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The anomalous Nernst effect, which generates an out-of-plane charge voltage in response to a thermal gradient perpendicular to the magnetization of a ferromagnet, can play a significant role in many spintronic devices where large thermal gradients exist. Since they typically include features deep within the submicron regime, nonlocal spin valves can be made very sensitive to this effect by lowering the substrate thermal conductance. Here, we use nonlocal spin valves suspended on thin silicon nitride membranes to determine the temperature dependence of the anomalous Nernst coefficient of 35 nm thick permalloy (Ni80Fe20) from 78 K to 300 K. In a device with a simple ferromagnet geometry, the transverse Seebeck coefficient shows a weak temperature dependence, with values at all T near 2.5 μV/K. Assuming previously measured values of the Seebeck coefficient for permalloy, which has a near-linear dependence on T, leads to a low temperature upturn in the anomalous Nernst coefficient RN. We also show that the temperature dependence of this coefficient is different when a constricted nanowire is used as the ferromagnetic detector element.
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Apparatus for the room temperature measurement of low field Nernst and magneto-Seebeck coefficients
Nernst coefficient measurements are a classic approach to investigate charge carrier scattering in both metals and semiconductors. However, such measurements are not commonly performed, despite the potential to inform material design strategies in applications such as thermoelectricity. As dedicated instruments are extremely scarce, we present here a room temperature apparatus to measure the low field Nernst coefficient (and magneto-Seebeck coefficient) in bulk polycrystalline samples. This apparatus is specifically designed to promote accurate and facile use, with the expectation that such an instrument will make Nernst measurements de rigueur. In this apparatus, sample loading and electrical contacts are all pressure-based and alignment is automatic. Extremely stable thermal control (10 mK of fluctuation when ΔT = 1 K) is achieved from actively cooled thermoelectric modules that operate as heaters or Peltier coolers. Magneto-Seebeck measurements are integrated into the system to correct for residual probe offsets. Data from the apparatus are provided on bulk polycrystalline samples of bismuth, InSb, and SnTe, including raw data to illustrate the process of calculating the Nernst coefficient. Finally, we review how Nernst measurements, in concert with Seebeck, Hall, and electrical resistivity, can be analyzed via the Boltzmann equation in the relaxation time approximation to self-consistently predict the Fermi level, effective mass, and energy-dependent relaxation time.
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- Award ID(s):
- 2118201
- PAR ID:
- 10534757
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- Review of Scientific Instruments
- Volume:
- 95
- Issue:
- 8
- ISSN:
- 0034-6748
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0222406
