Towards hard-magnetic behavior of CoFe2O4 nanoparticles: a detailed study of crystalline and electronic structures, and magnetic properties

Abstract

We have used the coprecipitation and mechanical-milling methods to fabricate CoFe₂O₄ nanoparticles with an average crystallite size (d) varying from 81 to ∼12 nm when changing the milling time (t_m) up to 180 min. X-ray diffraction and Raman-scattering studies have proved the samples crystalizing in the spinel structure. Both the lattice constant and residual strain tend to increase when t_m(d) increases (decreases). The analysis of magnetization data has revealed a change in the coercivity (H_c) towards the hard-magnetic properties. Specifically, the maximum H_c is about 2.2 kOe when t_m = 10 min corresponding to d ≈ 29 nm; beyond this t_m(d) value, H_c gradually decreases. Meanwhile, the increase of t_m always reduces the saturation magnetization (M_s) from ∼69 emu g⁻¹ for t_m = 0 to 35 emu g⁻¹ for t_m = 180 min. The results collected as analyzing X-ray absorption data have indicated a mixed valence state of Fe^2+,3+ and Co²⁺ ions. We think that the migration and redistribution of these cations between the tetrahedral and octahedral sites together with lattice distortions and defects induced by the milling process have impacted the magnetic properties of the CoFe₂O₄ nanoparticles.