Synthesis and Size-Selective Precipitation of Monodisperse Nonstoichiometric MxFe3–xO₄ (M = Mn, Co) Nanocrystals and Their DC and AC Magnetic Properties

Spinel ferrite nanocrystals (NCs) have shown great promise for a wide variety of electromagnetic and medical applications. In this work, the AC magnetic properties of nonstoichiometric manganese and cobalt ferrites (M x Fe 3– x O 4, M = Mn, Co) NCs are systematically studied as a function of composition. Samples of very similar average size and shape, but different Mn to Fe and Co to Fe ratios are prepared to study the effect of composition. Conventional syntheses are combined with a size-selective precipitation method using oleic acid as an antisolvent yielding nearly monodisperse samples. DC and AC magnetic measurements shows that introducing Co to the ferrite crystal increases the blocking temperatures and magnetic anisotropies of the nanocrystals with corresponding shifts in AC magnetic susceptibilities, while manganese ferrites are relatively insensitive to the variation in compositions as size and shape dominate over crystal anisotropy. The systematic AC-magnetic characterizations of superparamagnetic Mn x Fe 3– x O 4 and Co x Fe 3– x O 4 NCs raise the importance of controlling chemical composition of ferrite NCs for AC magnetic applications.