THERMAL-BEHAVIOR AND MAGNETIC-PROPERTIES OF ACICULAR COPPER-COBALT FERRITE PARTICLES

CoxCu1-xFe2O4 (0 less than or equal to x < 0.3) ferrite particles were prepared by a soft chemistry method using oxalic precursors. The ferrite particles obtained by this process are acicular in shape (4.2 less than or equal to L/D less than or equal to 4.6) ranging in length between 1.5 and 0.8 mu m for 0 less than or equal to x less than or equal to 0.3. Two series of samples were investigated: a first sample (SC) grouped together the ferrites slowly cooled from 710 degrees C, and the second (Q) consisted of samples quenched from the same temperature. In contrast to the stability of the SC ferrites, the Q ferrites are subjected to two phenomena before reaching 400 degrees C. The first is ascribed to the migration of Cu2+ ions from tetrahedral to octahedral sites. The second is the oxidation of Cu+ ions formed above 600 degrees C during the preparation process and frozen by quenching at room temperature in the Q samples, Above approximately 400 degrees C, no differences appear to subsist between SC and Q samples. They are both subjected to diffusionless order-disorder transformation, due to the modified orientation of the Jahn-Teller distorsions, and to the migration of Cu2+ ions from octahedral to tetrahedral sites, before the Curie temperature is reached. Magnetic measurements performed on the samples prepared for the present work reveal remarkably high and stable coercivities. These magnetic properties are due not only to the structural anisotropy, but also to the morphological characteristics. Because of their high and stable coercivities, submicron copper ferrites could be interesting pigments for magnetic recording. (C) 1995 Academic Press, Inc.