Spring exchange mechanism and temperature dependent magnetic properties of BaFe12O19/CoFe2O4 nanocomposites

Bare BaFe12O19, CoFe2O4, and their nanocomposites with concentration percentage 30-70, 50-50 and 70-30 were synthesized by sol-gel approach. The average crystallite size < D > of bare BaFe12O19 nanoparticles (NPs) was 35 nm which gets reduced with the addition of CoFe2O4 phase and found minimum for bare CoFe2O4 nanoparticles, i.e. 12 nm. Transmission electron microscopy (TEM) image showed the agglomeration of nanocomposites and mean particle size was observed to be 44 nm as estimated from normal size distribution. The ZFC/FC curves exhibited that the blocking temperature T-B of all samples lies above room temperature. However, a small broad peak at similar to 75 K in pure BaFe12O19 NPs is attributed to the surface freezing peak (T-f). The flatness in FC curve at low temperature with the addition of CoFe2O4 phase revealed the strong inter-particle interactions. From M-H loops, the value of saturation magnetization (MS) for Ba70/Co30 was found to be maximum (100.82 emu/g) than their corresponding bare nanoparticles. The higher value of MS showed the presence of exchange spring mechanism in these samples. The coercivity of BaFe12O19 sample was observed to be 4262 Oe which get increased with the addition of CoFe2O4 phase and found maximum for CoFe2O4 (7349 Oe), which shows the hard magnetic nature of CoFe2O4 NPs. However, the smaller value of Bloch's constant "B" for BaFe12O19 phase (4.8 x10-6 K-3/2) showed the strong intra-particle interactions in the core of BaFe12O19 NPs. The dM/dH vs. H curves showed the well-established exchange coupling of both hard and soft magnetic phases for 50% concentration of both phases and may be beneficial for high density recording devices.