Stability of spontaneous polarization in ultrathin improper ferroelectrics.

The competition between the substrate-induced strain and the electrostatic polarization in improper ferroelectric films (hexagonal YMnO3) has been studied using optical second-harmonic generation techniques. Deposited on a (111) surface of yttria-stabilized zirconia substrate, the films exhibit (1) strong strain with negligible polarization, (2) relaxation accompanied by the nonlinear buildup of the polarization, and (3) bulklike polar behavior, in the order of increasing film thickness. The role of the substrate is distinctively different from that in proper ferroelectrics. [ABSTRACT FROM AUTHOR]