Strain-driven structure-ferroelectricity relationship in hexagonal TbMnO_3 films

摘要

Thin films and heterostructures of hexagonal manganites as promising multiferroic materials have attracted a considerable interest. We report structural transformations of high-quality strain-stabilized epitaxial hexagonal TbMnO_3/yttria stabilized zirconia( 111) (h-TMO) films, analyzed by means of various characterization techniques. A reversible structural phase transition from P6_3cm to P6_3/mmc structure at T_C ～ 800 K was observed in stoichiometric h-TMO films by temperature-dependent Raman spectroscopy and optical ellipsometry. The latter, directly probing the electronic system, indicates its modification at the structural phase transition, likely due to charge transfer from oxygen to Mn. A partially reversible phase transformation and stress relaxation was observed in h-TMO films with Tb excess after temperature cycling (300-1000-300 K) during Raman and ellipsometry. An inhomogeneous microstructure, containing ferroelectric and paraelectric nanodomains, was revealed by transmission electron microscopy in the Tb-rich film after annealing. The results obtained indicate a strong influence of stress, induced by temperature and by constrained sample geometry, onto the structure and ferroelectricity of strain-stabilized h-TMO thin films.