Diluted and undiluted monodispersed CoFe2O4 nanoparticles: the effects of post-annealing on magnetic properties

Well-dispersed uniform CoFe2O4 nanoparticles were synthesized by thermal decomposition of Fe(acac)3 and Co(acac)2 in a solution of oleic acid, oleylamine, and benzyl ether. We diluted some of the nanoparticles in a SiO2 matrix, and then annealed undiluted and diluted samples at 300–700 °C. For the undiluted samples, the interparticle dipolar interaction dominated the macroscopic magnetic properties: as the annealing temperature increased, the dipolar interaction strength decreased, increasing the coercivity H c from 1094 to 2297 Oe and the remanence ratio M r/M s from 0.39 to 0.56. Furthermore, as the measurement temperature decreased, both the field cooling and zero-field cooling magnetizations decreased, indicating superspin glass behavior caused by strong interparticle dipolar interaction. For the diluted samples, the strength of their interparticle dipolar interactions was lower than those of the corresponding undiluted samples; additionally, their thermal energy overcame their interparticle dipolar interaction energy, which made the CoFe2O4 nanoparticles tend toward superparamagnetism as well as the decreased H c and M r/M s ratio. Finally, our diluted CoFe2O4 nanoparticles retained higher saturation magnetization M s than those reported before, therefore they may hold promise in biomedical applications.