Strain-induced high coercivity in CoFe2O4 powders

Abstract

Three cobalt ferrite $\left({\mathrm{CoFe}}_2{\mathrm{O}}_4\right)$ powders were used as starting materials for mechanical milling. A mechanical milling for a short time resulted in a relatively large residual strain and a high density of defects in the micrometer-sized ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ powders. High coercivities (up to 5.1 kOe) were achieved in these samples, probably due to the stress anisotropy and pinning effects. A relatively high remanence ratio of 60% and a relatively large value of ${\left(\mathrm{BH}\right)}_{\max }$ of up to 2.0 MG Oe were obtained for the sample with the highest coercivity. A prolonged milling resulted in the formation of nanosized grains in an amorphous matrix, and the reduction of coercivity to a very low level. The initial particle/grain size played an important role in the microstructure evolution. No significant change in both microstructure and coercivity was found in the ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ sample consisting of nanoparticles after mechanical milling.