Influence of Co²⁺ on the structural and magnetic properties of substituted magnetites obtained by the coprecipitation method

Abstract

In this paper we report the effect of divalent cobalt on the structural and magnetic properties of substituted magnetites, Fe _3−x Co _x O ₄, with γ=Co ²⁺/Fe = 0, 5, 10, 15, 20 and 30 % wt, synthesized by the coprecipitation method. The samples were characterized by Atomic Absorption Spectroscopy, X-ray Diffraction, room temperature Mössbauer Spectroscopy and Vibrating Sample Magnetometry. The effect of Co ²⁺ was found to depend strongly of the concentration employed in the synthesis process. For γ≤15 % the Co ²⁺ promotes the formation of particles more crystalline and with higher saturation magnetization, remanence and coercivity than those obtained in absence of this cation. A sequential increasing of the lattice parameter is observed, as well as a reduction in the hyperfine magnetic field of the Fe ^2.5+sub spectrum, while the hyperfine magnetic field of the Fe ³⁺sub spectrum keeps almost constant. For γ=20 % and 30 % the crystallinity of the samples decreases, particle size distribution effects are evidenced and the saturation magnetization decreases drastically. The results suggest that for low Co ²⁺ contents the substitution of Fe ³⁺by Co ²⁺ at octahedral sites of the inverse spinel system is the dominant effect, while for the highest concentrations used the substitution of Fe ²⁺ by Co ²⁺ and the increasing of the particle size distribution are the dominant effects.