Ferromagnetism of Single Vertical Cobalt Nanotubes Grown by Focused Electron Beam Induced Deposition

Abstract

Three-dimensional (3D) nanomagnets provide a great potential to be implemented in future spintronic devices for high-density magnetic memories, nano-sensing or logic. Among these appealing architectures, ferromagnetic nanotubes (NTs) are currently being investigated for their fast and low-power domain wall conduit properties [1].The development of novel 3D nanostructures typically requires complex synthetic methods. Moving forward in this challenging task, Focused Electron Beam Induced Deposition (FEBID) has been explored for the growth of 3D ferromagnetic NTs. This unique one-step nanolithography technique, based on the decomposition of organometallic precursor gas molecules in the vicinity of a substrate [2,3], has demonstrated great versatility in terms of shape, composition and magnetic properties [4]. Following the strategy developed for the fabrication of core-shell nanowires [5], the growth of 3D Co NTs by FEBID has been performed for the first time [6]. The heterostructured design is composed by a vertical Pt-C nanowire (≤100 nm in diameter) acting as a core, and a Co coating forming the shell with a thickness down to ~11 nm. These are exotic magnetic nanostructures in which unconventional domain walls are formed, presenting high domain wall velocities without Walker breakdown phenomenon and where great stability during propagation has been predicted.Furthermore, Transmission Electron Microscopy (TEM) experiments reveal that these NTs present a nanocrystalline structure and a metallic content of ~70 at. %. In addition, a coercivity of ~16 mT has been obtained by Magneto-Optical Kerr Effect magnetometry, and magnetic characterization performed by Electron Holography evidences the ferromagnetic behaviour, estimating a remanent magnetic induction up to 1.3 T and detecting complex head-to-head magnetic domain walls. The application of magnetic fields inside the TEM has allowed the pinning and imaging of domain walls in these nanostructures. Finally, micromagnetic simulations have been performed for a deeper understanding of the magnetization dynamics [6]. **