A study on the heat-treatments of nanocrystalline nickel substituted BaW hexaferrite produced by low combustion synthesis method

The novel low temperature combustion synthesis (LCS) method for the preparation of nanocrystalline W-type BaW hexaferrite i.e. BaNi₂Fe₁₆O₂₇ has been carried out by citrate precursor using the sol-to-gel (S–G) followed by gel-to-nanocrystalline (G–N) conversion. Decomposition behaviors and the phases associated therein are investigated by means of thermal analysis (DTA/DTG/TG) and XRD, respectively. Atomic absorption spectroscopy (AAS) has been used to determine the elemental analysis in different conditions. Surface morphology of the nonporous ultra fine particles have been examined by SEM. The TEM micrographs show that the particles of the size of 10 nm were seemed to be agglomerated in the ‘as synthesized’ condition. Room temperature Fe-57 Mossbauer spectrum, MS has showed doublet of ‘as synthesized’ nanocrystalline powder that indicates the superparamagnetic behavior of the material. This effect is further confirmed by vibrating sample magnetometer (VSM) wherein it was noticed that the magnetic field (10 KG max) did not have any effect on the material. The material was annealed at 400, 700 and 1000 °C in the furnace for 4 h. The grain size is found to increase from 10 to 70 nm after annealing at 1000 °C for 4 h. MS after annealing at 700–1000 °C for 4 h, showed that the doublets of ‘as synthesized’ is further resolved into broad sextets due to the presence of both superparamagnetic and ferrimagnetic particles, in the wide size range from 10 to 70 nm. Only slight increase in particle size (from 10 to 15 nm) is noticed after the heat-treatment for 1–3 and 5 min in microwave oven (2.45 GHz with 760 W) but with predominant phase changes. TEM after the heat treatment revealed the presence of microcrystalline nature of grains of the size ∼70 nm. The transformation of the magnetic properties i.e. from superparamagnetic to ferrimagnetic behaviour after heating in microwave oven has been revealed by hysteresis loops under VSM study. The saturation magnetisation, M_s after heat treatment has been seen to increase from 26.7 to 44.5 emu/gm. Remanence and coercivity have also increased four and seven times, respectively. M_s of the as synthesised hexaferrite nano powder and heat-treated powder in microwave oven for 5 min show doublets, confirming the presence of superparamagnetic relaxation in the nano particles as only slight increase in the particle size is associated with the heat treatment.