Superparamagnetic ZnFe2O4 nanoparticles: The effect of Ca and Gd doping

ZnFe2O4 ferrite nanoparticles are arousing a great interest in the biomedical field, in particular for magnetic hyperthermia application, thanks to their superparamagnetic behaviour at room temperature. In order to better tuning the magnetic properties, different doping substitution and synthesis methods have been tried to reduce particle sizes and to vary the cation distribution on the spinel sites. In this paper, we focused on the microwave combustion synthesis method of Ca (on Zn site) and Gd (on Fe site) substituted ferrites. Undoped ZnFe2O4 and Sr and Al doped samples were also synthesized for comparison. The use of X-ray powder diffraction, microscopic and spectroscopic techniques allowed us to ensure the good quality of the spinel structure for all the investigated samples, to determine a homogeneous distribution of the dopants and an average particle size lower than 11 nm. In addition, we estimated the inversion degree of the spinels by using the RietVeld structural refinement and Raman spectroscopy. By means of SQUID magnetometry we found, for all the samples, a superparamagnetic behaviour with saturation magnetization between 6 and 10 emu/g at the maximum applied magnetic field of 3T, with a more effective role played by Ca ions with respect to Gd ions substitution. (C) 2017 Elsevier B.V. All rights reserved.