# Magnetic Nanowires
## 4 Discussion  
The research activity developed at UCLouvain over the last 25 years on magnetic nanowires opens up the possibility for a controlled template-assisted synthesis of complex nanowire-based architectures with excellent control over geometrical features, morphology and chemical composition, leading to tunable magnetic, magneto-transport and thermoelectric properties. Due to their relative ease of fabrication, magnetic nanowires are promising materials for a variety of applications, ranging from magnetic, high frequency and electronic devices to sensing and bio-applications. On the road towards the size reduction of microwave devices, ferromagnetic nanowires embedded into porous templates have also proven to be an interesting alternative route to ferrite based materials. The main advantages of magnetic nanowired substrates compared to classical ferrites are a higher operation frequency and a microwave absorption without application of an external magnetic field as these NWs behave as strong uniaxial nanomagnets. In contrast, standard ferrite devices such as non-reciprocal devices need to be biased by a magnetic field provided by a permanent magnet to operate. In close collaboration with Prof. [PERSON] at UCLouvain, we have successfully prepared a variety of FM nanowire based substrates and demonstrated the ability to build planar reciprocal and non-reciprocal devices for very high frequencies, such as phase shifters, isolators or circulator [46; 47; 48; 49; 50; 51; 52; 53; 54] (see Figure 17). More recently, we also demonstrated the operation of fully integrated and miniaturized waveguides based on ferromagnetic nanowires used as microwave isolators. Nanowires of various heights were selectively grown at dedicated areas into nanoporous alumina template in order to create the shielding walls of a Substrate Integrated Waveguide (SIW) topology and the ferromagnetic slab supporting a circularly polarized nonreciprocal propagation [52]. Tremendous advantages can be expected in terms of ease of integration and power consumption. Spin-valve nanopillars such as Co/Cu/Co and NiFe/Cu/NiFe are also ideal devices to achieve high injection current densities and are very promising for applications as spin transfer nano-oscillators (STNOs). The sustained gyrotropic mode of vortex magnetization in these segmented nanopillars provides improved spectral quality microwave signals without any static bias magnetic field. Lithography techniques allow you to connect a single or a desired number of spin-torque vortex oscillators from the dense forest-like structure of nanowires embedded in nanoporous alumina templates integrated onto Si wafers, with better compatibility to the standard CMOS processes. The prospect of obtaining the synchronization of a large number of STNOs, either along the same wire or in neighboring wires, is interesting in order to achieve the required power for applications. Magnetic nanowires embedded in a polymer membrane could also be used to develop various light weight and flexible magnetic sensors, spintronic devices and permanent magnets. The nanowires can be easily released into suspension by dissolving the membrane. Suspensions of magnetic nanowires  
Figure 16: **(a)** schematics of a set-up for direct observation of Peltier and magneto-Peltier effects; a Cernox thermometer is used to monitor the temperature changes at the junction between the NWs and the electrode; **(b)** measured temperature changes \(\Delta T_{H}\) at the Peltier junction in the Co/Cu CNWs (105 nm in diameter and packing density of 22%) during the magnetic field sweep for a DC current of \(-\)50 mA and +50 mA. The magneto-Peltier effect leads to heating and cooling at the saturation field depending on current flow direction (from [65]). figure adapted from ref. [65] with permission of the author. offer diverse opportunities for applications in biomedicine and biotechnology [114; 115; 116; 117; 118; 119]. Single wires could also be used as probes for high resolution magnetic force microscopy.