Coercivity analysis of cubic and tetragonal (Cu,Co) ferrite particles within the global model

Abstract

We analyzed the coercivity of both cubic and tetragonally distorted Co x Cu1−x Fe2O4 particles (x = 0.2 and 0.1, respectively) using the global model. According to magnetic viscosity measurements, the size of the activation volumes v a increases from around 2000 nm3 (v a 1/3 ≈ 13 nm) at 10 K for both samples, to close to 175 000 nm3 for the cubic sample and 5400 nm3 for the tetragonally distorted sample at 300 K. The microstructure-related phenomenological parameters obtained from the global model analysis are α= 0.29 and N eff = −0.01 and α= 0.39 and N eff = −0.24 for the cubic and tetragonal samples, respectively. High-resolution transmission electron microscopy images of the tetragonal sample showed that nanograins are connected through a thin amorphous layer (∼1 nm), which could account for the intergranular exchange interactions indicated by the negative value of N eff as well as values of M r/M s > 0.5 for such isotropic samples. Low temperature discrepancies are tentatively attributed to magnetic reversal being governed by a local heating process following thermal activation.