Enhanced magnetic properties of Bi-substituted cobalt ferrites

In this paper we present a magnetic study of Co_1-xBi_xFe₂O₄ nanoparticles obtained by applying magnetic fields up to 14 T and for temperatures in the range of 5 to 340 K. Hysteresis loops yield a saturation magnetization (M_s), coercive field (H_c), and remanent magnetization (M_r) that vary significantly with temperature and bismuth content. The T-dependence of M_s obtained for H = 5 T presents a maximum at 150 K and a minimum at 25 K that are also dependent on the value of x. However, for H = 14 T, this anomalous behavior disappears and the magnetization smoothly approaches saturation down to 5 K. The magnetic cubic anisotropy constant for different Bi contents, determined by a “law of approach” to saturation, was found to be smaller than those values for pure cobalt ferrite nanoparticles and strongly dependent on temperature. A discussion on the implications of the anomalous behavior in the determination of the anisotropy constant in these sample materials is also presented.