Dzyaloshinskii-Moriya interaction and spin-orbit torque at the Ir/Co interface

Yuto Ishikuro, Masashi Kawaguchi, Naoaki Kato, Yong-Chang Lau, and Masamitsu Hayashi

Phys. Rev. B 99, 134421 – Published 15 April 2019

Abstract

We studied the spin torque efficiency and the Dzyaloshinskii-Moriya interaction (DMI) of heterostructures that contain interface(s) of Ir and Co. The current-induced shifts of the anomalous Hall loops were used to determine the spin torque efficiency and DMI of $[Pt / Co / X]$ multilayers $(X = Ir, Cu)$ as well as Ir/Co and Pt/Ir/Co reference films. We find the effective spin Hall angle and the spin-diffusion length of Ir to be ∼0.01 and less than ∼1 nm, respectively. The short spin-diffusion length and the high conductivity make Ir an efficient spin sink layer. Such spin sink layer can be used to control the flow of spin current in heterostructures and to induce sufficient spin-orbit torque on the magnetic layer. The DMI of the Ir and Co interface is found to be in the range of ∼1.4 to $\sim 2.2 mJ / m^{2}$ , similar in magnitude to that of the Pt and Co interface. The Ir/Co and Pt/Co interfaces possess the same sign of DMI, resulting in a reduced DMI for the [Pt/Co/Ir] multilayers compared to that of the [Pt/Co/Cu] multilayers. These results show the unique role that the Ir layer plays in defining spin-orbit torque and chiral magnetism in thin-film heterostructures.

Received 10 January 2019
Revised 12 March 2019

DOI:https://doi.org/10.1103/PhysRevB.99.134421

Physics Subject Headings (PhySH)

Anomalous Hall effect Domain walls Dzyaloshinskii-Moriya interaction Spin Hall effect Spin Hall magnetoresistance Spin current Spin torque

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Yuto Ishikuro¹, Masashi Kawaguchi¹, Naoaki Kato¹, Yong-Chang Lau^1,2, and Masamitsu Hayashi^1,2,*

¹Department of Physics, The University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
²National Institute for Materials Science, Tsukuba 305-0047, Japan

^*hayashi@phys.s.u-tokyo.ac.jp

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Issue

Vol. 99, Iss. 13 — 1 April 2019

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Images

Figure 1
(a) Longitudinal resistance $R_{x x}$ of reference film D (Sub./1.5 Ta/2 Ir/1 CoFeB/2 MgO) plotted as a function of the angle $θ$ between the magnetic field and the film normal $(z$ axis). The applied magnetic field is 3 T. The inset shows the definition of the coordinate axis. (b) Spin Hall magnetoresistance $(Δ R_{x x}^{SMR} / R_{x x}^{z})$ plotted as a function of the Ir layer thickness $(d)$ for reference film D. The inset shows $L / (w R_{x x}^{z})$ vs $d$ for the same film. The results presented were obtained using Hall bars with $L$ ∼ 1.2 mm and $w \sim 0.4$ mm.
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Figure 2
(a) Optical micrograph of a representative Hall bar with the definition of the coordinate axis. (b) Anomalous Hall resistance $(R_{x y})$ vs $H_{z}$ for two different dc currents $(I_{DC} = \pm 12 mA)$ for film C $(X = Ir, N = 1, d \sim 0.6$ nm). The bias field along $x (H_{x})$ is fixed to ∼0.2 T. The definition of $H_{eff}^{z}$ is schematically illustrated. (c) $H_{eff}^{z}$ vs current density $J$ for the same film with $H_{x} \sim \pm 0.2 T$ . A linear function is fitted to the data to obtain the slope $H_{eff}^{z} / J$ . The fitting results are shown by the solid lines. The error bars show the standard deviation of $H_{eff}^{z}$ from repeated measurements. (d) The slope $H_{eff}^{z} / J$ plotted as a function of the bias field $H_{x}$ . $H_{DM}$ and $H_{eff}^{z} {/ J |}_{sat}$ , which is proportional to $ξ_{DL}$ , are extracted as schematically drawn (see text for the details).
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Figure 3
$H_{eff}^{z} / J$ vs $H_{x}$ for reference films (a) E [Sub./1.5 Ta/7 Ir/0.8 Co/2 MgO/1 Ta] and (b) F [Sub./3 Ta/2 Pt/1 Ir/0.9 Co/2 MgO/1 Ta].
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Figure 4
(a) Spin torque efficiency $ξ_{DL}$ and (b) spin Hall magnetoresistance $Δ R_{x x}^{SMR} / R_{x x}^{z}$ plotted as a function of the $X$ layer thickness $(d)$ for films A–C $({[0.6 Pt / 0.9 Co / d X]}_{N}$ multilayers). Black squares: film A $(X = Ir, N = 3)$ ; red circles: film B $(X = Cu, N = 3)$ ; and green triangles: film C $(X = Ir, N = 1)$ . The error bars in (a) represent standard deviation of the data used to obtain $ξ_{DL}$ from the plot of $H_{eff}^{z} / J$ vs $H_{x}$ with $| H_{x} | > | H_{DM} |$ [see, e.g., Fig. 2]. The error bars for the results shown in (b) are smaller than the symbol size.
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Figure 5
(a)–(d) Illustration of spin transport in the ${[Pt / Co / X]}_{N}$ multilayers $(X = Ir$ , Cu, $N$ >1). $\vec{m}, \vec{σ}$ and ${\vec{H}}_{SOT}$ indicate the magnetization direction of the Co layer, spin polarization of the conduction electrons diffusing in from the Pt layers via the spin Hall effect, and the spin-orbit effective field acting on the magnetic moments. ${\vec{H}}_{SOT}$ associated with the spin current from the top and bottom Pt layers are illustrated by the red and blue arrows, respectively. (a),(b) $X = Cu$ and (c),(d) $X = Ir$ . The Ir layer is assumed to absorb spin current that diffuses in from the top Pt layer. Co magnetization points along the film normal, i.e., along $z$ , for (a),(c), and along the film plane, i.e., along $y$ , for (b),(d). For the latter, charge current due to the inverse spin Hall effect is depicted by the dotted lines.
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Figure 6
The DM exchange constant $| D |$ vs $X$ layer thickness $(d)$ for films A–C $({[0.6 Pt / 0.9 Co / d X]}_{N}$ multilayers). Black squares: film A $(X = Ir, N = 3)$ ; red circles: film B $(X = Cu, N = 3)$ ; and green triangles: film C $(X = Ir, N = 1)$ . Broken lines, which are a guide to the eye, illustrate values of $| D |$ with large $d$ . The error bars represent the range of $H_{DM}$ when the range of linear fitting to $H_{eff}^{z} / J$ vs $H_{x}$ is varied.
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