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.

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The changes in crystal and magnetic structure of BiFeO3 produced by partial substitution of Bi ions by Ho ions has been studied with powder X-ray diffraction, neutron powder diffraction, dielectric and magnetization techniques. The present study demonstrates that Bi1-xHoxFeO3(x = 0.05, 0.10, 0.15, & 0.2) multiferroics shows change in crystal structure at x > 0.05. The sample with x = 0.05 exhibits rhombohedral structure (space group R3c), while the other three samples (x > 0.05) exhibit mixed phase with coexisting rhombohedral (R3c) and Orthorhombic (Pnma) structure. This change in the crystal structure is attributed to the distortion of FeO6 octahedra via substitution of Ho at phase boundaries. The magnetization studies indicate that doping of Ho in pristine BiFeO3 leads to enhancement in the ferromagnetic moment. We find that doping of Ho breaks the spin cycloid phase of BiFeO3 and creates a canted G-type antiferromagnetic structure in the hexagonal phase whereas the orthorhombic phase exhibits a collinear G-type AFM structure. The canting angle increases with increase in doping with Ho, leading to an enhancement in the ferromagnetic component in magnetization. Dielectric constant (ε′) and loss factor (tanδ) are measured in frequency range 1 kHz to 7 MHz and ε′ and tanδ show dispersion behaviour at low frequencies. The significant improvement in magnetization and dielectric properties is achieved by Ho substitution which in turn enhances the potential of BiFeO3 for multiferroics applications.

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