Mixed Bloch-Néel-Ising Character of 180° Ferroelectric Domain Walls
D. Lee, R. Berhera, P. Wu, H. Xu, Y. Li, S. Phillpot, S. Sinnott, L. Chen and V. Gopalan, Phys. Rev. B., 80, 060102(R) (2009). PDF
Ferroelectric 180° domain walls are well-known to be predominantly Ising-like. Using density functional theory, and molecular dynamics simulations, the 180° domain walls in prototypical ferroelectrics lead titanate (PbTiO3) and lithium niobate (LiNbO3) are shown to have mixed character; while predominantly Ising-like, they also manifest some Bloch- and Néel-like character. Phase-field calculations show that such mixed wall character can be dramatically enhanced in nanoscale thin film heterostructures such as BaTiO3 /SrTiO3, where the internal wall structure can form polarization vortices. Such mixed character walls can be expected to exhibit dynamical wall properties distinct from pure Ising walls.
FIG. 3. (Color online) (a) Structural arrangement of LiNbO3 near the lowest energy y-wall position projected in the (0001) plane; (b) the three polarization components, Pn, Pt, and Pz as a function of the normal coordinate xn across a single y-wall. The solid lines are fits to simulation results. (c) z-direction displacement of Li (uz,LiI =0.001 Å and uz,LiII =0.023 Å) and Nb (uz,NbI =0.019 Å and uz,NbII =0.016 Å) atoms in the near the y-wall; and (d) Wall normal (un) and parallel (ut) displacements of Li (un,LiI =0.026 Å, un,LiII =0.012 Å, ut,LiI =0.029 Å, and ut,LiII =0.088 Å), and Nb atoms (un,NbII =0.001 Å, un,NbI =0.014 Å, ut,NbI =0.012 Å, and ut,NbII =0.026 Å) near a y-wall.
FIG. 4. (Color online) (a) Phase-field modeling of ferroelectric polarization vector distribution in a commensurate periodic (001)
(SrTiO3)4 / (001) (BaTiO3)8 superlattice with four 180° domain walls indicated by vertical dashed lines, described in the text. The 24 rows are spaced every half a unit cell, and the 32 columns are spaced every 0.5 nm. (b) The polarization vector variation in the z direction for columns A-H across a single domain wall. The polarization components, Px (or Pxn), Py (or Pxt), and Pz, the total polarization, Ptotal, and the Bragg (?B) and Néel (?N) are shown as a function of coordinate xn for the center of a SrTiO3 layer (panels e, h) center of a BaTiO3 layer (panels c, f), and the interface layer (panels d, g).