Origin of superstructures in (double) perovskite thin films

Abstract

We have investigated the origin of superstructure peaks as observed by X-ray diffraction of multiferroic Bi(Fe0.5Cr0.5)O3 thin films grown by pulsed laser deposition on single crystal SrTiO3 substrates. The photon energy dependence of the contrast between the atomic scattering factors of Fe and Cr is used to rule out a chemically ordered double perovskite Bi2FeCrO6 (BFCO). Structural calculations suggest that the experimentally observed superstructure occurs due to unequal cation displacements along the pseudo-cubic [111] direction that mimic the unit cell of the chemically ordered compound. This result helps to clarify discrepancies in the correlations of structural and magnetic order reported for Bi2FeCrO6. The observation of a superstructure in itself is not a sufficient proof of chemical order in double perovskites.

Enhanced spontaneous polarization in double perovskite Bi2FeCrO6 films

“…However, for the current 300‐nm‐thick BFCO/STO system, the oxygen octahedral distortions can only play a trivial role, if any. So the strong superlattice peaks may be rooted in the displacements of the Bi 3+ ，Fe 3+ , and Cr 3+ cations along the [111], as suggested by Shabadi and coauthors . It should also be noted that such cation displacements could generally influence the polarization and ferroelectricity of perovskite thin films …”

“…In perovskite materials, superlattice peaks can arise not only due to cation ordering but also because of A-or B-site cation shifts and tilts of the oxygen octahedra. 21 Among these, the oxygen octahedra distortions may come from different symmetry between the substrate and the film or lattice mismatch. However, for the current 300-nm-thick BFCO/STO system, the oxygen octahedral distortions can only play a trivial role, if any.…”

“…So the strong superlattice peaks may be rooted in the displacements of the Bi 3+ , Fe 3+ , and Cr 3+ cations along the [111], as suggested by Shabadi and coauthors. 21 It should also be noted that such cation displacements could generally influence the polarization and ferroelectricity of perovskite thin films. [32][33][34][35] In order to investigate the relationship between the superlattice peaks and the ferroelectricity of BFCO thin films, the polarization-electric field (P-E) hysteresis loops of the (100)-, (110)-, and (111)-oriented BFCO films were probed at 10 K ( Figure 5A-C).…”

Unraveling the magnetic properties of BiFe0.5Cr0.5O3 thin films

“…These values are comparable with those obtained by magnetometric measurements in BFCO thin films with weak ferromagnetic behavior. 20,26,29 Similar measurements as in Figure 2 were repeated with the magnetic field oriented perpendicularly with respect to the sample surface, as shown in the supplementary material. 32 Also in this latter case, the magnetization of both Cr and Fe is oriented parallel to the field.…”

“…The interest of such material is that, if Fe and Cr sublattices are chemically ordered, the expected antiferromagnetic super-exchange coupling 17 between Fe and Cr 18 may yield a ferrimagnetic ordering with a net predicted magnetic moment of 2 µ B per unit cell. 19 However, the measured values of magnetization are widely spread, [20][21][22][23][24][25][26][27] from nearly zero with antiferromagneticlike behavior 20,26 to large magnetic moments and magnetic saturation at 0.5 T. 21,22 In the first case, the absence of a strong ferrimagnetic signal has been attributed to the lack of chemical order in the double-perovskite, with Fe and Cr atoms being randomly distributed. 26 All the reported hysteresis loops of BFCO thin films have been measured by integrating methods, like vibrating sample magnetometers, 22,23 SQUID, 21,25,26 or alternating gradient magnetometer.…”