Effect of co-doping on the multiferroic properties of BiFeO3

Abstract

The role of codoping with La in Bi-site and Ta in Fe-site on the multiferroic properties of BiFeO3 ceramic has been investigated in this research. Single phase Bi0.8La0.2Fe1-xTaxO3 (BLFTO) ceramics with x = 0.0, 0.01, 0.03 and 0.05 were synthesized by the conventional solid-state reaction method. The BLFTO dried pellets were calcined at 800oC for 2 hours and then sintered at temperatures in the range of 850-1000oC for holding times of 1 and 2 hours. Phase analysis by X–ray diffraction (XRD) indicated formation of single phase distorted R3c structure. Percentage theoretical density as high as 98% was achieved for Bi0.8La0.2FeO3 ceramics although this was associated with excessive grain growth. Microstructural investigation using the field emission scanning electron microscope (FESEM) showed that addition of Ta dramatically reduced the average grain size of Bi0.8La0.2FeO3 due to its strong pinning effect from 10.6 μm in Bi0.8La0.2FeO3 to 0.92μm in Bi0.8La0.2Fe0.95Ta0.05O3 at a sintering temperature of 1000oC for a holding time of 2 hours. A combination of good theoretical density, average grain size and optimum multiferroic properties were obtained at a sintering temperature of 1000oC for 2 hours. La and Ta codoped BFO showed superior values of dielectric constant and magnetic permeability. The best values of dielectric constant (> 2000) at room temperature were attained for the Ta doped ceramics at average grain sizes in the range of 0.92-0.98 μm. At higher temperature a considerable increase in the dielectric constant of Bi0.8La0.2FeO3 samples occurred due to space charge polarization. However, in Ta-substituted ceramics the stability of dielectric constant with temperature considerably improved. DTA analysis revealed that the peak for ferroelectric transition (TC) shifted towards higher temperatures for the Ta-substituted ceramics and reached 870oC for x=0.05. The real part of initial permeability, μi', was found to decrease with increasing Ta substitution due to decrease in grain size. Moreover it was observed that μi' increased with sintering temperature. For samples sintered at 1000oC for 2 hours, μi' of Bi0.8La0.2Fe0.97Ta0.03O3 and Bi0.8La0.2Fe0.95Ta0.05O3 ceramics remained almost constant with increase in frequency up to 15MHz. However, for Bi0.8La0.2FeO3 and Bi0.8La0.2Fe.99Ta.01O3, μi' remained constant up to the resonance frequency, fr, i.e. 10 MHz and 14MHz respectively after which there was a sharp decrease in μi' and increase in the imaginary part of permeability, μi'', due to domain wall relaxation.