Effect of doping rare earths on magnetostriction characteristics of CoFe2O4 prepared from spent Li-ion batteries

Highlights: • The cobalt ferrites were prepared by the sol–gel auto-combustion route. • Rare earth substitution cobalt ferrites were synthesized using used batteries. • The effect to magnetic properties was discussed. • Rare earth substitution improved magnetostriction characteristics at lower magnetic field strength. - Abstract: Recovering spent Li-ion batteries is beneficial to the economy and environment. Therefore, this study synthesized nanoparticles of cobalt ferrite doped with different rare earth ions (Nd, Ce, and Pr) by a sol-gel auto-combustion method using spent Li-ion batteries. The effect of the different doping elements on grain sizes, structure, magnetic and magnetostrictive properties, and strain derivative were confirmed by X-ray diffraction, scanning election microscopy, vibrating sample magnetometer, and a magnetostrictive coefficient measuring system. Substitution of a small amount of Fe³⁺ with RE³⁺ in CoRE_xFe_2-xO₄ (x = 0.025, 0.05, and 0.1) had a large effect on magnetostrictive properties and strain derivative, which was improved compared with pure cobalt ferrite at low magnetic field. The maximum strain derivative (dλ/dH = −1.49 × 10⁻⁹ A⁻¹ m at 18 kA m⁻¹) was obtained for Nd, x = 0.05. Changes in the magnetostriction coefficients and strain derivatives were correlated with changes in cation distribution, microstructure, and magnetic anisotropy, which depended strongly on RE³⁺ substitution and distribution in the spinel structure.