Impact of Stoichiometry and Size on the Magnetic Properties of Cobalt Ferrite Nanoparticles

Cobalt ferrite nanoparticles (NPs) are promising candidates for application as nanomagnets. The challenge is to lower the size of the cobalt ferrite NPs such that the remanent magnetization remains stable at room temperature and above. Wet chemical synthesis allows for the production of monodisperse cobalt ferrite (Co_xFe_3-xO₄) NPs in which the stoichiometry of the metal cations can be varied in a controlled way. Here we experimentally show that NPs with the cobalt stoichiometry in the range of 0.5 < x < 0.7 exhibit the largest magnetocrystalline anisotropy constant and room-temperature remanent magnetization. Moreover, we experimentally demonstrate that the commonly observed bimagnetic behavior of the magnetization in cobalt ferrite NPs is due to interparticle dipolar interactions. At the optimum cobalt stoichiometry, we tune the size of the cobalt ferrite NPs through a systematic change in the reaction parameters. We experimentally demonstrate that 10 nm is the smallest particle diameter at which cobalt ferrite NPs show remanent magnetization that is stable at room temperature. The smallest NPs size translates to a maximum packing density in excess of 10¹² nanomagnets/cm², which can be advantageously used for magnetic nonvolatile memories and mass information storage applications.