A Systematic Study of the Structural and Magnetic Properties of Mn-, Co-, and Ni-Doped Colloidal Magnetite Nanoparticles

A series of colloidal M x Fe 3– x O 4 (M = Mn, Co, Ni; x = 0–1) nanoparticles with diameters ranging from 6.8 to 11.6 nm was synthesized by hydrothermal reaction in aqueous medium at low temperature (200 °C). Energy-dispersive X-ray microanalysis and inductively coupled plasma spectrometry confirm that the actual elemental compositions agree well with the nominal ones. The structural properties of the obtained nanoparticles were investigated by powder X-ray diffraction, Raman spectroscopy, Mössbauer spectroscopy, X-ray and neutron pair distribution function analysis, and electron microscopy. The results demonstrate that our synthesis technique leads to the formation of chemically uniform single-phase solid solution nanoparticles with cubic spinel structure, confirming intrinsic doping. The local structure of the Fe 3 O 4 NPs is distorted with respect to the cubic inverse-spinel structure, while chemical substitution of Fe by Mn or Ni partially eliminates the local distortions. Magnetic studies showed that, in comparison to nondoped Fe 3 O 4, the saturation magnetization ( M s ) of M x Fe 3– x O 4 (M = Mn, Ni) decreases with increasing dopant concentration, while Co-doped samples showed similar M s . On the other hand, whereas Mn- and Ni-doped nanoparticles exhibit superparamagnetic behavior at room temperature, ferrimagnetism emerges for Co x Fe 3– x O 4 nanoparticles, which can be tuned by the level of Co doping.