Structure, surface composition, and magnetic properties of Mn2+-doped magnetite nanoparticles of the composition MnxFe3-yO4

We systematically investigated the core structure, surface composition, and magnetic properties of Mn2+doped magnetite nanocrystalline particles (∼ 8–23 nm) of the nominal composition MnxFe3-yO4 (x = 0.0–0.5; ) where the Fe3+, not Fe2+, ions are deliberately substituted with Mn2+ to create a unique defect structure formed of both substitutional and interstitial Mn2+ impurities. XRD and Raman data indicate that Mn2+ doping weakens the magnetite-to-maghemite transformation, halting it entirely for the MnxFe3-yO4 sample with x = 0.5. Rietveld analysis of XRD data favors a cationic distribution wherein Mn2+ ions exclusively substitute the tetrahedral Fe3+ ions in the spinel-related structure, removing ⅓ of them to interstitial tetrahedral sites at low x values and to both interstitial tetrahedral and octahedral sites at higher x values. XPS data support the finding that the magnetite-to-maghemite transformation is increasingly inhibited at higher x values. The nanoparticles exhibit complex magnetic behavior, which is a manifestation of the unique defect structure wherein the saturation magnetization initially decreases and then increases as x increases. The Verwey transition observed at very low temperatures (∼10 K) in MnxFe3-yO4 nanoparticles is attributed to particle size effects rather than Mn2+ doping. The changes in the magnetic properties are clearly related to the unique defect structure developed.