Polarization and Dielectric Properties of BiFeO3-BaTiO3 Superlattice-Structured Ferroelectric Films

Abstract

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO3 and BaTiO3 with a total thickness of 600 perovskite (ABO3) unit cells were grown on single-crystal SrTiO3 substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO3 and BaTiO3 have over 25 ABO3 unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO3 and Mn-BiFeO3 layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.

Ferroelectric polarization of tetragonal BiFeO3—an approach from DFT calculations for BiFeO3–BaTiO3 superlattices—

“…The experimentally observed P s for the films with N ⩾ 10 is smaller than the volume-averaged P s . 60) This is probably because a domain clamping is caused 60) where a (experiment) is determined by X-ray diffraction analysis. Geometrical optimizations were conducted for the BaTiO 3 and BiFeO 3 cells in tetragonal P4mm symmetry under the fixed a DFT , leading to c BiFeO3 and c .…”

“…68) The details are described in the preceding paper. 60) We employed the following choice: the Perdew-Burke-Ernzerhof gradientcorrected exchange-correlation functional revised for solids (PBEsol), 69) a plane-wave cut-off energy of 520 eV, the Γcentred k-point mesh (5 × 5 × 3 for all the supercells and 7 × 7 × 5 for all the density of states (DOS) and band structures, unless otherwise stated), the simplified generalized gradient approximation (GGA)+U approach 70) (the on-site Coulomb interaction parameters of U − J of 6 eV to Fe-3d).…”

“…41) Moreover, it was reported that ferroelectric nature disappears when x is increased over 40%-50%. 59) In our preceding paper, 60) we have reported the dielectric and polarization properties of superlattice BaTiO 3 -BiFeO 3 epitaxial films, where Mn(5%)-doped BiFeO 3 [61][62][63] is adopted instead of BiFeO 3 to suppress the influence of oxygen vacancy as much as possible. 64,65) The total number of ABO 3 unit cells in the thickness direction are fixed at 600, and the average composition remains unchanged with x = 0.5, while the number of the unit cells in one layer (N) changes from 300 down to 1.…”

Origin of Ferroelectricity in BiFeO3-Based Solid Solutions

“…Ferroelectric BiFeO 3 has been intensively studied from the viewpoints not only of multiferroic nature [ 15 , 16 , 17 , 18 ] but also of a Pb-free piezoelectric material [ 19 , 20 , 21 , 22 ]. In BiFeO 3 [ 18 , 23 ], P s coexists with an incommensurate spin structure, which can be approximated as an antiferromagnetic spin configuration [ 15 , 17 ].…”

“…A formation of composition-driven phase boundaries, such as MPBs, is expected in the systems of BiFeO 3 –REFeO 3 [ 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 ] (RE: rare-earth elements) and BiFeO 3 –BaTiO 3 [ 20 , 21 , 36 , 37 ]. The simplest solid solution is the BiFeO 3 –LaFeO 3 system with a composition of Bi 1− x La x FeO 3 , the detail of which has been comprehensively summarized in the review paper [ 27 ].…”