Anisotropy of Assemblies of Densely Packed Ferromagnetic Nanoparticles Embedded in Carbon Nanotubes

Abstract

We report the results related to the impact of magnetic anisotropy on the properties of a new type of nanocomposite consisting of ferromagnetic nanoparticles (NP) embedded in carbon nanotubes (CNT). Samples were synthesized by chemical vapor deposition. We found that for low NP concentration, NPs are intercalated mainly inside CNTs and the extended magnetic order, up to hundreds of nanometers, presents in samples. It is shown by analyzing the correlation functions of the magnetic anisotropy axes that the extended order is not simply due to random anisotropy but is associated with the coherent magnetic anisotropy. With increasing temperature, the extended magnetic order is lost, the exchange coupling becomes stronger, but the coherent anisotropy still occurs. The magnetic coupling between NPs distant from each other for tens and hundreds of nanometers could occur via the RKKY interaction. The magnetic relaxation measurements confirmed the importance of the magnetic anisotropy at low temperatures. For the first time, we have been able to analyze the relaxation data using the temperature dependence of the magnetic anisotropy. We demonstrate further that, when each nanotube contains only one ferromagnetic NP, the agnetoelastic anisotropy plays an important role and leads to the formation of densely packed array of magnetically isolated nanoparticles.