Local Spin Seebeck Imaging with a Scanning Thermal Probe

Alessandro Sola, Craig Barton, Vittorio Basso, Carsten Dubs, Massimo Pasquale, and Olga Kazakova

Phys. Rev. Applied 14, 034056 – Published 22 September 2020

Abstract

We present the results of an experiment to locally resolve the spin Seebeck effect in a high-quality yttrium iron garnet–Pt sample. We achieve this by using a locally heated scanning thermal probe to generate a highly local nonequilibrium spin current. To support our experimental results, we also present a model based on the nonequilibrium thermodynamic approach that is in a good agreement with the experimental findings. To further corroborate our results, we index the locally resolved spin Seebeck effect with that of the local magnetization texture by magnetic force microscopy and correlate corresponding regions. We hypothesize that this technique allows imaging of magnetization textures within the magnon diffusion length and hence characterization of spin-caloritronic materials at the nanoscale.

Received 19 May 2020
Revised 17 August 2020
Accepted 26 August 2020

DOI:https://doi.org/10.1103/PhysRevApplied.14.034056

Physics Subject Headings (PhySH)

Magnetic domains Metrology Spin Seebeck effect Spin caloritronics Spin current

3-dimensional systems Magnetic insulators Magnetic multilayers

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Alessandro Sola ^1,*, Craig Barton ², Vittorio Basso ¹, Carsten Dubs ³, Massimo Pasquale ¹, and Olga Kazakova ²

¹Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Turin, Italy
²National Physical Laboratory, Teddington TW11 0LW, United Kingdom
³INNOVENT e.V., Technologieentwicklung, Prüssingstraße 27B, 07745 Jena, Germany

^*a.sola@inrim.it

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Vol. 14, Iss. 3 — September 2020

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Images

Figure 1
Longitudinal spin Seebeck measurement configurations. (a) Standard configuration with uniform heating (the red arrows at the YIG/Pt interface represent the hot side; the cold side is at the right-hand side of the YIG slab). (b) Local heating from the $Pt$ side generates a magnetic moment current density $j_{M}$ in spherical symmetry. The projections of the experimental scheme on the $x$ - $z$ and $y$ - $z$ planes are presented on the right side of the panel: the local heat current $i_{q}$ from the thermal probe spreads over a circle whose dimensions ( $r_{0}$ ) limit the spatial resolution. For the YIG/Pt sample that we investigate, the dimensions are $t_{Pt} = 5$ nm, $t_{YIG} = 0.5$ mm, and $L_{z}^{Pt} = 150 μ m$ .
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Figure 2
Experimental setup for the local SSE measurements. (a) Macroscopic schematic representation of the YIG/Pt sample with the nanovoltmeter connected at the edges of the $Pt$ strip. The images in (b)–(e) are examples of measurements obtained from a $80 \times 80 μ m^{2}$ area of the $Pt$ film with an applied magnetic field of approximately $8$ mT. (b) MFM image of the same $80 \times 80 μ m^{2}$ area. (c) Spin Seebeck voltage originating from the scanning thermal probe. (d) AFM image obtained both from the MFM map and the local spin Seebeck map. (e) Three-dimensional image obtained by our overlapping the MFM map and the spin Seebeck map, taking into account the information on their mutual shift provided by the AFM images.
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Figure 3
Measured voltage as consequence of the local heating of the sample at magnetic saturation: spin and ordinary Seebeck voltage as a function of the temperature difference (red points). The dashed blue lines are fitting lines after the compensation of the ordinary Seebeck component. The direction of the saturating magnetic field is shown as a sketch in each panel.
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Figure 4
MFM micrograph of the same area of the YIG/Pt surface at magnetic saturation (a) and at approximately $8$ mT (b).
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Figure 5
Spin Seebeck maps of one $80 \times 80 μ m^{2}$ area of the YIG/Pt sample surface obtained with local heating generated by 0.5, 1, 1.5, 1.8, and 2.2 V on the heater. (a) Spin Seebeck coefficients measured at each pixel of the map. (b) Map of the spurious offset-voltage component and (c) map of the ratios between the local spin Seebeck voltage and the corresponding quantity at magnetic saturation; the colour scale in (c) represents the magnetization as a percentage of the magnetic saturation.
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Figure 6
Local spin Seebeck voltage maps (right columns) obtained at approximately $8$ mT with 1.5 V at the heater, shown together with the MFM micrographs (left columns). The five pairs of maps correspond to the areas of the sample represented in the scheme, where the edge of the $Pt$ film is shown, together with the direction of the applied magnetic field.
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