In-Situ Investigation of the Ferroelectric Phase Transition in Improper Ferroelectric YMnO₃ Thin Films by Electron Energy Loss Spectroscopy

The functional properties of many materials are closely related to symmetry-changing phase transitions. In particular, many perovskite materials undergo a temperature-driven phase transition at the so-called Curie temperature (<i>T</i>_c) from a non-polar paraelectric (PE) phase at high temperature to a lower-temperature, non-centrosymmetric polar ferroelectric (FE) phase. The spontaneous polarization exhibited by ferroelectric materials below <i>T</i>_c has made them promising candidates for non-volatile memories. In improper ferroelectrics, the phase transition is governed by a primary order parameter, which is independent of electrostatics, and ferroelectric polarization arises as a secondary effect of this order parameter. As a result, in contrast to proper ferroelectrics, the ferroelectric properties of improper ferroelectrics are expected to be robust against the detrimental effects of the depolarizing field, which is important for the continued miniaturization of ferroelectric devices, down to the ultrathin limit.<br/>Of the various known improper ferroelectrics, the hexagonal YMnO₃ (YMO) has attracted much attention due to its multiferroic properties, vortex-antivortex topological domain configurations, conducting domain walls and magnetoelectric coupling. In its paraelectric phase, it crystallizes in the centrosymmetric <i>P</i>6₃/<i>mmc</i> space group, consisting of corner-sharing MnO₅ bipyramids alternating with Y³⁺ ion layers along the <i>c</i>-axis. A structural phase transition to the noncentrosymmetric <i>P</i>6₃<i>cm</i> space group occurs at a <i>T</i>_c of ∼997 °C, when the unit cell triples as a result of a zone-boundary K₃ phonon-mode condensation driven by a tilting of the MnO₅ bipyramids around the Y³⁺ ions and a buckling of the Y layers. However, the exact details of the electronic structure in YMO during the phase transition have remained unclear to date [1-4].<br/>In this talk, we discuss how the electronic structure of YMO epitaxial thin films changes across the PE-FE phase transition, as observed by in-situ heating experiments in the transmission electron microscope. Specifically, our electron energy loss spectroscopy observations clarify some of the remaining uncertainties about the electronic structure of YMO at the PE-FE phase transition. This information is crucial for the control of exotic polarization states and the development of emerging ferroelectric-based electronics [5].<br/><br/>[1] B. B. Van Aken<i> et al.</i> <i>Nat. Mater.</i> <b>3</b>, 164 (2004).<br/>[2] D.-Y. Cho <i>et al. Phys. Rev. Lett. </i><b>98</b>, 217601 (2007).<br/>[3] J. Kim <i>et al.</i> <i>Appl. Phys. Lett.</i> <b>95</b>, 132901 (2009).<br/>[4] Y. Kumagai <i>et al.</i> <i>Phys. Rev. B</i> <b>85</b>, 174422 (2012).<br/>[5] A. Vogel <i>et al.</i> <i>Phys. Rev. B</i> <b>107</b>, 224107 (2023).