Jahn-Teller Effect in Ultrafine Copper Ferrite Particles

The magnetic properties of ultrafine copper ferrite particles have been investigated, and to examine the possibility of enhanced coercive force in ultrafine particles by Jahn-Teller effect. Ultrafine copper ferrite particles prepared by coprecipitation have been heated at various temperatures. X-ray diffraction patterns indicate that the crystal structure of ultrafine copper ferrite particles obtained by heating at a temperature above ∼400°C is tetragonal with the axial ratio (c-a)/a of∼5.8%, which is nearly equivalent to that of the bulk material, while that of the ultrafine particles obtained by heating at a temperature below ∼300°C is cubic. The maximum coercive force obtained in this work has been ∼1500 Oe at heating temperature of∼700°C. It has been believed that Jahn-Teller effect obviously occur in ultrafine copper ferrite particles with the particle size of 12∼40 nm. The enhanced coercive force can be mainly attributed to uniaxial magnetic anisotropy produced by the crystal distortion.