Structural and Magnetic Properties of Dilute Ca2+ Doped Iron Oxide Nanoparticles

Undoped and calcium substituted hematite (alpha-Fe2O3) nanoparticles are synthesized by surfactant directed co-precipitation and post annealing method. The annealed nanoparticles were found to be in single phase in nature and crystallize in the rhombohedral structure with space group R3c as confirmed by Rietveld refinement of the X-ray diffraction (XRD) data. Average crystallite sizes are calculated to be 20 to 30 nm and 50 to 60 nm for the nanoparticles annealed at 400 and 600 degrees C respectively. Mossbauer spectra for all the nanoparticles could be fitted with a sextet corresponding to the single magnetic state of the iron atoms in its Fe3+ state in the hematite matrix. The FTIR and Raman spectra of all the samples correspond to specific modes of alpha-Fe2O3. UV-Vis spectra of annealed samples showed broad peaks in the range of 525-630 nm resulting from spin-forbidden ligand field transition together with the spin-flip transition among the 2t(2g) states. The estimated band gap energies were in the range of 1.6 to 1.9 eV which are much lower than the reported values for nano hematite. From the room temperature magnetic hysteresis loop measurements, weak ferromagnetic behavior is observed in all undoped and Ca2+ doped hematite samples. Morin temperature (T-M) is calculated to be 257 and 237 K for 1.45% doped samples with particle size 54 and 27 nm respectively. The sample with Ca content of 1.45 wt% when annealed at 400 degrees C showed that the particles were of different shapes which included both quasi spherical and rod shaped. On annealing the same sample at 600 degrees C, the nanorods collapsed to form bigger spherical and ellipsoidal particles.