Effects of Bi doping on structural and magnetic properties of cobalt ferrite perovskite oxide LaCo0.5Fe0.5O3

Literature reports on magnetic transition of LaFe0.5Co0.5O3 are highly ambiguous. The present study is an endeavour to address that particular issue of this ferrite perovskite. Here we report the low temperature sol-gel synthesis of pure and Bi-doped La1-xBixFe0.5Co0.5O3 (x = 0, 0.1 & 0.2) perovskites. All the samples were characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, energy dispersive X-ray (EDX) analysis and magnetization studies. LaFe0.5Co0.5O3 crystallizes in a single phase rhombohedral (R-3c) structure. Substitution of Bi at the A-site results in structural transition. La0.9Bi0.1Fe0.5Co0.5O3 exhibits mixed rhombohedra (R-3c) and orthorhombic (Pbnm) phases, whereas La0.8Bi0.2Fe0.5Co0.5O3 is single phase orthorhombic (Pbnm). Favourable grain growth and good crystallinity observed in Bi doped samples are possibly due to the low melting temperature of Bi containing La(1-x)BixFe(0.5)-Co0.5O3. EDX analysis confirms the chemical homogeneity of the samples. XPS studies and bond valence sum calculations confirm the nominal cationic oxidation states of the ions. Magnetization measurements reveal the weak ferromagnetic nature of La1-xBixFe0.5Co0.5O3 (x = 0, 0.1 & 0.2) which is associated to the canted anti ferromagnetic structure. The magnetically active subsystem of Fe3+ ions are responsible for weak ferromagnetism which is explained by the antisymmetric Dzyaloshinskii-Moriya interaction, whereas the Co3+ ions remain predominantly in the low spin state. The increase in magnetic transition and the lower magnetic moment with an increase in Bi content can be related to the weakening of Dzyaloshinskii-Moriya interaction due to structural modification and local lattice distortion associated to the stereochemically active 6s(2) lone pair electrons of the Bi3+ ions. Our results confirm the high structure sensitivity of magnetic properties of cobalt ferrite perovskite.