Anisotropy of Assemblies of Densely Packed Co-Alloy Nanoparticles Embedded in Carbon Nanotubes

Abstract

We report the magnetic properties of an array of binary CoFe, CoNi and CoPt nanoparticles (NPs) embedded inside vertically oriented carbon nanotubes (CNTs). Samples were synthesized by chemical vapor deposition activated by current discharge plasma and hot filament. Assemblies of Co-based catalytic NPs have been preliminary formed on SiO2/Si substrates by sputtering of ultrathin films followed by reduction in H2/NH3 mixture. As a result, each CNT contained only one ferromagnet top-located NP. Magnetic hysteresis loops for both parallel and perpendicular magnetic field orientations revealed anisotropy of magnetic properties. For all samples the easy axis of magnetization were oriented along the CNT axis. Using the obtained experimental data and the random anisotropy model, the magnetic parameters like the effective anisotropy constant, the contribution of dipole interaction, shape, magnetocrystalline and magnetoelastic anisotropy were estimated. It is shown that the latter contribution of anisotropy is decisive. From the obtained magnetoelasticity the stresses in the CNT array with studied NPs were determined. Finally, the magnetization distribution in CoFe, CoNi and CoPt NPs was simulated considering the magnetoelastic contribution.