Ferromagnetic resonance study on the influence of the electrolytic bath acidity on the magnetic anisotropy of Ni nanowires

Abstract

In this work, a ferromagnetic resonance study on the influence of the electrolytic bath acidity on the magnetic anisotropy and hardness parameter of arrays of 50 nm diameter Ni nanowires in porous alumina membranes, is reported. Fine tuning the effective magnetic anisotropy of the arrays has been achieved by progressively changing the nanowires microstructure from polycrystalline textured to single-crystalline as the electrolyte acidity is reduced. The micro-structural analysis carried out by X-ray diffraction and high resolution transmission electron microscopy has revealed that a preferred crystal texture is not a sufficient condition for the enhancement of the magnetic anisotropy. Instead, a correlation between the increase of crystallite size and quality, and the increase of both the magnetic anisotropy and hardness parameter has been shown. Ferromagnetic resonance and alternating gradient magnetometry experiments have shown that no additional magnetic contributions take place in Ni NWs grown from highly acidic electrolytes (pH 1.6), whereas a significant additional anisotropy contribution progressively appears as the electrolyte acidity is reduced (pH 5.0). Besides the careful adjustment of electrodeposition conditions, achieving a very long length for the nanowires is also a key requirement to have a precise control on the effective magnetic anisotropy energy, which can be fine tuned in the wide range from 64-167 kJ·m−3 as a result of an energy enhancement of magnetoelastic origin. Ferromagnetic resonance measurements have demonstrated that nanocomposites based on arrays of nanowires made of low cost and abundant elements like Ni exhibit hardness parameters in the range 0.47–0.75 within the semihard region, where those with κ>0.5 are suitable for permanent magnet applications. These features make arrays of low diameter Ni nanowires very appealing for their use in the development of agile microwave and spintronic devices.