Magneto-Seebeck microscopy of domain switching in collinear antiferromagnet CuMnAs

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Magneto-Seebeck microscopy of domain switching in collinear antiferromagnet CuMnAs

T. Janda et al.

Phys. Rev. Materials 4, 094413 – Published 28 September 2020

Abstract

Antiferromagnets offer spintronic device characteristics unparalleled in ferromagnets owing to their lack of stray fields, THz spin dynamics, and rich materials landscape. Microscopic imaging of antiferromagnetic domains is one of the key prerequisites for understanding physical principles of the device operation. However, adapting common magnetometry techniques to the dipolar-field-free antiferromagnets has been a major challenge. Here we demonstrate in a collinear antiferromagnet a thermoelectric detection method by combining the magneto-Seebeck effect with local heat gradients generated by scanning far-field or near-field techniques. In a 20-nm epilayer of uniaxial CuMnAs we observe reversible $180^{\circ}$ switching of the Néel vector via domain wall displacement, controlled by the polarity of the current pulses. We also image polarity-dependent $90^{\circ}$ switching of the Néel vector in a thicker biaxial film, and domain shattering induced at higher pulse amplitudes. The antiferromagnetic domain maps obtained by our laboratory technique are compared to measurements by the established synchrotron-based technique of x-ray photoemission electron microscopy using x-ray magnetic linear dichroism.

Received 5 June 2020
Accepted 12 August 2020

DOI:https://doi.org/10.1103/PhysRevMaterials.4.094413

Physics Subject Headings (PhySH)

Antiferromagnetism Magnetic domains Seebeck effect Spintronics

Antiferromagnets

Atomic force microscopy Near-field optical spectroscopy Scanning techniques

Condensed Matter, Materials & Applied Physics

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Vol. 4, Iss. 9 — September 2020

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Images

Figure 1
Comparison between laboratory optical and synchrotron x-ray images of the domain structure in bar-shaped patterned antiferromagnetic CuMnAs. (a) Schematics of the measurement setup for the laboratory SFOM-MSE technique (top panel). A focused laser beam creates a local thermal gradient. When scanned over an antiferromagnetic (purple and gray arrows) texture, the in-plane components of the thermal gradient generate a voltage across the bar due to the MSE (bottom panel). (b) Optical micrograph of four $50 − μ m$ -long and $5 − μ m$ -wide bars patterned from a 45-nm-thick CuMnAs epilayer along [100], $[1 \bar{1} 0]$ , [110], and [010] crystallographic axes of CuMnAs. (c) Comparison between SFOM-MSE and XMLD-PEEM measurements in the four microbars. Antiferromagnetic domain structure is observed by XMLD-PEEM for x-ray polarization $E ∥ [1 \bar{1} 0]$ crystal axis. The single- and double-headed arrows in panel (c) indicate the in-plane projection of the x-ray propagation vector and the x-ray polarization vector, respectively. The light (dark) contrast corresponds to antiferromagnetic domains with the Néel vector oriented perpendicular (parallel) to the x-ray polarization.
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Figure 2
Current-pulse-induced modification of the domain structure detected by MSE scans and compared to AMR measurements. (a) SEM micrograph of a $5 − μ m$ -wide cross bar patterned from a 45-nm-thick CuMnAs epilayer. The MSE scans have been performed within the area of $25 \times 25 μ m^{2}$ , indicated by the yellow dashed line. (b) MSE signal measured along the vertical bar after seven trains of positive pulses (left) followed by 10 trains of negative pulses (right). Each train of pulses contains six individual pulses. (c) MSE signal simultaneously measured along the horizontal bar. (d) Corresponding variation of the magnetoresistance measured in a four-point geometry along the vertical bar. Red (blue) data points correspond to resistance measurements after current pulses of positive (negative) polarity with $| j_{p} | = 9.6 \times 10^{10} A / m^{2}$ and $τ_{p} = 20 ms$ .
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Figure 3
Reproducibility of antiferromagnetic texture after pulsing with alternating polarity. (a) Optical micrograph of a $10 − μ m$ -wide cross bar, showing the measurement contacts geometry used. [(b)–(e)] Sequence of MSE maps of the vertical thermoelectric voltage and horizontal thermoelectric voltage measured simultaneously for alternating pulses. The MSE scans were performed within the area highlighted by the yellow dashed line in panel (a). (f) Simulated MSE maps of $V_{T}^{V}$ (upper graph) and $V_{T}^{H}$ (lower graph) for a domain configuration of a geometrically pinned domain wall (middle schematics) by taking into account the experimental conditions of a focused laser spot with Gaussian profile of $1.5 − μ m$ FWHM and $5 − mW$ laser power and by assuming a magneto-Seebeck coefficient of $Δ S = 4 μ V / K$ .
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Figure 4
Shattering of magnetic domains after an electrical pulse. (a) SFOM-MSE scan of a $5 − μ m$ -wide bar device prior to any electrical pulse. (b) After a pulse with a current density of $1.3 \times 10^{11} A / m^{2}$ , the contrast is lost with the main features absent. The loss of contrast is ascribed to the shattering of the magnetic domains into smaller domains, which become significantly smaller than the spatial resolution of the SFOM-MSE. Hence, the total SFOM-MSE signal over many domain walls averages nearly to zero. (c) The SFOM-MSE signal 1 week after the electrical pulse, where a similar pattern of large domains has reappeared and is resembling the initial state in panel (a). (d) XMLD-PEEM measurements prior the pulse, (e) just after the pulse with a current density of $1.2 \times 10^{11} A / m^{2}$ , and (f) 4 h after the pulse.
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Figure 5
SNOM-MSE scan of a bar device patterned from a uniaxial CuMnAs layer. (a) Schematics of the SNOM-MSE setup. The thermal gradient is created when a metal-coated AFM tip interacts with an infrared laser, inducing an optical near field at the apex of the tip. A $180^{\circ}$ domain wall appears in the MSE maps as two features of opposite sign together, as illustrated in the bottom panel. (b) Micrograph of the scanned bar-device, where the AFM tip is also visible. (c) From left to right: topography map, magnitude of the MSE signal, and its sign.
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Figure 6
Reversible switching measured in SNOM-MSE. (a) Maps of the magnitude of the MSE signal, $| V_{T} |$ , after applying current pulses of amplitude $| j_{p} | = 2.5 \times 10^{11} A / m^{2}$ in opposite directions indicated by the red and blue arrow on top. (b) Maps of the corresponding $s g n (V_{T})$ values. [(c), (d)] Resistance variations associated with the corresponding switching states for low and high current densities. Current density values shown are in $10^{11} A / m^{2}$ .
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