N\'eel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures

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Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures

Tae-Eon Park et al.

Phys. Rev. B 103, 104410 – Published 5 March 2021

Abstract

Since the discovery of ferromagnetic two-dimensional (2D) van der Waals (vdW) crystals, significant interest on such 2D magnets has emerged, inspired by their appealing physical properties and integration with other 2D family for unique heterostructures. In known 2D magnets, spin-orbit coupling (SOC) stabilizes perpendicular magnetic anisotropy down to one or a few monolayers. Such a strong SOC could also lift the chiral degeneracy, leading to the formation of topological magnetic textures such as skyrmions through the Dzyaloshinskii-Moriya interaction (DMI). Here, we report the experimental observation of Néel-type chiral magnetic skyrmions and their lattice (SkX) formation in a vdW ferromagnet ${Fe}_{3} Ge {Te}_{2}$ (FGT). We demonstrate the ability to drive an individual skyrmion by short current pulses along a vdW heterostructure, $FGT / h − BN$ , as highly required for any skyrmion-based spintronic device. Using first principle calculations supported by experiments, we unveil the origin of DMI being the interfaces with oxides, which then allows us to engineer vdW heterostructures for desired chiral states. Our finding opens the door to topological spin textures in the 2D vdW magnet and their potential device application.

Received 13 November 2019
Revised 5 February 2021
Accepted 8 February 2021

DOI:https://doi.org/10.1103/PhysRevB.103.104410

Physics Subject Headings (PhySH)

Magnetic texture Magnetism Skyrmions Spintronics

Condensed Matter, Materials & Applied Physics

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Vol. 103, Iss. 10 — 1 March 2021

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Figure 1
Crystal structure and the Hall measurement of a van der Waals ${Fe}_{3} Ge {Te}_{2}$ . (a) An atomic structure of a ${Fe}_{3} Ge {Te}_{2}$ (FGT) monolayer (left) and the structure of a FGT bilayer with an interlayer van der Waals (vdW) gap (right). ${Fe}^{III}$ and ${Fe}^{II}$ represent the two inequivalent Fe sites in the +3 and +2 valence states, respectively. (b) Cross-sectional high-resolution transmission electron microscopy (HRTEM) image of the FGT with a Pt capping Hall-bar device fabricated on 100-nm-thick ${Si}_{3} N_{4}$ membrane substrate (right, scale bar is 20 nm). Oxidized FGT is indicated as O-FGT. The enlarged panel shows the high angle annular dark field (HAADF) image in scanning TEM mode of the red-dashed highlighted area in (b) (left, scale bar is 1 nm). (c) Temperature dependent Hall resistance $(R_{x y})$ as a function of applied out-of-plane magnetic field $B_{z}$ . (d) The measured normalized Hall resistance $(R_{x y} / R_{x y, sat})$ as a function of magnetic field, $B_{z}$ , at 120 K, where red and blue dashed rectangular boxes represent the areas of multidomains in the hysteresis loops (left). Bottom two hysteresis loops exhibit the magnetic field sequences used for the generation of multidomains near zero fields from (i) $- B_{z}$ saturation and (ii) $+ B_{z}$ saturation, respectively. Black dashes lines are included to show the original full saturation hysteresis loops.
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Figure 2
Microscopy imaging of domain structures using scanning transmission x-ray microscopy. (a) Schematic of scanning transmission x-ray microscopy (STXM) experimental setup used for magnetic domain imaging and simultaneous electrical pulse injections. The inset shows scanning electron microscopy (SEM) image of the measured device with Hall bar geometry. Scale bar, 4 μm. Two electrode pads on horizontal $x$ axis were used for electrical pulse applications, and oscilloscopes before and after device were used to verify the pulse profiles before and after device, respectively. (b) Exemplary STXM images acquired as a function of increasing magnetic field from $B_{z} = 0 mT$ to $B_{z} = 80 mT$ at 120 K. Dark and bright contrast correspond to magnetization of Fe atoms oriented down $(- M_{z})$ and up $(+ M_{z})$ , respectively. Scale bar, 1 μm.
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Figure 3
Generation and stabilization of magnetic skyrmion lattice phase. (a) The two images on the left side were acquired at $B_{z} = - 40 mT$ at 120 and 100 K after the initial saturation at $B_{z} = + 200 mT$ , respectively, where initial labyrinth domain states were stabilized. The right two images at $B_{z} = - 40 mT$ were acquired after the application of bipolar pulse bursts at 120 and 100 K, respectively, and the other two images at $B_{z} = 0 mT$ were acquired after removing magnetic fields. Scale bar, 1 μm. (b) Representative STXM image of skyrmion crystal (SkX) stabilized over the whole FGT device at $B_{z} = 0 mT$ and $T = 100 K$ . Scale bar, 2 μm. For clarity, the enlarged image of SkX was obtained at $B_{z} = - 80 mT$ . Scale bar, 1 μm. The hexagonal white lines are drawn to guide eye for the ordered SkX, and inset shows the fast-Fourier transform (FFT) image. The right schematic represents the exemplary magnetic configuration of SkX found in chiral magnets for comparison. Note that skyrmion polarity in (b) ( $- M_{z}$ core) is different from (a) ( $+ M_{z}$ core), as the initial field-sweep procedure of reversed field direction was used before the pulse application: $B_{z} = - 200 mT \to + 40 mT$ . (c) Experimental phase diagram of magnetic configurations as a function of temperature and magnetic field. Experimentally measured positions are marked with open circles, and star symbols correspond to exemplary images shown on the right side of the phase diagram. Three representative images show each magnetic configuration state: SkX, SkX + multidomains, and saturated ferromagnet (FM). Scale bar, 1 μm. Black dashed lines in phase diagram are guide to the eyes to indicate the phase boundaries.
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Figure 4
Lorentz transmission electron microscopy (LTEM) measurements of skyrmion crystals (SkX). (a) LTEM images of magnetic configurations in the FGT flake on off-axis at underfocus (left), in focus (middle), and overfocus (right) acquired at zero field and 160 K, respectively. Scale bar, 500 nm. (b) LTEM images of SkX taken at the flake with tilting angle of $- 20^{\circ}$ (left), $0^{\circ}$ (middle), and $20^{\circ}$ (right) with respect to $x$ axis at $B_{z} = - 40 mT$ and 160 K after the field-cooling (FC) process, respectively. The defocus values are ±3 mm for observing over/underfocus image. The striplike contrasts in (b) at zero-tilt angle indicate bending contours of the flake. White hexagonal-shaped lines are eye guides for the unit of the SkX. Scale bar, 200 nm. Upper panels in (b) are schematic drawings of the flake orientations with respect to the incident electron beam. Solid red lines in (b) guide the line-scan positions for contrast profiles. (c) Experimentally measured (symbols) and simulated (solid lines) contrast profiles across a single skyrmion as shown in (b). Note that the simulated profiles were obtained for a 100-nm-size Néel-type skyrmion.
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Figure 5
Current-driven motion of skyrmions. (a) Schematic image of the FGT device used for the experiment. Two bottom inset images show the optical microscopy image of a FGT device capped by $h$ -BN used for the STXM experiment (left, scale bar is 10 μm), and the cross-sectional high-resolution transmission electron microscopy (HRTEM) image of the device (right, scale bar is 20 nm). (b) Sequential STXM images showing skyrmion configurations in the FGT device, where each STXM image was acquired after injecting five unipolar current pulses along the $+ x$ direction (electron flow to $- x$ direction), with an amplitude $J_{e} = 1.4 \times 10^{11} A / m^{2}$ and duration 50 ns. All images were obtained at the oblique field of −20 mT and 100 K. Individual skyrmions are outlined in colored circles for clarity (Sk 1, Sk 2, and Sk 3). Scale bar, 1 μm. (c) The average velocities for two representative skyrmions, Sk2 and Sk3, where error bars denote the standard deviation of multiple measurements.
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Figure 6
First principles calculations of Dzyaloshinskii-Moriya interaction (DMI) in FGT crystal and interfaces. (a) Atomic concentration distribution of Fe, Ge, Te, and O atoms across sample thickness within the FGT crystal used for STXM measurements. (b) Relative distribution of Te and O atomic concentrations, i.e., their sum and difference across the FGT sample thickness. (c) Top views of relaxed crystal of oxygen-substituted ${Fe}_{3} Ge {Te}_{2 - x} O_{x}$ ( $x = 1$ ) relaxed structure showing DMI vectors in (d) [100] and (e) [110] directions. (f) The calculated DMI parameters in [100] and [110] in-plane directions as a function of oxygen substitution concentration of ${Fe}_{3} Ge {Te}_{2 - x} O_{x}$ monolayer ( $x = 0$ , 0.25, 0.5, 0.75, 1). Solid symbols show the DMI values for O-FGT/FGT bulk structures. (g), (h) The side view of oxygen-added FGT structures along [100] and [110] directions. (i) The same as (f) for oxygen addition concentration. (j) Side view of bulk O-FGT/FGT[100] structure and layer-resolved SOC energy difference, $Δ E_{soc}$ , associated with DMI distribution.
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Physical Review B

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Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures

Tae-Eon Park et al.

Phys. Rev. B 103, 104410 – Published 5 March 2021

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Néel-type skyrmions and their current-induced motion in van der Waals ferromagnet-based heterostructures

Tae-Eon Park et al.

Phys. Rev. B 103, 104410 – Published 5 March 2021

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