Investigating negative magnetization and blocking temperature in aggregates of ferrite nanoparticles

Blocking temperatures of aggregates of Mn_0.5Zn_0.5Gd_xFe(_2-x)O₄ ferrite nanoparticles, with x = 0.02, 0.05, 0.11, 0.15, and 0.2, were obtained from the zero-field-cooled (ZFC) magnetization measurements. We found a nonmontonic behavior of the blocking temperature with increasing size of the particles. The effective magnetic anisotropy was calculated and found to have two distinct rates of increase with decreasing the size of the particles. These results were attributed to the strong inter-particle interactions in the aggregated nanoparticles and to the enhanced role of surface anisotropy with the decrease of the size of the particles. In three samples, the ZFC magnetization was found to exhibit a significant negative magnetization in a considerable part of the low temperature region. To our knowledge, this is the first time that negative magnetization is reported in such nanoparticles. These peculiar results are discussed and are currently under investigation.