Temperature dependent Anomalous Nernst effect in magnetic metallic film by ac higher harmonics technique*

Thermoelectric effects in magnetic materials can generate a transverse electric voltage from a heat flow. Anomalous Nernst effect (ANE), as a transverse thermoelectric effect, has been receiving increasing attention because it provides insights on the topological character of the band structure and leads to the simple construction of thermoelectric devices for measuring heat fluxes. Typical measurements of the ANE require a separate heater element as the source of the temperature gradient. We present an ac higher harmonics technique to detect ANE in patterned metal lines with the temperature gradient induced by self-heating. The temperature gradient can be calculated from the dissipated power and the thermal conductivity of the metal. The ANE is observed in the second harmonic, and therefore separable from longitudinal magnetoresistance signal, using lock-in detection with a bridge circuit. Using this ac higher detection method, we measured an anomalous Nernst coefficient of 0.3 μV/K  in a Ni film at room temperature. Typically, large ANE responses are detected in materials with nonzero Berry curvature. We further investigated the temperature dependence of ANE in disordered Ni₃Pt films prepared by sputter deposition. Ni₃Pt has a moderate ANE coefficient (0.8 μV/K ) at room temperature. The ANE coefficient increases linearly with temperature for T < 50 K , peaks at approximately 200 K at 1.2 μV/K and gradually decreases to zero at the Curie temperature. We tune the composition of Ni_3-xFe_xPt films to move the peak in the ANE coefficient to room temperature. This ac higher harmonics technique gives us a new and simple method to probe the ANE in ferromagnetic and non-collinear antiferromagnetic thin metallic films, facilitating the search for materials with large ANE.