Electrical switching of perpendicular magnetization in a single ferromagnetic layer

Rapid Communication

Electrical switching of perpendicular magnetization in a single ferromagnetic layer

Liang Liu, Jihang Yu, Rafael González-Hernández, Changjian Li, Jinyu Deng, Weinan Lin, Chenghang Zhou, Tiejun Zhou, Jing Zhou, Han Wang, Rui Guo, Herng Yau Yoong, Gan Moog Chow, Xiufeng Han, Bertrand Dupé, Jakub Železný, Jairo Sinova, and Jingsheng Chen

Phys. Rev. B 101, 220402(R) – Published 2 June 2020

Abstract

We report on the efficient spin-orbit torque (SOT) switching in a single ferromagnetic layer induced by a new type of inversion asymmetry, the composition gradient. The SOT of 6- to 60-nm epitaxial FePt thin films with a $L 1_{0}$ phase is investigated. The magnetization of the FePt single layer can be reversibly switched by applying electrical current with a moderate current density. Different from previously reported SOTs which either decreases with or does not change with the film thickness, the SOT in FePt increases with the film thickness. We found the SOT in FePt can be attributed to the composition gradient along the film normal direction. A linear correlation between the SOT and the composition gradient is observed. This Rapid Communication introduces a platform to engineer large SOTs for lower-power spintronics.

Received 27 November 2019
Revised 21 April 2020
Accepted 14 May 2020

DOI:https://doi.org/10.1103/PhysRevB.101.220402

Physics Subject Headings (PhySH)

Composition Magnetization switching Spin torque Spin-orbit coupling Spintronics

Ferromagnets

Inversion symmetry

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Liang Liu^1,*, Jihang Yu^1,*, Rafael González-Hernández^2,3, Changjian Li¹, Jinyu Deng¹, Weinan Lin¹, Chenghang Zhou¹, Tiejun Zhou¹, Jing Zhou¹, Han Wang¹, Rui Guo¹, Herng Yau Yoong¹, Gan Moog Chow ¹, Xiufeng Han⁴, Bertrand Dupé ^2,†, Jakub Železný⁵, Jairo Sinova², and Jingsheng Chen ^1,6,‡

¹Department of Materials Science and Engineering, National University of Singapore, Singapore 117575
²Institut für Physik, Johannes Gutenberg Universität Mainz, D-55099 Mainz, Germany
³Department of Physics, Universidad del Norte, Barranquilla, Colombia
⁴Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁵Institute of Physics of the Czech Academy of Sciences, Cukrovarnická 10/112, Prague 16200, Czech Republic
⁶Suzhou Research Institute, National University of Singapore, Suzhou 215123, China

^*These authors contributed equally to this work.
^†Fonds de la Recherche Scientifique (FNRS), Bruxelles, Belgium; Nanomat/Q-mat/CESAM, Universite de Liege, B-4000 Sart Tilman, Belgium.
^‡msecj@nus.edu.sg

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Issue

Vol. 101, Iss. 22 — 1 June 2020

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Images

Figure 1
Current-induced magnetization switching in the $L 1_{0}$ FePt single layer. (a) Crystal structure of $L 1_{0}$ FePt. (b) Scanning transmission electron microscopy (STEM) image of the FePt film in the [100]-[001] plane. The brighter atoms are Pt whereas the darker atoms are Fe. The scale bar is 2 nm. (c) and (d) Current-induced magnetization switching in the 6-nm FePt film on $SrTi O_{3}$ under in-plane magnetic fields of (c) −1000 Oe and (d) 1000 Oe. (e)–(j) Polar MOKE images showing the magnetization switching process by increasing $I_{pulse}$ from 0 to $2.6 \times 10^{7} A / c m^{2}$ . The light dashed lines in (e) indicate the Hall bar edges. (k)–(p) Polar MOKE images showing the magnetization switching process by changing $I_{pulse}$ from 0 to $- 2.6 \times 10^{7} A / c m^{2}$ . The corresponding processes of (e)–(j) and (k)–(p) are indicated by the arrows in (d).
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Figure 2
Field dependence of the SOT switching in FePt on $SrTi O_{3}$ substrates. (a) and (b) SOT switching loops obtained under varying $H_{x}$ for (a) 6-nm and (b) 20-nm FePt films. (c) Critical switching current density $(J_{c})$ as a function of $H_{x}$ . (d) Current-induced multilevel switching in 6-nm FePt film with controlled series of current pulses (see the Supplemental Material [16]). Six intermediate states ( $- 0.45 Ω$ , $- 0.3 Ω$ , $- 0.15 Ω$ , $0.1 Ω$ , $0.3 Ω$ , and $0.4 Ω$ ) are observed. (e) Histogram of the six resistance states in (d), obtained from 50 repetitions of 3 positive + 3 negative pulse current sequence.
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Figure 3
Estimation of spin-orbit effective fields in 20-nm FePt films by harmonic Hall measurements. (a) Schematic of the spin-orbit effective fields in FePt. (b)–(d) Field dependence of (b) first- and (c) and (d) second-harmonic signals for 20-nm FePt film $(I_{a c} = 10 mA)$ . The blue and orange colors in (b)–(d) correspond to $+ M$ and $- M$ , respectively. (e) Longitudinal and transverse spin-torque efficiencies for the 20-nm FePt films grown on $SrTi O_{3}$ , MgO, and TiN (5 nm)/MgO substrates. (f) Ordering parameter dependence of longitudinal (dark symbols) and transverse (blue symbols) spin-torque efficiencies in 20-nm FePt films. The round symbols represent FePt films grown on $SrTi O_{3}$ substrates with different deposition temperatures. The triangular symbols are for the FePt films grown on TiN (5 nm)/MgO substrates.
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Figure 4
Composition gradient in the 60-nm FePt film. (a) Cross-sectional high-angle annular dark-field-STEM image of a 60-nm FePt film on TiN (5 nm)/MgO. (b) and (c) Zone-in STEM images at the (b) bottom and the (c) top of the 60-nm FePt film as indicated in (a). The brighter atoms are Pt whereas the darker atoms are Fe. (d) Schematic of a stoichiometry gradient in the FePt structure where certain Fe atoms are replaced by Pt in the Fe atomic layers. (e) Overall energy dispersive x-ray (EDX) intensity along the vertical direction for Fe (upper panel) and Pt (lower panel).
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Figure 5
The dependence of SOT effective fields on composition gradient. (a) Thickness dependence of longitudinal and transverse spin-torque efficiencies for FePt films on TiN (5 nm)/MgO. (b) Longitudinal and transverse spin-torque efficiencies as a function of the composition gradient. The data points are for the 20-, 30-, and 60-nm FePt samples with Pt/Fe composition gradients of 0.31, 0.53, and 0.71%/nm, respectively.
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