Single-spin sensing of domain-wall structure and dynamics in a thin-film skyrmion host

Alec Jenkins, Matthew Pelliccione, Guoqiang Yu, Xin Ma, Xiaoqin Li, Kang L. Wang, and Ania C. Bleszynski Jayich

Phys. Rev. Materials 3, 083801 – Published 1 August 2019

Abstract

Skyrmions are nanoscale magnetic structures with features promising for future low-power memory or logic devices. In this work, we demonstrate scanning techniques based on nitrogen vacancy center magnetometry that simultaneously probe both the magnetic dynamics and structure of room temperature skyrmion bubbles in a thin-film system Ta/CoFeB/MgO. We confirm the handedness of the Dzyaloshinskii-Moriya interaction in this material and extract the magnitude of the helicity angle of the skyrmion bubbles. Our measurements also show that the skyrmion bubbles in this material change size in discrete steps, dependent on the local pinning environment, with their average size determined dynamically as their domain walls hop between pinning sites. In addition, an increase in magnetic field noise is observed near skyrmion bubble domain walls. These measurements highlight the importance of interactions between internal degrees of freedom of skyrmion bubble domain walls and pinning sites in thin-film systems. Our observations have relevance for future devices based on skyrmion bubbles where pinning interactions will determine important aspects of current-driven motion.

Received 8 December 2018
Revised 7 May 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.3.083801

Physics Subject Headings (PhySH)

Domain walls Nitrogen vacancy centers in diamond Skyrmions

Optically detected magnetic resonance Scanning probe microscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Alec Jenkins¹, Matthew Pelliccione¹, Guoqiang Yu², Xin Ma³, Xiaoqin Li⁴, Kang L. Wang⁵, and Ania C. Bleszynski Jayich^1,*

¹Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA
²Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
³Department of Electrical and Computer Engineering, University of California, Santa Barbara, Santa Barbara, California 93106, USA
⁴Department of Physics, The University of Texas at Austin, Austin, Texas 78712, USA
⁵Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA

^*ania@physics.ucsb.edu

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Vol. 3, Iss. 8 — August 2019

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Images

Figure 1
Scanning NV-based imaging of skyrmions. (a) Diagram of the experimental setup. A single-crystal diamond probe containing a single NV center near the apex of its tip is scanned above the multilayer sample. Simultaneous optical and RF excitation of the NV gives an ESR signal (b), which is used to measure the stray magnetic field at each position in the scan. (b) Example of an NV ESR signal: The NV fluorescence rate decreases when applied microwaves are on resonance with either of the NV's two spin transitions ( $m_{s} = 0 \to + 1$ and $m_{s} = 0 \to - 1$ ). The splitting of the two peaks is used to calculate $B_{NV}$ , the magnetic field along the NV center axis. Plotted is the ratio of the NV fluorescence to its off-resonant value. (c) NV magnetic image of a skyrmion bubble with an external magnetic field of 10.0 Oe perpendicular to the film plane. (d) Magnetic contour image with applied microwave frequency = 2.870 GHz and with 6.5 Oe perpendicular to the film plane.
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Figure 2
Field contour images of magnetic phases in the Ta/CoFeB/MgO system acquired with the NV microscope. The applied microwave frequency is 2.870 GHz for all images. An external magnetic field applied normal to the sample plane is increased from 0–9 Oe in (a)–(d). As the field increases, the magnetic order evolves from stripes at 0 Oe (a) into a mixed skyrmion/stripe phase at 3 Oe (b) and a skyrmion phase at 6 (c) and 9 (d) Oe. At these fields, zero-field contours mark the approximate domain-wall positions. The scale bar for all images is shown in (d).
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Figure 3
Domain wall fluctuations and evolution with $B_{ext}$ imaged by NV center microscopy. (a) 2.870 GHz contour images of a skyrmion bubble, each labeled by the corresponding $B_{ext}$ applied during that scan. As $B_{ext}$ increases, a section of the domain wall evolves into a bistable configuration, seen most clearly in the (d) and (e) scans with 6.5 and 7.0 Oe. The scale bar for (a)–(g) is shown in (a). (h) 2.870 GHz contour images of another skyrmion bubble taken at $B_{ext} = 7.0 G$ . (i)–(m) NV center ESR spectra taken with the NV fixed at the location indicated by the red dot, each labeled by $B_{ext}$ . The multiple pairs of ESR splittings in each spectrum correspond to different positions of the domain wall.
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Figure 4
Enhanced magnetic fluctuations near skyrmion domain walls. (a) Spatial map of the magnetic field along the NV axis. The external field, $B_{ext} = 9.5$ Oe, is normal to the film plane. (b) NV ESR width averaged over both ESR dips, showing enhanced magnetic fluctuations at the skyrmion domain wall.
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Figure 5
Telegraph switching of the NV fluorescence signal in the presence of a skyrmion bubble switching between a bubble and a saturated state. (a) 2.870 GHz contour image of the skyrmion bubble shown in Fig. 4 taken at $B_{ext} = 9.2 G$ . (b) 2.870 GHz contour image of the skyrmion bubble shown in Fig. 4 taken at $B_{ext} = 9.5 G$ . As the field is increased, the bubble state becomes unstable and switching between the saturated and bubbles sates is observed as a decrease in the NV ESR contrast. (c) The NV fluorescence collected in 5 msec bins when fixing the NV location at the position indicated by the red dot in (b) and fixing the drive microwave frequency at 2.871 GHz. A characteristic hopping frequency of 14 Hz is measured over the full 30 second measurement window.
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Figure 6
Reconstruction of helicity angle. (a) Linecuts at various angles $ϕ$ across a magnetic bubble, comparing the measured magnetic field shown in Fig. 4 to the simulated field for four domain-wall types—right-handed Néel, Bloch, left-handed Néel, and a domain wall with $| ψ_{h} | = 58^{\circ}$ . The NV height is calibrated to be $58 \pm 5 nm$ using the method described in Ref. [35]. The NV angle relative to the film normal is measured as $60^{\circ} \pm 4^{\circ}$ using an external field aligned along the film normal. The in-plane NV angle is extracted from the $B_{NV}$ bubble image by finding the direction of maximum field gradient at the domain wall and is $16^{\circ} \pm 2^{\circ}$ relative to the image $x$ axis. (b) The simulated magnetic field along the NV axis for the best fit helicity angle $58^{\circ}$ . (c) Schematic of the magnetization of a skyrmion bubble with a fixed helicity angle $| ψ_{h} | = 58^{\circ}$ , viewed from above. The magnetization transitions from pointing upward outside the bubble (red) to pointing downward inside (blue) with a rotation direction between that of Bloch and Néel-type domain walls.
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