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Effects of structural inhomogeneity in CoNi nanowires on its magnetic configurations

par PREVOTS Evelyne, PREVOTS Evelyne - publié le , mis à jour le

Depending its Co content, magnetic CoNi nanowires may display two types of crystal phases : a face-centered cubic (fcc) and a hexagonal close packed (hcp) structure. When the magnetocrystalline easy axis of the hcp phase is oriented perpendicular to the nanowire axis, transversally oriented magnetic domains appear with an exotic chain of vortex-like magnetic configuration. Homogeneous magnetic domains with magnetization running parallel to the wire axis are obtained in the fcc areas.

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Image MET d’un nanofil de CoNi présentant des grains cfc et hcp, (repérés en fausses couleurs bleu et rouge, respectivement), et image de la phase magnétique obtenue par holographie électronique. La configuration magnétique de la région hcp consiste en une chaîne de vortex, confirmée par des simulations micromagnétiques.

Due to magnetostatic energy, the magnetization in magnetic nanowires (NWs) often aligns along the NWs axis. However, in hexagonal close packed NWs whose “c” direction is oriented perpendicular to the NW axis, the magnetocrystalline anisotropy may be strong enough to compete with the large shape anisotropy. In this study, we have found a chain of transversal vortex-like magnetic structures in a hexagonal close packed grain of a CoNi nanowire whose close-packed direction is oriented perpendicular to the nanowire axis.

The magnetic configurations of cylindrical Co-rich CoNi nanowires have been quantitatively studied at the nanoscale by electron holography and correlated to structural and chemical properties locally at the same positions of the sample. The nanowires display a mixture of crystal grains of both face-centered cubic (fcc) and hexagonal close packed (hcp) crystal structures, with grain boundaries oriented parallel to the nanowire axis direction. Study by electron holography reveal the existence of a complex magnetic configuration that is characterized by two very different types of magnetic configurations coexisting within a single nanowire : a chain of periodical vortices separating small transverse domains in regions with mainly hcp grains with easy axis orientation perpendicular to the nanowire axis, and a more longitudinal magnetic configuration parallel to the nanowire axis in regions fcc grains. The experimental results have been compared to micromagnetic simulations, which has confirmed that the vortex chain is formed within a hcp phase of the Co-rich CoNi nanowire.

Deviations in the local chemical composition are at the origin of these magnetic configuration changes. Variations in the chemical composition lead to a change of the crystalline orientation and/or crystal structure, and give rise to a change in magnetic anisotropies.



Exotic Transverse-Vortex Magnetic Configurations in CoNi Nanowires
ACS Nano, 2019





Mia Andersen
Christophe Gatel 
Etienne Snoeck