Finite size effects on ferroelectricity and spin-orbit coupling in the Aurivillius phases

Ferroelectric oxides show promise for use in nonvolatile ferroelectric random access memory (FRAM). Materials which retain ferroelectric polarization in ultrathin films improves scalability of FRAM devices. Previous research found that ferroelectric BaTiO3 loses ferroelectricity at a critical thickness owing to a depolarizing field[1]. Bismuth tungstate, Bi2WO6, is the n=1 member of the ferroelectric layered Aurivillius phases (Bi2O2)(An-1BnO3n+1). Through first principles calculations based on density functional theory, we identified several polymorphs of Bi2WO6 in close energetic competition. We determined the impact of strain on the polymorphs and their properties. By simulating ferroelectric capacitors of varying numbers of layers, we examined whether a critical thickness similarly exists for Bi2WO6, and describe its physical mechanism. Due to broken inversion symmetry and large spin-orbit coupling, Bi2WO6 exhibits unidirectional out-of-plane spin texture, which is reversible under polarization switching[2]. Through finite thickness simulations we will determine how the Rashba spin-splitting, which is linked to polarization, may be impacted by finite size effects.

1. J. Junquera, and P. Ghosez. Nature, vol. 422, no. 6931, 2003.

2. H. Djani, et al. Npj Quant Mat, vol. 4, no. 1, 2019.