Magnetic interactions and hysteresis loops study of Co/CoFe2O4 nanoparticles

Co/CoFe2O4 nanoparticles with the mean size of 8.8, 10.8 and 16.9 nm were prepared by thermal decomposition of metal salts in the presence of citric acid. The X-ray diffraction patterns and Rietveld refinements confirmed coexistence of Co-ferrite and metallic cobalt phases in the nano-powders. Scanning electron microscope images showed an increase in particles aggregates mean diameter with increasing the annealing temperature. Magnetic hysteresis loops showed a demagnetization jump at low fields, which was attributed to different reversal fields of ferrite and the cobalt phases. Field-dependent behavior of maximum magnetization (Mmax), remanence (Mr), squareness (S) and coercivity (Hc) were studied through minor loops measurements. The calculated S value of the loops showed a maximum, between anisotropy and coercive fields. A sharp increase in Hc of larger particles was observed with increasing the applied field when compared to smaller particles. Henkel plots showed that the samples are interacting. Negative deviation of Henkel plots from linear behavior and negative δm plots revealed the dominant role of dipole-dipole interactions in the nano-aggregates.