The Effects of Multiphase Formation on Strain Relaxation and Magnetization in Multiferroic BiFeO3 Thin Films

Multiferroic materials simultaneously display ferroelectric and ferromagnetic properties in the same phase. [1] In the 1960s to 1970s, these materials were first investigated to understand the magnetoelectric (ME) coupling effect. Recently, there is increased interest in multiferroic materials because of their potential applications in novel devices using the magnetoelectric effects including switching of the electric polarization controlled by an applied magnetic field. Very few intrinsic multiferroic compounds such as HoMnO3 and TbMn2O5 exist in nature or have been synthesized. Furthermore, these materials exhibit the ME coupling at low temperatures. BiFeO3 has attracted great attention because it is multiferroic well above room temperature. However, the small magnetization and narrow growth window of BiFeO3 impede its use in practical applications. To enhance the magnetic and ferroelectric properties of BiFeO3, chemical modification, and controlled stress through growth of heteroepitaxial films and variation in thickness have been attempted with varying degrees of success. In Bi-Fe-O, however, there are several phases of different structures, compositions, and electrical and magnetic properties which can co-exist. The presence of these phases can substantially affect the electrical and magnetic properties of the overall material. There has been a report on the coexistence of BiFeO3 and c-Fe2O3 phases in Bi-Fe-O films grown by pulsed laser deposition (PLD). We have recently reported on the coexistence of BiFeO3 and Fe2O3 phases (a and c) with a systematically varying degree of mixture controlled by the oxygen partial pressure during deposition. In the present work, we investigate the microstructural evolution in Bi-Fe-O films which form nanocomposites consisting of more than one phase when the thickness of the film is changed. We focus on characterization of the secondary phases and the effects of these phases on the relaxation of the lattice misfit strain and the magnetic properties of the overall films. Figure 1a shows X-ray diffraction (XRD) spectra obtained from films of different thicknesses, t, grown at an oxygen partial pressure of 5 mTorr. Diffraction was performed using the x-scan mode, and the intensities were integrated in v between 85 and 95 degrees. The corresponding TEM cross-sectional images of some of the films are shown in Figure 1b. In our previous report, thin films of pure BiFeO3, which has a perov-