Temperature controlled evolution of monoclinic to super-tetragonal phase of epitaxial BiFeO3 thin films on La0.67Sr0.33MnO3 buffered SrTiO3 substrate

摘要

Epitaxial BiFeO₃ thin films of 130nm were deposited by pulsed laser deposition (PLD) technique on La_0.67Sr_0.33MnO₃ buffered SrTiO₃ (001) substrate at various temperatures under different ambient oxygen pressures. Reciprocal space mapping reveals that, with decreasing temperature and oxygen pressure, the broadly reported monoclinic phase (M_A) of BiFeO₃ thin film initially transforms to a tetragonal phase (T₁) with c/a =1.05 (1) in a narrow girth of deposition condition and then to a super-tetragonal phase (T₂) with giant c/a = 1.24 (1), as confirmed by reciprocal space mapping using high resolution x-ray diffraction. The surface morphology of the films reveals the island growth of the BiFeO₃ films deposited at low temperatures. We propose that the transformation from monoclinic to the super-tetragonal phase is essentially due to the manifestation of excess local strain as a result of the island growth. This study offers a recipe to grow the super-tetragonal phase of BiFeO₃, with giant c/a =1.24 (1) which exhibits exceptionally large ferroelectric polarization, on ferromagnetic layer La_0.67Sr_0.33MnO₃. This phase of BiFeO₃ can be utilized for the ferroelectric control of magnetism at the interface of BiFeO₃ and La_0.67Sr_0.33MnO₃.