Validity of the Neel-Arrhenius model for highly anisotropic CoxFe3-xO4 nanoparticles

摘要

We report a systematic study on the structural and magnetic properties of CoxFe3-xO4 magnetic nanoparticles with sizes between 5 and 25 nm, prepared by thermal decomposition of Fe(acac)(3) and Co(acac)(2). The large magneto-crystalline anisotropy of the synthesized particles resulted in high blocking temperatures (42K = 5 nm) revealed the existence of a magnetically hard, spin-disordered surface. The thermal dependence of static and dynamic magnetic properties of the whole series of samples could be explained within the Neel-Arrhenius relaxation framework by including the thermal dependence of the magnetocrystalline anisotropy constant K-1(T), without the need of ad-hoc corrections. This approach, using the empirical Brukhatov-Kirensky relation, provided K-1(0) values very similar to the bulk material from either static or dynamic magnetic measurements, as well as realistic values for the response times (tau(0) approximate to 10(-10) s). Deviations from the bulk anisotropy values found for the smallest particles could be qualitatively explained based on Zener's relation between K-1(T) and M(T). (C) 2015 AIP Publishing LLC.