_{2} O_{3}$ bilayers where the $α − {Fe}_{2} O_{3}$ is an antiferromagnetic (AFM) insulator. Systematic field and temperature dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin orbit torque due to the thermal random field. However, unlike its ferromagnetic counterpart, theoretical modeling reveals that the USMR in $Pt / α − {Fe}_{2} O_{3}$ is determined by the antiferromagtic magnon number with a non-monotonic field dependence. Our findings extend the generality of the USMR which pave the ways for the highly sensitive detection of AFM spin state.

Received 8 August 2022
Accepted 31 January 2023
Corrected 1 May 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.086703

Physics Subject Headings (PhySH)

Spin dynamics Spintronics

Antiferromagnets

Epitaxy Film deposition Landau-Lifschitz-Gilbert equation Sputtering Transport techniques

Condensed Matter, Materials & Applied Physics

Corrections

1 May 2023

Correction: A production processing flaw rendered the setting of two diacritics in Eqs. (5) and (6) in the PDF version incorrectly. These diacritics have been fixed and were set without incident in the HTML version.

Authors & Affiliations

Yang Cheng^1,*, Junyu Tang ², Justin J. Michel³, Su Kong Chong¹, Fengyuan Yang³, Ran Cheng ^2,4, and Kang L. Wang^1,†

¹Department of Electrical and Computer Engineering, and Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA
²Department of Physics and Astronomy, University of California, Riverside, California 92521, USA
³Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
⁴Department of Electrical and Computer Engineering, University of California, Riverside, California 92521, USA

^*Corresponding author. cheng991@g.ucla.edu
^†Corresponding author. wang@ee.ucla.edu

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Issue

Vol. 130, Iss. 8 — 24 February 2023

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Images

Figure 1
Experimental geometry and first harmonic results. (a) Schematics of a $Pt / α − {Fe}_{2} O_{3}$ Hall bar with a $5 μ m$ width and $10 μ m$ length. (b) In plane angular dependence of first harmonic voltage $V_{x x}^{1 ω}$ (blue curve) and $V_{x y}^{1 ω}$ (green curve) for a $Pt (5 nm) / α − {Fe}_{2} O_{3} (30 nm)$ bilayer at 300 K with 2 T applied field. (c) Field dependence of (transverse) spin Hall magnetoresistance voltage $V_{(T) SMR}$ extracted from the fitting in (b) by Eq. (1) where the inset of (c) shows the ratio of $V_{SMR}$ and $V_{TSMR}$ .
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Figure 2
(a) Schematic of current induced spin orbit torque in two spin sublattices $m_{A (B)}$ . In-plane angular dependence of second harmonic voltage (b) $V_{x x}^{2 ω}$ and (c) $V_{x y}^{2 ω}$ for the $Pt (5 nm) / α − {Fe}_{2} O_{3} (30 nm)$ bilayer at 300 K with 2 T applied field. The blue, green, and black curves are contributions from the fieldlike torque, spin Seebeck effect, and USMR, respectively. The red curves are the total fit by Eqs. (3) and (4).
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Figure 3
In-plane angular dependence of second harmonic Hall voltage (a) $V_{x x}^{2 ω}$ and (b) $V_{x y}^{2 ω}$ at different magnetic fields for the $Pt (5 nm) / α − {Fe}_{2} O_{3} (30 nm)$ bilayer at 300 K. (c) Field dependence of fieldlike torque contribution in $V_{x x}^{2 ω}$ (blue curve) and $V_{x y}^{2 ω}$ (green curve). The solid line is the $1 / H$ fit. (d) Field dependence of spin Seebeck effect contribution in $V_{x x}^{2 ω}$ (blue curve) and $V_{x y}^{2 ω}$ (green curve). The solid line is the linear fit. (e) The ratio of $V_{x x, FL}$ and $V_{x y, FL}$ (blue curve) and the ratio of $V_{x x, SSE} + V_{x x, USMR}$ and $V_{x y, SSE}$ (green curve), where the magnitude is calculated from (c) and (d). The ratio of $V_{x x, SSE} + V_{x x, USMR}$ and $V_{x y, SSE}$ is greater than 2, which indicates the presence of USMR. Various contributions in (c) to (e) are obtained by fitting like those in Fig. 2.
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Figure 4
(a) The extracted magnetic field dependence of USMR contribution in the $V_{x x}^{2 ω}$ . (b) Temperature dependence of USMR in $V_{x x}^{2 ω}$ at 2 T. The error bars are extracted by fitting the data in Fig. 3.
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Figure 5
(a) Magnetic field dependence of antiferromagtic magnon number difference between $φ_{H} = \pm π / 2$ and (b) between $φ_{H} = 0$ , $π$ . In the insets, the light color arrows (red and blue) represent the sublattice magnetic moments of AFM after rotating $H_{0}$ (green arrow) to the opposite direction. The magnetic fluctuation originates from the thermal random fields in the two orthogonal ${\hat{e}}_{φ}$ and ${\hat{e}}_{r}$ directions. $σ_{T}^{2}$ is the thermal coefficient.
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