Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe₂O₄ Nanoparticles

In this work, we show the enormous size effect on the ordering transition temperature, T O, in samples of CoFe 2 O 4 nanoparticles with diameters ranging from 1 to 9 nm. Samples were characterized by HRTEM and XRD analyses and show a bimodal particle size distribution centered at 3 nm and around 6 nm for “small” and “large” particles, respectively. The results and concomitant interpretation were derived from studies of the magnetization dependence of the samples on temperature at low and high magnetic fields and relaxation times using both dc and ac fields. The large particles show a typical superparamagnetic behavior with blocking temperatures, T B, around 100 K and a Curie temperature, T C, above room temperature. The small particles, however, show a colossal reduction of their magnetic ordering temperature and display paramagnetic behavior down to ∼10 K. At lower temperatures, these small particles are blocked and show both exchange and anisotropy field values above 5 T. The order of magnitude reduction in T O demonstrates a heretofore unreported magnetic behavior for ultrasmall nanoparticles of CoFe 2 O 4, suggesting its further study as an advanced material.