Sign reversal of unidirectional magnetoresistance in monocrystalline Fe/Pt bilayers

Xiang Zhou, Fanlong Zeng, Mengwen Jia, Haoran Chen, and Yizheng Wu

Phys. Rev. B 104, 184413 – Published 8 November 2021

Abstract

The discovery of unidirectional magnetoresistance (UMR) in ferromagnet/heavy metal heterostructures provides one possibility to detect the magnetization direction with a simple two-terminal geometry. We investigated the temperature and thickness dependence of UMR in single-crystalline Fe/Pt bilayers and discovered an obvious sign reversal when increasing the Fe thickness at low temperature. Meanwhile, the same phenomenon is observed when increasing temperature for thick Fe samples. All the UMR mechanisms are quantitatively analyzed and the UMR contribution induced by the thermal effect shows a thickness-dependent sign change, thus the sign reversal of UMR is attributed to the competition between the contributions on UMR from the thermal effect and the spin accumulation induced by the spin Hall effect in the Pt layer. Our results emphasize the thermal contribution to the UMR in ferromagnet/heavy metal heterostructures and suggest a promising way to tune the UMR, which will promote its application in information storage devices.

Received 17 August 2021
Revised 29 October 2021
Accepted 1 November 2021

DOI:https://doi.org/10.1103/PhysRevB.104.184413

Physics Subject Headings (PhySH)

Spin accumulation Spintronics

Ferromagnets

Hall bar Resistivity measurements

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Xiang Zhou ^1,2, Fanlong Zeng¹, Mengwen Jia¹, Haoran Chen¹, and Yizheng Wu ^1,3,*

¹Department of Physics, State Key Laboratory of Surface Physics, Fudan University, Shanghai 200433, China
²ShanghaiTech Laboratory for Topological Physics & School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China
³Shanghai Research Center for Quantum Sciences, Shanghai 201315, China

^*wuyizheng@fudan.edu.cn

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Issue

Vol. 104, Iss. 18 — 1 November 2021

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Images

Figure 1
Schematic diagrams of UMR mechanisms in FM/HM heterostructures: (a) the spin accumulation, (b) the electron-magnon scattering, (c) the SOT effect, and (d) the thermal effect. $J_{c}$ is the current density by SHE, $J_{s}$ is the induced spin current density by SHE or Rashba effect, M is the magnetization of the FM layer, $σ$ is the orientation of the spin polarization, $Δ T$ indicates the temperature gradient in the FM/HM heterostructure, and $E$ is the additional electric field due to ANE.
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Figure 2
(a) RHEED pattern of Fe(10 nm)/MgO(001) with the electron beam directed along MgO $〈 110 〉$ . (b) Illustration of the side view of the wedged sample. (c) Schematic illustration of Hall bar structure and measurement geometry.
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Figure 3
(a) $R_{2 ω}^{x x} - H$ curves from the Fe(1 nm)/Pt sample measured with different currents at 300 K and (b) the extracted $Δ R_{2 ω}^{x x}$ as a function of the current density $J_{P t}$ in the Pt layer from the Fe(1 nm)/Pt sample at 300 K. The solid line in (b) is the linear fitting. (c) The measured Fe-thickness-dependent $ξ_{UMR}$ at 10 K and 300 K, respectively. The inset shows two representative $R_{2 ω}^{x x} - H$ curves with the opposite UMR sign. (d) Temperature-dependent $ξ_{UMR}$ of the $Fe (t_{F e}) / Pt$ bilayers with different $t_{F e}$ .
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Figure 4
Normalized $R_{2 ω}^{x x}$ versus $H$ curves for different Fe thicknesses at (a) 10 K and (b) 300 K. The extracted $Δ R_{2 ω}^{x x}$ as a function of $J_{P t}$ for different $t_{F e}$ at (c) 10 K and (d) 300 K, respectively. The solid lines in (c) and (d) are the linear fitting of the measured data. All the results are measured by PPMS.
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Figure 5
(a),(b) Angular dependence of $R_{2 ω}^{x x}$ at 10 K and 300 K for (a) $t_{F e} = 0.6$ nm and (b) $t_{F e} = 1.8$ nm, respectively. The solid lines are the fitting curves according to the $sin θ$ dependence. (c),(d) Angular dependence of $R_{2 ω}^{x y}$ for (c) $t_{F e} = 0.6$ nm and (d) $t_{F e} = 1.8$ nm, respectively. The solid lines are the fitting curves with the $cos θ$ dependence. (e),(f) Field dependence of $R_{2 ω}^{x y}$ for (e) $t_{F e} = 0.6$ nm and (f) $t_{F e} = 1.8$ nm with the field $H$ applied perpendicular to the current. (g),(h) The measured $R_{2 ω}^{x y}$ as a function of ${(H + H_{k}^{e f f})}^{- 1}$ for (g) $t_{F e} = 0.6$ nm and (h) $t_{F e} = 1.8$ nm. The solid lines in (g) and (h) are the linear fits to the data and the intercept of the fitted line represents the contribution of the thermal effect. The green up triangles and blue down triangles represent the $R_{2 ω}^{x y}$ data obtained from the $R_{2 ω}^{x y} - θ$ curves under different $H$ . The circles and squares are the measured $R_{2 ω}^{x y}$ from the $R_{2 ω}^{x y} - H$ curves in (e) and (f) for $H > 1.5$ T.
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Figure 6
Thickness dependent $ξ_{magnon + F L}, ξ_{thermal}$ , and $ξ_{spin}$ measured at (a) 10 K and (b) 300 K, respectively. (c) Temperature dependence of $ξ_{magnon + F L}, ξ_{thermal}$ , and $ξ_{spin}$ for $t_{F e} = 1.8$ nm. (d) Plot of the critical thickness of the sign reversal of $ξ_{thermal}$ and $ξ_{UMR}$ as a function of temperature. The solid lines serve a guide to the eye and the red arrows in (a)–(c) indicate the sign reversal of $ξ_{thermal}$ .
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