Spin-dependent Seebeck effect in 3D interconnected nanowire network

Track-etched polymer membranes with crossed nanochannels have been successful templates for the growth of three-dimensional interconnected magnetic nanowire networks through metals electrodeposition inside the pores. This reliable technique has enabled to produce unique nanowire structures while keeping an accurate control over the morphology, the material composition as well as the density of nanowires. This innovative interconnected architecture combines mechanical stability and electrical conductivity within the self-supported sample together with flexibility and thermal stability provided by the polycarbonate template. The electrons flow along the nanowires axis while the global transport of heat and charges takes place in the centimeter scale film plane. From this configuration results the possibility to perform suitable macroscopic electrical and thermal measurements after removing a part of the cathode by ions sputtering to create a two-probe design. The successive ferromagnetic (NiFe) and non-magnetic (Cu) layers with the alloy composition ranging from 40% to 93% of Ni allow to switch from a natural antiparallel arrangement of the magnetic moments to a parallel alignment when applying an external magnetic field. These two arrangements have shown different behaviors for the spin-up and spin-down electrons resulting in an alteration of the transport properties. An appropriate magnetic field is therefore at the origin of the relatively tunable electrical resistance and Seebeck coefficient characterizing the thermoelectric power. Results have demonstrated a giant magnetoresistance of 20% as well as a magneto-thermopower of 26% at room temperature for Ni80Fe20/Cu, these values have been drastically outperformed when decreasing the temperature reaching both 60% at respectively 15 K and 70 K. Moreover, properly designed experiments allow to extract large spin-dependent Seebeck coefficients exhibiting a gap of -13.57 µV/K and -17.18 µV/K at temperatures of 300 K and 320 K for Ni93Fe7/Cu. In addition, this switch of magnetic state is very sensitive since it only requires an external magnetic field of around 2 kOe. Theory suggests that this difference could be even larger for a further Ni enrichment giving rise to exciting perspectives. All these benefits make these samples good candidates for light and flexible spintronics, thermoelectric, and spin caloritronics systems with large magnetic control over the network transport properties. This technology is then promised to a brilliant future with advanced applications such as magnetic modulated active cooling devices embedded into flexible films. Keywords: Nanostructures, 3D nanowire networks, Nickel-iron alloys, Giant magnetoresistance, Spin-dependent Seebeck coefficient