Change in the coercivity of CoFe2O4 nano-particles verified to result from a change in their domain structure

CoFe2O4 nano-particles were synthesized using the combustion method. The samples obtained were annealed at different temperatures in order to increase their sizes. Based on the characteristics of the particle size and the magnetic property, a monotonic increase in saturation magnetization from 53.2 to 89.8 emu/g occurred when the particle size increased from 15 nm to >1 µm. However, the coercivity (HC) did not change monotonically with particle size and it achieved its maximum of 1546 Oe at around 40 nm, which was accompanied by the appearance of a Barkhausen jump. This indicates that a non-monotonic change in HC with particle size should be attributed to the transition of the domain structure from single domain to multi-domain.