Spin caloritronics has recently emerged from the combination of spintronics and thermoelectricity. Here, we show that flexible, macroscopic spin caloritronic devices based on large-area interconnected magnetic nanowire networks can be used to enable controlled Peltier cooling of macroscopic electronic components with an external magnetic field. We experimentally demonstrate that three-dimensional CoNi/Cu multilayered nanowire networks exhibit an extremely high, magnetically modulated thermoelectric power factor up to 7.5 mW/K2m and large spin-dependent Seebeck and Peltier coefficients of −11.5 μV/K and −3.45 mV at room temperature, respectively. Our investigation reveals the possibility of performing efficient magnetic control of heat flux for thermal management of electronic devices and constitutes a simple and cost-effective pathway for fabrication of large-scale flexible and shapeable thermoelectric coolers exploiting the spin degree of freedom.

of reproducibility in the experiments has limited the application as heat harvesters. Although there have been some initial studies on the thermoelectric analogues of giant magnetoresistance (GMR) in magnetic multilayers with current inplane configuration, [9][10][11][12] the effects of interfaces make the interpretation of the results more delicate than in the simpler current-perpendicular-to-plane (CPP) configuration. [13] Indeed, in the limit of no-spin relaxation, most of the CPP-GMR data can be understood using a simple two-current series-resistor model, in which the resistance of layers and interfaces simply add and where "up" and "down" charge carriers are propagating independently in two spin channels with large spin asymmetries of the electron's scattering. [14,15] Similarly, the spin-dependent thermoelectric effects exploit the fact that the Seebeck coefficients for spin-up and spin-down electrons are also different. The diffusion thermopower arises from the diffusion of charge carriers opposite to the temperature gradient. It is related to the energy dependent conductivity of the material σ(ε ) by Mott's formulawith L 0 = 2.44 · 10 −8 V 2 K −2 the Lorenz number and e the electron charge (positive). According to Einstein's relation for a metal or alloy with isotropic properties, the conductivity is proportional to the density of states N(ε ) and to the scattering time τ(ε ), where both terms in Equation (1) are to be evaluated at the Fermi level ϵ F . Because of the pronounced structure of the d-band and the high energy derivative of the density of states at the Fermi level in 3D ferromagnetic metals, large diffusion thermopowers are obtained (e.g., S ≈ −30 µV K −1 in cobalt at room temperature (RT)). Moreover, significantly different Seebeck coefficients for spin-up and spin-down electrons, S ↑ and S ↓ , are expected because the d-band is exchange split in these ferromagnets, as suggested from previous works performed on dilute magnetic alloys. [16,17] To date, most of the investigations of thermoelectric transport in CPP-GMR systems were performed on lithographically defined nanopillars, single nanowire (NW), and parallel Spin-related effects in thermoelectricity can be used to design more efficient refrigerators and offer promising applications for the harvesting of thermal energy. The key challenge is to design structural and compositional magnetic material systems with sufficiently high efficiency and power output for transforming thermal energy into electric energy and vice versa. The fabrication of large-area 3D interconnected Co/Cu nanowire networks is demonstrated, thereby enabling the controlled Peltier cooling of macroscopic electronic components with an external magnetic field. The flexible, macroscopic devices overcome the inherent limitations of nanoscale magnetic structures that are caused by insufficient power generation capability limiting the heat management applications. From properly designed experiments, large spin-dependent Seebeck and Peltier coefficients of −9.4 µV K −1 and −2.8 ...

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.

Track-etched polymer membranes with crossed nanochannels have been revealed to be most suitable as templates to produce large surface area and mechanically stable 3D interconnected nanowire (NW) networks by electrodeposition. Geometrically controlled NW superstructures made of NiCo ferromagnetic alloys exhibit appealing magnetoresistive properties. The combination of exact alloy compositions with the spatial arrangement of NWs in the 3D network is decisive to obtain specific magnetic and magneto-transport behavior. A proposed simple model based on topological aspects of the 3D NW networks is used to accurately determine the anisotropic magnetoresistance ratios. Despite of their complex topology, the microstructure of Co-rich NiCo NW networks display mixed fcc-hcp phases with the c-axis of the hcp phase oriented perpendicular to their axis. These interconnected NW networks have high potential as reliable and stable magnetic field sensors.