Evolution of topological skyrmions across the spin reorientation transition in Pt/Co/Ta multilayers

Min He, Gang Li, Zhaozhao Zhu, Ying Zhang, Licong Peng, Rui Li, Jianqi Li, Hongxiang Wei, Tongyun Zhao, X.-G. Zhang, Shouguo Wang, Shi-Zeng Lin, Lin Gu, Guoqiang Yu, J. W. Cai, and Bao-gen Shen

Phys. Rev. B 97, 174419 – Published 21 May 2018

Abstract

Magnetic skyrmions in multilayers are particularly appealing as next generation memory devices due to their topological compact size, the robustness against external perturbations, the capability of electrical driving and detection, and the compatibility with the existing spintronic technologies. To date, Néel-type skyrmions at room temperature (RT) have been studied mostly in multilayers with easy-axis magnetic anisotropy. Here, we systematically broadened the evolution of magnetic skyrmions with sub-50-nm size in a series of Pt/Co/Ta multilayers where the magnetic anisotropy is tuned continuously from easy axis to easy plane by increasing the ferromagnetic Co layer thickness. The existence of nontrivial skyrmions is identified via the combination of in situ Lorentz transmission electron microscopy (L-TEM) and Hall transport measurements. A high density of magnetic skyrmions over a wide temperature range is observed in the multilayers with easy-plane anisotropy, which will stimulate further exploration for new materials and accelerate the development of skyrmion-based spintronic devices.

Received 23 July 2017
Revised 19 April 2018

DOI:https://doi.org/10.1103/PhysRevB.97.174419

Physics Subject Headings (PhySH)

Domains Magnetic anisotropy Magnetic phase transitions Skyrmions Spintronics Topological Hall effect

Magnetic multilayers Transition metals

Lorentz microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Min He^1,2, Gang Li^1,2, Zhaozhao Zhu^1,2, Ying Zhang^1,*, Licong Peng^1,2, Rui Li^1,2, Jianqi Li^1,2, Hongxiang Wei¹, Tongyun Zhao¹, X.-G. Zhang³, Shouguo Wang⁴, Shi-Zeng Lin⁵, Lin Gu^1,2, Guoqiang Yu¹, J. W. Cai^1,2,†, and Bao-gen Shen^1,2

¹Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
²School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
³Department of Physics and the Quantum Theory Project, University of Florida, Gainesville, FL 32611, United States
⁴Institute of Advanced Materials, Beijing Normal University, Beijing 100875, China
⁵Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

^*zhangy@iphy.ac.cn
^†jwcai@iphy.ac.cn

Article Text (Subscription Required)

Click to Expand

Supplemental Material (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 97, Iss. 17 — 1 May 2018

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
The magnetic anisotropy together with the corresponding skyrmion distribution in Pt/Co/Ta multilayers. (a) (Top) The DMI for two magnetic atoms (grey spheres) close to an atom (blue sphere) with a large spin-orbit coupling. (Bottom) Schematic multilayers made of six repetitions of the Pt/Co/Ta trilayer. (b)–(d) Room-temperature magnetic hysteresis loops measured along the in-plane and out-of-plane directions respectively. Each loop is normalized to saturate magnetization. (e)–(g) Magnetic skyrmion distribution while completely evolved from stripe domain at the magnetic fields of 680, 930, and 1040 Oe, respectively. The skyrmion density gets higher with thicker Co layer. The scale bar in (e)–(g) is 200 nm.
Reuse & Permissions
Figure 2
The analysis of topological Hall resistivity (THR). (a) Out-of-plane magnetic hysteresis loop, (b) total Hall resistivity ( $ρ_{x y}$ ), and (c) THR ( $ρ_{TH}$ ) extracted by subtracting ordinary resistivity ( $R_{0} H$ ) and anomalous Hall resistivity ( $R_{S} M$ ) from the experimental $ρ_{x y} - H$ curve in the representative Pt/Co(1.95)/Ta multilayers. (d) The summarized THR for the multilayers with various Co-layer thicknesses. (e) The maximum THR value dependent on the Co layer thickness.
Reuse & Permissions
Figure 3
Identification of skyrmion spin configuration by tilting samples in real-space L-TEM at RT. (a) Schematic diagram of titling process, and α is the tilt angle around $x$ axis. The angle α for Pt/Co(1.85)/Ta is (b) −18°, (c) 0° and (d) 18°, respectively. The enlarged views of a single skyrmion in the insets (b) and (d), with bright and dark reversed magnetic contrast for the opposite tilting angle, are identified as the Néel-type skyrmion. The tilting angle α are (e) −18°, (f) 0°, and (g) 18° for Pt/Co(1.95)/Ta, respectively. The tilting angle α are (h) −20°, (i) 0°, and (j) 20° for Pt/Co(2.1)/Ta, respectively. Scale bars in (b)–(j) correspond to 50 nm.
Reuse & Permissions
Figure 4
The correlation between THR asymmetry and microscopic skyrmion evolution during increasing/decreasing magnetic fields in a representative Pt/Co(2.0)/Ta multilayers with easy-plane anisotropy. (a) THR hysteresis loop. L-TEM images for the corresponding skyrmion evolution at different magnetic fields (b) 0, (c) 750, (d) 950, and (e) 1320 Oe, while increasing the magnetic field to saturation state, and (f) 920 and (g) 750 Oe, while decreasing the magnetic field. The scale bar in (b)–(g) is 200 nm.
Reuse & Permissions
Figure 5
Thermal stability of high-density skyrmions in Pt/Co(2.1)/Ta multilayers with easy-plane anisotropy. L-TEM Fresnel images of magnetic skyrmions at different temperatures (a) 12, (b) 100, and (c) 300 K. (d) The contour mapping of skyrmion density as a function of applied magnetic fields ( $H$ ) and the temperature ( $T$ ). The black dots show the experimental points, from which the skyrmion density map is extrapolated. The color scale indicates the skyrmion numbers per square micrometer. S for the stripe, SKX for skyrmions, and FM for ferromagnetic state. The scale bar in (a)–(c) is 200 nm.
Reuse & Permissions

Physical Review B

covering condensed matter and materials physics