Influence of the ZnO buffer layer on the magnetic properties of the BiFeO$_3$ thin film on sapphire

Thin layers of bismuth ferrite on an Al$_2$O$_3$ (C-plane) sapphire substrate with a ZnO zinc oxide buffer layer were synthesized by gas-discharge high-frequency cathode sputtering of a ceramic target of stoichiometric composition BiFeO$_3$ in an oxygen atmosphere. The structure, magnetic and optical properties of the BiFeO$_3$/ZnO/Al$_2$O$_3$ film are investigated. X-ray diffraction measurements and Raman spectra of the deposited films confirm the formation of a crystalline phase of thin layers of BiFeO$_3$ and ZnO. Based on the results of optical studies, the band gap for the direct transition is 2.51eV. Magnetic hysteresis loops (M – H) of a thin BiFeO$_3$ film with a ZnO buffer layer exhibit the behavior characteristic of ferromagnets. It is shown that the magnetization of BiFeO$_3$ is significantly higher (almost five times) in a BiFeO$_3$/ZnO/Al$_2$O$_3$ film compared to a film without a buffer layer, which indicates the important role of the ZnO buffer layer in improving the magnetic properties of bismuth ferrite for practical applications.