Concentration and temperature dependent double energy gap characteristic properties of hexagonal YMnO₃-xBiFeO₃ films

The study of hexagonal (1-x)YMnO3-xBiFeO3 (0 > x > 2.5%) (YBMF x) films can realize the modulation of h-YMO films, especially for the significant modulation of crystal orientation and surface. The choice of the solvent and solution, and the co-doping of Fe and Bi are two important facts of realizing the modulation. The results of x-ray diffraction and atomic force microscopy measurements indicated that there are the internal relations of the lattice orientation and the surface morphology for highly c-axis-orientation (HCO) and no preferred orientation (NPO), respectively. X-ray photoelectron spectroscopy confirmed that manganese ions show a valence of +3 in YMnO3 film. The peak positions of the Mn 2p level move to higher energy with increasing concentration. Furthermore, there is an optimal doping concentration of x = 1.5% for the YBMF x (0 > x > 2.5%) films by taking the shapes of P-E loops, the values of 2Pr and 2Ec into consideration. The transmittance spectra at room temperature pointed out that the double energy gaps (∼1.25 eV and 3.3 eV) corresponded to significant absorption edges, the band gap (Eg1) and subband gap (Eg2), respectively. The different dependency of Eg1 at the lower temperature can be explained by the ionized impurity scattering. A critical temperature (∼100 K) of Eg2 can be attributed to the relaxation of antiferromagnetic phase transformation. The abnormal shrinkage of Eg2 below 100 K can be explained by the pinning effect of composite domain walls and the accumulation of discrete oxygen vacancies at composite domain walls. The present study sheds light on the understanding of the electronic band structure, antiferromagnetic phase transformation and their relation in h-YMO. Moreover, it contributes to the verification and improvement of Katsufujis model (Katsufuji et al 2001 Phys. Rev. B 64 104419).