Mechanism of the formation of nanoscale M-type barium hexaferrite in the citrate precursor method

Nanoscale particles of BaFe12O19 in the size range 10-20 nm have been prepared at a substantially low temperature of 425 degrees C by a citrate precursor method. The low-temperature processes involved in the precursor synthesis are examined and compared with the processes involved in other chemical methods and conventional ceramic methods to show how ferrite formation becomes possible at relatively lower temperatures in the precursor method. The precursor decomposes at 425 degrees C to form a metastable spinel-like structure which undergoes time/temperature-dependent changes to transform to the final hexagonal structure. Using XRD and IR data it is shown that the spinel-like structure is actually a 'masked' hexaferrite structure where the Ba2+ ion in the hexagonal layer of the unit cell is engaged due to the presence of traces of carbon. When the Ba2+ ions are freed by the removal of carbon, the hexaferrite structure is completed. The Mossbauer spectra illustrate how various sublattice sites gradually develop with increasing heat treatment temperature and particle size to complete the complex barium ferrite structure. The magnetic measurements show how the above changes in structure are reflected as changes in magnetization and as increases in T-C. The increase in T-C is attributed to possible changes in the number of effective magnetic interactions.