Enhanced Anisotropy in Tetragonalized (Cu, Co) Fe2O4 Particles via the Jahn-Teller Effect of Cu2+ Ions

We studied the Jahn-Teller (JT) effect of Cu2+ ions on the tetragonalization of CuFe2O4 and (Cu, Co) Fe2O4 particles. Sub-micrometer-sized CuFe2O4 and (Cu, Co) Fe2O4 particles were synthesized via coprecipitation and flux methods, followed by a heat-treatment process. From the X-ray diffraction patterns, all particles were found to be cubic spinel after flux treatment. The tetragonalization process was carried out by a heat-treatment process in air, where the temperature was varied between 700 degrees C and 900 degrees C, followed by furnace cooling. We confirmed that the ideal temperature for the tetragonalization of both CuFe2O4 and (Cu, Co) Fe2O4 is 900 degrees C. The obtained single-phase tetragonal spinel-structured CuFe2O4 and (Cu, Co) Fe2O4 particles indicate tetragonal distortion as a consequence of the JT effect of the Cu2+ ions. The results of saturation magnetization suggest that most of the divalent ions are in the B sites. The lengthening of the c-axis due to the JT effect increased the coercivity of the CuFe2O4 and (Cu, Co) Fe2O4 particles during the cubic-tetragonal phase transformation. The tetragonal (Cu, Co) Fe2O4 particles showed a saturation magnetization of 26 emu/g and a coercivity of 2200 Oe.