Size-dependent magnetic properties of γ-Fe₂O₃ nanocrystallites.

A route for synthesizing monodisperse magnetic nanocrystallites of maghemite, $\gamma$-Fe$_{2}$O$_3$, with various sizes has been revisited. A systematic investigation of three $\gamma$-Fe$_2$O$_3$ nanocrystalline samples by different techniques has been performed to characterize their size-dependent magnetic properties. Zero-field-cooled and field-cooled magnetization measurements reveal that the superparamagnetic blocking temperatures are around 230~K, 170~K, and 50~K for the 15.0~nm, 11.8~nm, and 6.1~nm nanocrystallites, respectively. Low-temperature M{\"o}ssbauer spectra show that all three nanocrystallites have the maghemite structure with all the vacancies in the B-sites. Furthermore, detailed analysis shows that there are more vacancies on the B-sites for the 6.1~nm nanocrystallites compared to 0.33 for the bulk maghemite.