Towards hard-magnetic behavior of CoFe 2 O 4 nanoparticles: a detailed study of crystalline and electronic structures, and magnetic properties.

We have used the coprecipitation and mechanical-milling methods to fabricate CoFe ₂ O ₄ 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 Co ²⁺ 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 CoFe ₂ O ₄ nanoparticles.
Competing Interests: The authors declare no competing financial interest.
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