Microstructural and magnetic properties of BiFeO3/Bi2Fe4O9 binary nanoparticles with a bimodal size distribution

Abstract

Binary nanoparticles composed of BiFeO3 and Bi2Fe4O9 have attracted considerable interest due to their magnetic properties and exchange bias effect. In this paper, two samples of well-crystallized BiFeO3/Bi2Fe4O9 binary nanoparticles with varying compositions, containing approximately 3 % and 32 % of the Bi2Fe4O9 crystalline phase, were investigated to understand the impact of microstructural variations on the magnetic behavior of the samples. Both BiFeO3 and Bi2Fe4O9 phases of binary nanoparticles were synthesized in the same synthesis process by calcining amorphous precursors obtained after solvent evaporation from aqueous solutions containing iron and bismuth nitrates and tartaric acid. The identification and quantification of crystalline phases of the produced samples were performed using the X-ray diffraction technique and multiphase Rietveld refinement, respectively. The structure and microstructures of the synthesized BiFeO3/Bi2Fe4O9 nanoparticles were carried out using transmission electron microscopy techniques. The atomic composition of the samples was investigated by energy-dispersive X-ray. The magnetic properties were studied by the DC magnetization technique. Transmission electron microscopy analyses showed that both samples exhibit a bimodal particle size distribution, with average particle sizes of ∼ 116 nm and ∼ 6.5 nm for the ∼ 3 % Bi2Fe4O9 sample. In contrast, for the ∼ 32 % Bi2Fe4O9 sample the average particle sizes were ∼ 108 nm and ∼ 7 nm, respectively. Energy-dispersive X-ray analyses revealed a Fe-rich impurity phase in both samples. Temperature-dependent zero-field-cooled and field-cooled magnetization curves present a large separation between them below 300 K and zero-field-cooled curves are consistent with a broad (bimodal) size distribution of the nanoparticles for both samples. Magnetic hysteresis loops measured at 5 K after the samples are cooled from 400 K in a zero magnetic field show weak ferromagnetism for the ∼ 3 % Bi2Fe4O9 sample, whereas the ∼ 32 % Bi2Fe4O9 sample presents an improved ferromagnetism. An exchange bias-type effect was observed in both samples. The results suggest that the increase of ultrasmall Bi2Fe4O9 nanoparticles leads to an improvement in the coercivity and remanence of BiFeO3/Bi2Fe4O9 binary nanoparticles.