Crystal structure and magnetic properties of Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics using neutron diffraction and Mossbauer spectroscopy

Bi0.8A0.2FeO3 (A = La, Ca, Sr, Ba) multiferroics were studied using x-ray, neutron diffraction and magnetization techniques. All the samples crystallized in rhombohedral structure with space group R3c. The compounds exhibit antiferromagnetic (AFM) ordering at 300 K and no evidence of further structural or magnetic transition was observed on lowering of temperature below it. The magnetic structure of these substituted compounds are found to be collinear G-type AFM structure as against the non collinear incommensurate magnetic structure reported in the case of parent compound. The moments on Fe at 6 K are aligned along the a-axis in the case of Ca-doped sample. With increase in the ionic radii of dopant, the moments are found to be aligned in the ac plane and the angle of tilt away from the a-axis increases. The observed change in the magnetic structure with substitution is attributed to the intrinsic structural distortion as evidenced by the change in the bond angle (Fe-O-Fe) and bond distances (Bi-O, Fe-O). It has been found that heterovalent substitution A2+ results in the formation of oxygen vacancies in the parent lattices as the possibility of Fe4+ ruled out by Mössbauer spectra recorded at room temperature. Higher value of remnant magnetization (0.4187 emu/g) and coercivity (4.7554kOe) is observed in Bi0.8Ba0.2FeO3 sample in comparison to other substituted samples revealing a strong correlation between ionic radii and magnetization