Recently, interconnected nanowire networks have been found suitable as flexible macroscopic spin caloritronic devices. The 3D nanowire networks are fabricated by direct electrodeposition in track-etched polymer templates with crossed nano-channels. This technique allows the fabrication of crossed nanowires consisting of both homogeneous ferromagnetic metals and multilayer stack with successive layers of ferromagnetic and non-magnetic metals, with controlled morphology and material composition. The networks exhibit extremely high, magnetically modulated thermoelectric power factors. Moreover, large spin-dependent Seebeck coefficients were directly extracted from experimental measurements on multilayer nanowire networks. This work provides a simple and cost-effective way to fabricate large-scale flexible and shapeable thermoelectric devices exploiting the spin degree of freedom.

NiFe alloy and NiFe/Cu multilayered nanowire (NW) networks were grown using a template-assisted electrochemical synthesis method. The NiFe alloy NW networks exhibit large thermopower, which is largely preserved in the current perpendicular-to-plane geometry of the multilayered NW structure. Giant magneto-thermopower (MTP) effects have been demonstrated in multilayered NiFe/Cu NWs with a value of 25% at 300 K and reaching 60% around 100 K. A large spin-dependent Seebeck coefficient of –12.3 μV/K was obtained at room temperature. The large MTP effects demonstrate a magnetic approach to control thermoelectric properties of flexible devices based on NW networks.

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Magnetoresistance and thermopower of crossed NiCr and CoCr nanowire networks have been measured as a function of temperature and chromium content in dilute alloys. At low temperatures, it is found that the impurity effect leads to negative anisotropic magnetoresistance, an observation that even persists until room temperature in diluted CoCr alloy nanowires. The addition of a small amount of Cr in nickel nanowires also abruptly reverses the sign of the thermopower from −20 μV/K for pure Ni up to +18 μV/K for the dilute alloys, implying the switching from n- to p-type conduction. These results are consistent with pronounced changes in the density of states for the majority spin electrons. The high room-temperature power factors of these magnetic nanowire networks (in the range of 1–10 mW/K2 m) provide interesting perspectives for designing n- and p-type legs for flexible spin thermoelectric devices.

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