Coupling of Blocking and Melting in Cobalt Ferrofluid

Zero-field-cooling and field-cooling (FC) measurements were performed on ferrofluids of cobalt magnetic nanoparticles (MNPs) in various organic solvent. Two peaks, one broad peak corresponding to the blocking transition ( T B ), and one sharp peak corresponding to the melting of the solvent ( T M ), were observed. Furthermore, for a given MNP size, when the blocking and melting transitions were superposed by choosing an appropriate solvent, the strongest intensity of the sharp peak at the melting point of the organic solvent was obtained. This observation is explained by applying the M spectrum theory. Additionally, a first order, melting-induced magnetic phase transformation was observed at the melting point of the solvent. Associated with the first order phase transition and the supercooling effect, a thermal hysteresis loop in the FC curve was observed.