Here we report on a mixed oxide system, γ-Fe2O3 nanoparticles doped with Mn(III), where the transition from the cubic to the more stable hexagonal α-Fe2O3 structure is suppressed. When amorphous Fe2O3 is heated at 300 °C for 3 h, ferrimagnetic γ-Fe2O3 is observed as the sole product. On the other hand, when the temperature is raised to 500 °C, one observes only antiferromagnetic α-Fe2O3 as the product. However, upon doping with 8.5 wt % Mn(III), the amorphous nanoparticles crystallized to mainly the γ-Fe2O3 matrix after heating at 500 °C for 3 h, and need to be heated to >650 °C for the complete transition to the α-Fe2O3 structure to take place.
Designing nanoparticles for practical applications requires knowledge and control of how their desired properties relate to their composition and structure. Here, we present a detailed systematic study of mixed iron-manganese oxide nanoparticles, showing that ultrasonication provides the high-energy reaction conditions required for complete atomic level mixing of Fe(III) and Mn(III) when amorphous Fe 2 O 3 nanoparticles are irradiated in the presence of Mn 2 (CO) 10 in ambient atmosphere. X-ray diffraction (XRD) results reveal that the crystal structure of manganese iron mixed oxide nanoparticles changes from spinel to bixbyite with increasing of Mn(III) content. The results of room-temperature magnetization curves are consistent with the XRD patterns and spin density from electron paramagnetic resonance measurements, showing samples converting from superparamagnetic to antiferromagnetic, when the crystal structures of these samples transform from spinel to bixbyite.
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