Fig. 1
Evolution of improper polarization in YMnO3 thin films. a Schematic of the ferroelectric P63cm YMnO3 crystal structure. The lattice trimerization Q is given by the tilt amplitude Q and azimuthal tilt angle φ of the MnO5 bipyramid (as defined in the inset viewed along [001]), and equivalently by the sinusoidal corrugation pattern of Y atoms. The dashed line outlines the YMnO3 unit cell. b Temperature dependence of the ISHG intensity from a 10-unit-cell YMnO3 film grown on YSZ upon cooling in the thin-film growth environment. The dashed line indicates the background level caused by surface contributions. The onset of Ps at the ferroelectric critical temperature TCfilm (indicated by the arrow) breaks inversion symmetry, giving rise to an ISHG wave with an amplitude proportional to Ps. c Ferroelectric transition temperatures for different YMnO3 film thicknesses. TCfilm is extracted from ISHG measurements as in a. Films below three unit cells show no spontaneous polarization down to room temperature, indicated by the hatched gray area. Note that the thin-film crystalline quality could be maintained up to a thickness of 120 unit cells (<140 nm). Within this investigated range, TCbulk = 1259 K (gray horizontal line) has not yet been reached. The dashed line, serving as a guide to the eye, highlights the thickness trend.
