Large Spin-Dependent Thermoelectric Effects in NiFe-based Interconnected Nanowire Networks

摘要

Thermoelectric effects in spintronic materials are actively studied in the emerging field of spin caloritronics due to their unique physical properties including spin Seebeck effects, thermally generated spin current and thermal-assisted spin-transfer torque [ – ]. Also, the thermoelectric analogs of the magnetoresistive effects in magnetic multilayers, spin valves, and tunneling junctions such as the giant magneto-Seebeck and magneto-Peltier effects are of special interest, as they could be used to enable magnetic control of heat flow and thermoelectric voltages for waste-heat recovery from electronic circuits [ , – ]. The large spin-dependent thermoelectric effects achieved by modifying appropriately the magnetization configurations of the multilayer with an external magnetic field exploit the fact that the Seebeck coefficients for spin-up and spin-down electrons are significantly different. This difference of Seebeck coefficients is ascribed to the d-band exchange splitting in transition ferromagnetic (FM) metals, as suggested from previous works performed on dilute magnetic alloys [ , ]. When considering the Peltier effect, it means that different amount of heat is carried by the spin-up and spin-down electrons. It was recently demonstrated that interconnected magnetic nanowire (NW) networks fabricated by electrochemical deposition in 3D nanoporous polymer host films provide an attractive pathway to fabricate light, robust, flexible, and shapeable spin caloritronic devices in versatile formats that meet key requirements for electrical, thermal, and mechanical stability [ , ]. In addition, electrochemical synthesis is a powerful method for fabricating multicomponent nanowires with different metals due to its engineering simplicity, versatility, and low-cost [ – ]. In such centimeter-scale nanowire networks, electrical connectivity is essential to allow charge flow over the whole sample sizes. The nanowire-based system overcomes the lack of reliability and reproducibility of the results obtained in metallic nanopillars and magnetic tunnel junctions [ , , , ], which can be mainly attributed to the thermal contact resistance between the nanoscale samples and the thermal baths which generate the temperature gradient. The 3D nanowire networks hold promise for flexible thermoelectric generators exhibiting extremely large and magnetically modulated thermoelectric power factor. The conventional thermoelectric modules consist of coupled n- and p-type thermoelectric materials or legs. While initial work has focused on n-type NW systems made of Co/Cu and CoNi/Cu multilayers [ , ], it was recently shown that dilute NiCr alloys are promising for the fabrication of p-type nanowire-based thermoelectric legs [ ]. In the present work, we report on experimental results obtained on other n-type thermoelectric films based on interconnected Ni, NiFe alloys, and Ni Fe /Cu multilayered NW networks. Nickel-iron is an important soft magnetic material that is widely used in magnetic data storage technologies. NiFe alloys with optimized sample compositions also exhibit large thermopower near room temperature. In addition, NiFe/Cu multilayers are well-known giant magnetoresistance (GMR) systems [ ]. The physical origin of GMR is the different conduction properties of the majority and minority spin electrons in magnetic multilayers. Through magneto-thermopower measurements and exploiting the fact that the branched nanowire architecture of these multilayer NW networks allows electrical measurements in the current perpendicular to the plane (CPP) geometry, a precise determination of spin-dependent Seebeck coefficients in permalloy (Ni Fe ) is obtained.