Abstract
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.
Highlights
In nanoscale spintronic circuits, large charge current densities can create large thermal gradients that impact the resulting signal through various thermoelectric and thermal spin effects
The anomalous Nernst effect (ANE) portion of the NLSV response can be enhanced by dramatically decreasing the substrate thermal conductance such that the majority of the thermal gradient produced by the high charge current density in the injector nanowire lies in the plane of the detector FM magnetization. We demonstrated this in a previous work by fabricating NLSVs on thin amorphous silicon-nitride scitation.org/journal/adv (Si–N) membranes with very low thermal conductance,29 and we show a simple model demonstrating this effect in Figs. 1(c)– 1(f)
This shows a fairly typical NLSV response, and this case does not isolate thermal effects, a small hysteresis contribution that originates in the ANE is already visible
Summary
Large charge current densities can create large thermal gradients that impact the resulting signal through various thermoelectric and thermal spin effects. We indicate calculated values of thermal gradients for these conditions at key locations, which show that the membrane-supported NLSV has approximately one order of magnitude lower ∇Tz at the heated FM1/NM interface, while keeping a 38× higher ∇Tx across the detector FM2 This provides our rationale for using the Si–N membrane NLSVs to study the ANE as a function of temperature since the relative reduction of ∇Tz reduces thermal spin injection, or the spin dependent Seebeck effect (SDSE), and allows us to record large ANE signals from NLSVs. The nearly two-dimensional nature of the circuits allows us to simplify our analysis by performing 2D finite-element modeling to calculate the in-plane thermal gradients. We present evidence that NLSV circuit geometry has a significant and unusual effect on the relative thermal profiles across the detector contacts
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