Multi-domain structures in spheroidal Co nanoparticles

Abstract

Multi-domain structures in spheroidal co nanoparticles n. A. Usov 1,2 ✉ & M. S. nesmeyanov 2 the structure of multi-domain micromagnetic states in hcp cobalt nanoparticles of spheroidal shape has been studied using numerical simulation in the range of diameters 20-200 nm. The single-domain diameters of the particles are determined depending on their aspect ratio. the complicated vortex structure of domain walls for two-and three-domain micromagnetic configurations is investigated. it has been shown that three domain states are actually strongly deformed two vortex states. in hcp cobalt particles of sufficiently large sizes two types of three-domain micromagnetic states with close total energies have been obtained. They differ in different magnetization directions of the exchange cores of the vortex domain walls. The remanent magnetization of particles has been calculated for twoand three-domain micromagnetic states. the single-domain diameters of fcc cobalt nanoparticles with cubic type of magnetic anisotropy were also calculated. Cobalt nanoparticles attract substantial interest due to their technological significance 1. They were also the first magnetic nanoparticles extensively studied theoretically 2-5. However, the theory of small magnetic particles developed by Kittel 2 , Stoner and Wohlfarth 3 , Brown 4 , and Aharoni 5 is only capable to describe the magnetic properties of single-domain nanoparticles. Meanwhile, magnetic nanoparticles of submicron sizes are often found in practice. For example, the assemblies of magnetic nanoparticles with a wide size distribution are frequently obtained in chemical synthesis 6 , so that the sizes of some particles may exceed the corresponding single domain diameter. Particles of sufficiently large sizes can arise during prolonged annealing of thin-film samples 7 , they can be created as a result of implantation process 8 , etc. It is worth noting that the magnetic properties of submicron-sized nanoparticles play an important role in paleo-magnetism 9. The magnetic properties of nanoparticles in inhomogeneous micromagnetic states differ significantly from those of single-domain ones 1,4,5. However, the characteristics of nanoparticles of sufficiently large sizes are still poorly investigated. In particular, the dependence of the single-domain diameter of a spheroidal cobalt nanoparticle on its aspect ratio is unknown. The upper and lower analytical estimates for the single-domain diameter of magnetic nanoparticle given by Brown 10 are not always close enough. Therefore, in most cases a detailed numerical simulation is necessary to determine the single-domain diameter of particles of various magnetic materials. At present, domain structures in submicron cobalt nanoparticles and cobalt nanowires are experimentally studied using magnetic force microscopy 11,12. In addition, modern electronic holography technique 13-17 allows to study experimentally inhomogeneous micromagnetic distributions in magnetic nanoparticles and nanowires. This revives interest in detailed micromagneti...

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