Chemical pressure induced near-complete suppression of spin-wave excitations in Bi0.9A0.1FeO2.95 (A = Ba, Ca)

We have probed the changes in the phonon and magnon modes of BiFeO3 on substitution of Ba and Ca at the A-site using infrared reflectance spectroscopy. The systematic hardening of the phonon modes in the doped samples is seen to correlate well with the volume reduction of the unit cell observed from the Rietveld measurements. This is attributed to the generation of chemical pressure in these samples on account of substitution of the dopants. In addition, a near-complete suppression accompanied by a complete lifting of the degeneracy of the magnon modes as compared to the pristine BiFeO3 is detected for the Ca-doped samples. These results suggest that aliovalent doping at A-site affects the crystal field of FeO6 octahedra of BiFeO3, which eventually modifies its magnetic nature. These doping-induced modifications are mainly responsible for the improved magnetic and magnetoelectric properties of the Ca-doped BiFeO3 samples.