Effect of sintering temperature on structural, elastic, and hyperfine features of nickel ferrite nanoparticles

Abstract

Temperature is one of the key parameters in the synthesis of a system of magnetic nanoparticles and can affect considerably the arrangement of atoms in the crystal structure. Consequently, the magnetic, elastic, and structural properties are strongly impacted. In the magnetically soft NiFe1.8O4 spinel ferrite, the relative number of Ni and Fe ions in each of the two possible different geometries, tetrahedral (T) and octahedral (O), is of extreme importance. In this work, this material was studied on products of co-precipitation synthesis followed by a temperature dependent sintering. We performed 57Fe Mössbauer spectroscopy (MS), X-ray diffraction (XRD), scanning and transmission electron microscopies (SEM and TEM), X-ray absorption spectroscopy (XAS) using synchrotron radiation at Ni K- and L2-edge, and vibrating sample magnetometry (VSM) measurements. From XAS measurements it was found that Ni atoms prefer an octahedral coordination when the sintering temperature is increased. MS showed that the T-sites are more populated by Fe ions than the O-sites, a fact that can be explained by the presence of Fe ion vacancies in the O- sites. However, as the sintering temperature increases, we found that there is also a migration of Fe atom to the O-sites. Additionally, based on MS measurements in 78–292 K region, we estimated the Debye temperatures of the Fe ions in the tetra- and octahedral geometries and found that the recoiless f-factor depends on the sintering temperature of the nickel ferrite powders. The ratio fO/fT is about 0.98 and 0.95 at room temperature for the samples sintered at 1000 °C and 600 °C, respectively.