Evidence of the Topological Hall Effect in Pt/Antiferromagnetic Insulator Bilayers

Yang Cheng, Sisheng Yu, Menglin Zhu, Jinwoo Hwang, and Fengyuan Yang

Phys. Rev. Lett. 123, 237206 – Published 5 December 2019

Abstract

The topological Hall effect has been a primary indicator of nontrivial spin textures in magnetic materials. We observe the evidence of the topological Hall effect in $Pt / {Cr}_{2} O_{3}$ bilayers grown on ${Al}_{2} O_{3} (0001)$ and ${Al}_{2} O_{3} (11 \bar{2} 0)$ , where the ${Cr}_{2} O_{3}$ epitaxial film is an antiferromagnetic insulator. The $Pt / {Cr}_{2} O_{3}$ bilayers exhibit topological Hall resistivity for ${Cr}_{2} O_{3}$ thicknesses below 6 nm near and above room temperature, which is above the Néel temperature of ${Cr}_{2} O_{3}$ , revealing the key role of thermal fluctuations in the formation of spin textures. The similarity of topological Hall signals in (0001)- and $(11 \bar{2} 0)$ -oriented ${Cr}_{2} O_{3}$ films indicates that the emergence of spin textures is insensitive to crystalline orientation.

Received 20 May 2019
Revised 4 November 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.237206

Physics Subject Headings (PhySH)

Antiferromagnetism Skyrmions Spin Hall effect Spin texture Spin torque

Monte Carlo methods Sputtering

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yang Cheng^1,*, Sisheng Yu^1,*, Menglin Zhu², Jinwoo Hwang², and Fengyuan Yang¹

¹Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA
²Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43212, USA

^*Y. C. and S. Y. contributed equally to this work.

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Vol. 123, Iss. 23 — 6 December 2019

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Images

Figure 1
(a) $2 θ / ω$ XRD scans of a 30 nm ${Cr}_{2} O_{3}$ epitaxial film grown on ${Al}_{2} O_{3} (0001)$ and ${Al}_{2} O_{3} (11 \bar{2} 0)$ substrates. (b) XRD rocking curve of the ${Cr}_{2} O_{3}$ (0006) peak for the film on ${Al}_{2} O_{3} (0001)$ . (c) XRR scan of a 30 nm ${Cr}_{2} O_{3}$ film on ${Al}_{2} O_{3} (0001)$ , where the fitting (black curve) gives a ${Cr}_{2} O_{3}$ thickness of 30 nm and a roughness of 0.1 nm. (d) AFM image of a 30 nm ${Cr}_{2} O_{3}$ film on ${Al}_{2} O_{3} (0001)$ with a surface roughness of 0.1 nm. (d) STEM image of a 30 nm ${Cr}_{2} O_{3}$ film on ${Al}_{2} O_{3} (0001)$ .
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Figure 2
Topological Hall (TH) measurements of a $Pt (2 nm) / {Cr}_{2} O_{3} (5 nm)$ bilayer on ${Al}_{2} O_{3} (0001)$ . (a) Hall resistivity $ρ_{x y}$ of $Pt (2 nm) / {Cr}_{2} O_{3} (5 nm) / {Al}_{2} O_{3} (0001)$ at 345 K, where the black curve is the fitting for $ρ_{AH}$ with the Langevin function and linear background due to $ρ_{OH}$ . (b) TH resistivity after subtraction of the fitting curve for $ρ_{AH}$ and $ρ_{OH}$ in (a). (c) $ρ_{x y} - ρ_{OH}$ at various temperatures from 250 to 385 K, where the black curves are the fitting for $ρ_{AH}$ with the Langevin function. (d) $ρ_{TH}$ after subtraction of the fitting curve for $ρ_{AH}$ in (c) at each temperature. Inset: $ρ_{TH}$ as a function of temperature.
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Figure 3
(a) Hall resistivity $ρ_{x y} - ρ_{OH}$ of $Pt (2 nm) / {Cr}_{2} O_{3} (t)$ bilayers on ${Al}_{2} O_{3} (0001)$ at 345 K with different thickness ( $t$ ) of ${Cr}_{2} O_{3}$ , where the black curves are the fitting for $ρ_{AH}$ with the Langevin function. (b) $ρ_{TH}$ of the bilayers after subtraction of the fitting curve in (a). (c) $ρ_{TH}$ as a function of ${Cr}_{2} O_{3}$ thickness measured at 345 K. (d) Monte Carlo simulations of field dependence of topological charges $Q$ at $T = 1.05 T_{N}$ ( $K = | J_{1} | = 1$ and $D = 0.5 | J_{1} | = 0.5$ with out-of-plane easy axis anisotropy). (e) Contour plot of $ρ_{TH}$ of $Pt (2 nm) / {Cr}_{2} O_{3} (t)$ bilayers on ${Al}_{2} O_{3} (0001)$ as a function of temperature and ${Cr}_{2} O_{3}$ thickness. (f) Contour plot of topological charges $Q$ as a function of temperature $T / T_{N}$ and $1 / D$ based on Monte Carlo simulation. The red circles in (e) and (f) are the data points.
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Figure 4
(a) Hall resistivity $ρ_{x y} - ρ_{OH}$ of a $Pt (2 nm) / {Cr}_{2} O_{3} (5 nm)$ bilayer on ${Al}_{2} O_{3} (11 \bar{2} 0)$ at various temperatures from 250 to 345 K, where the black curves are the fitting for $ρ_{AH}$ with the Langevin function. (b) TH resistivity $ρ_{TH}$ after subtraction of the fitting curve for $ρ_{AH}$ in (a) at each temperature. Inset: $ρ_{TH}$ as a function of temperature.
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Figure 5
(a) Hall resistivity $ρ_{x y} - ρ_{OH}$ of $Pt (2 nm) / {Cr}_{2} O_{3} (t)$ bilayers on ${Al}_{2} O_{3} (11 \bar{2} 0)$ at 345 K with different ${Cr}_{2} O_{3}$ thicknesses where the black curves are the fitting for $ρ_{AH}$ with the Langevin function. (b) $ρ_{TH}$ of the bilayers after subtraction of the fitting curve in (a). (c) $ρ_{TH}$ as a function of ${Cr}_{2} O_{3}$ thickness measured at 345 K. (d) Monte Carlo simulations of field dependence of topological charges $Q$ at $T = 1 ∶ 05 T_{N}$ ( $K = | J_{1} | = 1$ and $D = 0.5$ $| J_{1} | = 0.5$ with easy in-plane anisotropy). (e) Contour plot of $ρ_{TH}$ of $Pt (2 nm) / {Cr}_{2} O_{3} (t)$ bilayers on ${Al}_{2} O_{3} (11 \bar{2} 0)$ as a function of temperature and ${Cr}_{2} O_{3}$ thickness. (f) Contour plot of topological charges $Q$ as a function of temperature $T / T_{N}$ and $1 / D$ based on Monte Carlo simulation. The red circles in (e) and (f) are the data points.
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