Effect of cobalt-doping on dielectric, magnetic and optical properties of BiFeO3 nanocrystals synthesized by sol – gel technique

Abstract BiFe1-xCoxO3 (x = 0.00, 0.01, 0.03, 0.05 and 0.07) nanoparticles (NPs), were prepared by ethylene-glycol-based sol-gel technique. Phase purity of the NPs was confirmed by x - ray diffraction (XRD) technique. The average size of pristine BiFeO3 (BFO) NPs was found to be 46 nm, which initially increased to 50 nm for 1% Co doping and then decreases to 48, 45 and 43 nm for 3%, 5% and 7% Co-doped BFO NPs, respectively. X-ray photoelectron spectroscopy (XPS) and Mossbauer spectroscopy confirmed that the oxidation state of Fe and Co is +3. In the low frequency region, the dielectric constant (e‘) shows a maximum decrease by 96% (137 → 5.1) on 5% Co-doping concomitant with 62% decrease in dielectric loss (0.32 → 0.12) which indicates substantial reduction in leakage current density. Further, the pristine BFO shows a well-defined dielectric loss peak at 40 Hz (tan δ = 0.4). In contrast, twin dielectric loss peaks are uniquely observed for all the Co doped BFO NPs. 1% doped NPs show first and second peaks at 7 Hz and 20,400 Hz, respectively, which shifts to higher frequencies of 245 Hz and 154,000 Hz, on 5% doping. The magnetic measurements (M − H) revealed the ferromagnetic behaviour of the NPs. On Co doping, saturation magnetization (Ms) increased with decrease in coercive field (Hc). A maximum enhancement of 226% (0.93 → 3.05 emu/g) in Ms values concomitant with decline of 61% in Hc values (80 → 15 Oe) was observed for 7% doping. The reduction in leakage current and enhancement of magnetic properties makes the NPs better-suited for device applications.