Critical properties of nanoscale asymmetric ferroelectric tunnel junctions or capacitors

Critical properties, such as the phase transition temperature, critical thickness and Curie Weiss-type relation, of nanoscale asymmetric ferroelectric tunnel junctions or capacitors are investigated, considering the effects of size, surfaces, asymmetric interfaces and electrodes on the stability and magnitude of the two spontaneous polarization states. Using the modified thermodynamic model and taking into account contributions of the depolarization field, built-in electric field, interface and surface energies to the thermodynamic potential, explicit expressions of the critical properties are derived. For the asymmetric ferroelectric tunnel junction or capacitor, the results illustrate two important behaviors of vanishing critical thickness for the spontaneous polarization and smearing of the phase transitions, respectively. In addition, other critical properties are discussed as functions of the ambient temperature, misfit strain, surface coefficients, work function steps, dielectric constants and screening lengths of electrodes. Owing to the high-sensitivity of the critical properties to structures of asymmetric interfaces and electrodes, the results also suggest that the critical and other functional properties of nanoscale asymmetric ferroelectric tunnel junctions or capacitors can be completely controlled by adjusting the difference between asymmetric interfaces or electrodes. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.