Investigating magnetic and optical properties of hybrid magnetic-noble metallic nanowires synthesized by electrodeposition

The structural and magnetic properties of Ni nanowires (NWs) electrodeposited in anodic aluminum oxide (AAO) membranes with diameters in the range from 35 to 75 nm and an interpore distance of 105 nm have been investigated. The expected interplay between the shape anisotropy and the dipole-dipole interactions between the NWs of different aspect ratios and packing density is analyzed. The growth rate, crystallinity, and magnetic properties of the NWs with diameter 45 nm are compared as a function of different electrodeposition potentials (-0.8V to -1.5V vs. the Ag/AgCl) to optimize the deposition process. It is observed that the average growth rate of the synthesis increases nearly linearly by increasing the deposition potential. The synthesized NWs with different deposition potentials are uniform and mostly polycrystalline but with larger grain sizes up to 50 nm for nanowires synthesized at -1 V and -1.1 V. The magnetic properties (coercivity and remanence) of the NWs (fcc structure) with the same aspect ratio are independent on the deposition potentials. It is found that for NWs with 40 nm diameter, the shape anisotropy dominates the magnetic properties. Annealed Ni NWs (650◦C under Ar atmosphere for 24h) show a smaller coercivity than as-prepared samples which suggests a lower defect density inside the nanowires. The FMR measurements on two samples of Ni NWs with different diameters (35 nm and 45 nm) but with the same length (1.6 µm), and the interpore distance of 105 nm reveal the effect of the magnetic dipole-dipole interactions between the nanowires on the effective anisotropy field. The effective field of 35 nm diameter, with the dipolar field of 68.5 mT, is µ0Heff=215 mT, while for 45 nm diameter, with the dipolar field of 109.5 mT, is µ0Heff=160 mT. In a second study, hybrid nanowires consisting of Ni and Ag segments have been synthesized and characterized by structural, magnetic, and optical techniques. The electrodeposition process was optimized to create Ni-Ag NWs with sharp interfaces. It is demonstrated that the localized surface plasmon resonance transverse mode of the Ag segment (75(±5) nm diameter and the length of 210(±20) nm) changes by 12 nm with the wavelength of 438 nm due to the contact to ferromagnetic Ni. It is also revealed that higher order transverse plasmon modes of the Ag segment can clearly be identified as a distinct peak at 366 nm in the hybrid of Ag/Ni NWs rather than for individual Ag NWs that appeared as a shoulder at 375 nm. In addition, one high intensity plasmon peak at 313 nm related to Ni segments is observed. In a third study, a new approach for the design, synthesis and optimization of complex Ni capped Ag nanohybrids, was investigated. This technique is carried out using a multi-step process combining electrochemical deposition and partial removal of the membrane

Magnetic hardening of Fe30Co70nanowires

3d transition metal-based magnetic nanowires (NWs) are currently considered as potential candidates for alternative rare-earth-free alloys as novel permanent magnets. Here, we report on the magnetic hardening of FeConanowires in anodic aluminium oxide templates with diameters of 20 nm and 40 nm (length 6 μm and 7.5 μm, respectively) by means of magnetic pinning at the tips of the NWs. We observe that a 3-4 nm naturally formed ferrimagnetic FeCo oxide layer covering the tip of the FeCo NW increases the coercive field by 20%, indicating that domain wall nucleation starts at the tip of the magnetic NW. Ferromagnetic resonance (FMR) measurements were used to quantify the magnetic uniaxial anisotropy energy of the samples. Micromagnetic simulations support our experimental findings, showing that the increase of the coercive field can be achieved by controlling domain wall nucleation using magnetic materials with antiferromagnetic exchange coupling, i.e. antiferromagnets or ferrimagnets, as a capping layer at the nanowire tips.We acknowledge funding from the European Community's Seventh Framework Programme (FP7-NMP) under grant agreement no. 280670 (REFREEPERMAG)