Coercivity anomaly in the superferromagnetic state of an ensemble of nanoparticles with oriented anisotropy

Abstract

An anomalous angular dependence of the coercive field of physically nonpercolated granular CoFeB–SiO2 films with oriented anisotropy of the granules, where the presence of the superferromagnetic phase has been established earlier, is observed experimentally at temperatures above the blocking temperature. The angular dependence of the coercive field is similar to that typically observed in the case of magnetization reversal through an inhomogeneous magnetic state of single granules in the ensemble. As the temperature becomes lower than the blocking temperature, the angular dependence changes to that typical of the magnetization reversal through a coherent rotation of granules’ magnetic moments. It is shown theoretically that the anomaly is determined by a singularity of the angular dependence of the critical field causing the absolute loss of the superferromagnetic phase stability in the ensemble of weakly interacting ferromagnetic nanoparticles.