Multiphase Formation In Bi based Multiferroic Thin Films: Structural Change And Strain Relaxation

Bi based multiferroic epitaxial thin films like, Bi-Fe-O and Bi-Mn-O were grown on SrTiO$_{3}$(001) substrates by pulsed laser deposition under various oxygen partial pressures (0.1 -- 20 mTorr) and thickness (35 -- 500 nm). The structural properties of impurity phases in these systems were characterized by transmission electron microscopy, x-ray diffraction, and energy dispersive x-ray spectroscopy. In the Bi-Fe-O system growth at oxygen partial pressures lower than 5 mTorr gives rise to the formation of the canted antiferromagnetic phase, $\alpha $-Fe$_{2}$O$_{3}$ with a columnar structure in a matrix of BiFeO$_{3}$. Simultaneously, ferromagnetic phase, $\gamma $-Fe$_{2}$O$_{3}$ precipitates with plate-like structure in the $\alpha $-Fe$_{2}$O$_{3}$ matrix. Similarly, Mn$_{3}$O$_{4}$ impurity phase forms in the Bi-Mn-O system when the films are grown at low oxygen partial pressure. In Bi-Fe-O film, the friction of $\gamma $-Fe$_{2}$O$_{3}$ enhances the magnetic properties of the film, while $\alpha $-Fe$_{2}$O$_{3}$ reduces the compressive stress in BiFeO$_{3}$ due to its smaller volume compared to that of BiFeO$_{3}$. We also report structural and magnetic properties of BiCrO$_{3}$ thin film, a new multiferroic material.