Magnetic Control of Flexible Thermoelectric Devices Based on Macroscopic 3D Interconnected Nanowire Networks

Abstract

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 ...

Magnetic Nanowires

“…Despite their complex nano-architecture, interconnected NW networks made of Co/Cu, CoNi/Cu and Py/Cu multilayers were found to exhibit very large CPP-GMR responses reaching values of 40% at RT and 86% at T = 15 K for CoNi/Cu [64]. The same features were observed on the Co/Cu and Py/Cu CNW samples [65,67], as shown in Figure 10a-c.…”

“…Next, the template-assisted electrodeposition method was used for the synthesis of well-defined 3D architecture consisting of crossed nanowire (CNW) and nanotube (CNT) networks [60][61][62][63]66]. In addition, magnetic and non-magnetic layers were electrochemically stacked to make interconnected multilayered nanowires [64,65,67]. Figure 4 illustrates the various 3D nanofiber architectures which have so far been investigated.…”

“…In addition, since the geometrical features of the crossed core(Cu)/shell(Ni) nanocables can be carefully modified via the control over the reduction or deposition potential, Ni CNT networks with different wall thickness in the range 10-50 nm were obtained after carrying out the dealloying step of the Cu core [63]. In addition, CNWs with multilayer structure of FM/Cu, with FM = Co, CoNi and NiFe alloys were fabricated in the host 3D porous templates [64,65,67]. Interestingly, this fabrication method appears as a very convenient approach for large scale production of current-perpendicular to-plane (CPP) giant magnetoresistance (GMR) films based on CNW networks, as shown in Sections 3.3 and 3.5.…”

“…Another difficulty is the lack of experimental approaches for reliable measurements of spin caloritronic material parameters. It has recently been shown that arrays of interconnected nanowires embedded in porous polymer films provide a simple and cost-effective way to fabricate flexible spin caloritronic devices at the macroscopic scale [64,65].…”

“…The research activity at UCLouvain on magnetic NWs were also motivated by related applications in microwave devices [46][47][48][49][50][51][52][53][54] and multiferroic nanocomposites [55][56][57][58][59]. Recently, we demonstrated the suitability of three-dimensional (3D) nanoporous polymer templates to form dense interconnected magnetic NW networks with tunable geometrical parameters in terms of nanowire size, density, and orientation opens up the possibility for a controlled synthesis of a large variety of complex 3D networks of high aspect-ratio nanostructures with different geometries and materials [60][61][62][63][64][65][66][67]. Their unique architectures and high degree of nanowire interconnectivity facilitate the ability to perform magneto-transport measurements and to investigate the interplay between electronic and thermoelectric transport and magnetic properties.…”