Magneto-structural properties of rhombohedral Ni and Ni–B nanotubes deposited by electroless-plating in track-etched mica templates

Abstract

The intriguing properties of three-dimensional ferromagnetic nanotubes have attracted considerable attention, which is further boosted by their manifest use as building blocks for nanoarchitectures, magnetic devices, and potential applications in spin-wave spintronics, nanoelectronics, sensing, and biomedicine. Despite their peculiar magnetic properties and technological relevance, scarce studies have been reported in ferromagnetic nanotubes, and usually remain restricted to cylindrical morphologies. However, only a few studies have addressed alternative three-dimensional geometries, and most of them are focused on nanotubes with regular cross-sections, while the capabilities and additional degrees of freedom of three-dimensional nanotubes with irregular cross-sections remain still unexplored. Here, we present a synthetic platform for creating novel ferromagnetic nanotubes of diamond-shaped cross-section and a high aspect ratio, which is based on applying an electroless-plating deposition method to ion-track etched muscovite mica templates, providing a precise control over their geometry. Nanotubes with a wall thickness of 45 ± 5 nm were plated using both nanocrystalline nickel and semi-amorphous nickel–boron alloys. Whereas the Ni–B deposit exhibited reduced saturation magnetization and acted as an ensemble of weakly interacting, quasi-superparamagnetic nanoparticles, pure nickel nanotubes showed an archetypal ferromagnetic behavior dominated by shape anisotropy, whose defined structure is reflected in three distinct magnetic directions: an easy magnetization axis lying parallel to the nanotubes, and two perpendicular, increasingly hard magnetization directions connecting the opposing corners of the rhomboidal nanotubes. As such, the rhombohedral ferromagnetic nanotubes offer additional degrees of freedom for designing three-dimensional nanoarchitectures with controlled specific tailored-made configurational magnetic anisotropy.