# Magnetic Nanowires
## 3 Results
### Spin Caloritronics in Nanowire Networks
The same conclusion is drawn when the FM layer is formed from alloys such as CoNi and NiFe because of highly contrasting thermal conductivity values between the alloy and Cu. In contrast, the Seebeck coefficient of a FM/Cu multilayer stack in the direction parallel to the layers is given by  
\[S_{\parallel}=\frac{S_{\text{Cu}}\rho_{\text{FM}}+\lambda S_{\text{FM}}\rho_{ \text{Cu}}}{\lambda\rho_{\text{Cu}}+\rho_{\text{FM}}}, \tag{8}\]  
showing that large thermopower can be obtained only in case the thickness ratio \(\lambda\) is very large. The contrasting behavior between layer parallel and perpendicular directions is illustrated in Figure 13a for Co/Cu multilayers using Equations (7) and (8) and the literature resistivity and thermopower values at RT. The device configuration to measure the Seebeck coefficient and the magnetothermoelectric effect is schematically represented in Figure 13b. In such centimeter-scale nanowire networks, electrical connectivity is essential to allow charge and heat transfer over the whole sample sizes. Heat flow is generated by a resistive element and a thermoelectric voltage \(\Delta V\) is created by the temperature difference \(\Delta T\) between the two metallic electrodes that is measured by a thermocouple (see [64] for details). For conducting thermoelectric and magnetoresistance measurements, the cathode was locally removed by plasma etching to create a two-probe design, with the flow of current restricted along the nanowire segments, thus perpendicularly to the plane of the layers in FM/Cu CNWs. As shown in Figure 13c,d, the resistance and thermopower of the low-packing density (\(P=\) 3%) Co/Cu NW sample (80 nm in diameter) show the same magnetic field dependencies and similar relative changes of \(\sim\)25% at \(H=\) 8 kOe at RT. The samples are nearly magnetically isotropic, as observed from the magneto-transport curves obtained with the applied magnetic field along the OOP and IP directions of the NW network films (see Figure 13c,d). This behavior corresponds to the one expected considering the CNW architecture and magneto-static arguments when using similar magnetic and non-magnetic layer thicknesses [32]. In the following, only the measurements obtained in the plane of the CNW network films are reported. Recent estimations of the thermoelectric power factor at RT for both Co/Cu and CoNi/Cu CNW networks lead to values around 5 mW/K\({}^{2}\)m in the saturated state [64; 65], which are even larger than the PF of the widely used thermoelectric material, bismuth telluride (in the range 1-5 mW/K\({}^{2}\)m) [103]. In addition, the PF values obtained for FM/Cu NW networks embedded in polymer membranes are at least one order of magnitude larger than those of flexible thermoelectric films based on optimized conducting polymers, namely PEDOT-Tos films [104]. Very large magneto-power factor, reaching 100% at RT, was obtained for the Co/Cu CNWs. On the other hand, the efficiency of a material's thermoelectric energy conversion is determined by its figure of merit \(ZT=S^{2}\sigma T/\kappa\). Due to the very low thermal conductivity of polycarbonate (\(\kappa=\) 0.2 W/m\(\cdot\)K at RT), the contribution of the polymer matrix to heat transport is much smaller than that of the metallic nanowire network. In this case, the figure of merit is reduced to \(ZT=S^{2}/L_{0}\), thus leading to \(ZT\approx\) 1.6 \(\times\) 10\({}^{-2}\) (\(ZT\approx\) 1.9 \(\times\) 10\({}^{-2}\)) for the Co/Cu (CoNi/Cu) NW network sample at RT [64; 65]. Although the figure of merit is more than one order of magnitude smaller than those of state-of-the-art thermoelectric materials (\(ZT\approx\) 1 in BiTe alloys), it is comparable to those of thermocouple alloys (\(ZT\approx\) 6 \(\times\) 10\({}^{-2}\) and \(ZT\approx\) 1.4 \(\times\) 10\({}^{-2}\) in constantan and chromel, respectively) and can be used in applications for devices with low energy requirements when the supply of heat essentially is free as with waste heat.