Rietveld refinement, Raman, optical, dielectric, Mossbauer and magnetic characterization of superparamagnetic fcc-CaFe2O4 nanoparticles

This article describes synthesis of superparamagnetic fcc-CaFe(2)O(4 )nanoparticles by a metal nitrate-citrate monohydrate sol-gel route and characterization using X-ray diffractometry, Raman spectroscopic, UV-Vis-NIR optical absorption spectroscopic, Mossbauer spectroscopy, dielectric and SQUID magnetometry measurements. Rietveld refinement of the X-ray diffraction pattern and observation of active A(1g), T-2g & E-g modes in the Raman spectrum confirmed formation of single phase CaFe2O4 nanoparticles in the spinel ferrite type fcc structure without impurity. Mossbauer and Rietveld data analysis revealed that nanocrystalline CaFe2O4 is dominantly an inverse ferrite in which 85% Ca atoms preferentially occupy the octahedral site in the fcc symmetry and the Fe ions are in high spin Fe+3 state. Nanocrystalline CaFe2O4 significantly absorbs optical light below 500 nm and the direct band gap energy is estimated to similar to 1.83 eV, which is higher than 1.26 eV reported for orthorhombic CaFe2O4. Temperature and field dependent magnetic studies showed that fcc-CaFe(2)O(4 )nanoparticles exhibit superparamagnetism at room temperature with high saturation magnetization of 1.07 mu(B). The blocking temperature is similar to 53 K at 1000 Oe and similar to 72 K at 500 Oe clearly shows lowering of blocking temperature at higher magnetic field. Interestingly below the blocking temperature at 20 K, CaFe2O4 nanoparticles behave as non collinear soft-ferrimagnetic material with a saturation magnetization of 1.16 mu(B), coercivity of 150 Oe and remanence of 4.45 emu/g. The magnetic momenta of Fe+3 ions residing at the octahedral sub-lattice are canted at similar to 25 degrees.