Engineering of relaxor-ferroelectric thin films via mechanical strain
For applications of ferroelectrics it is important to shift the ferroelectric phase transition Tc towards room temperature. It is demonstrated that Tc of epitaxially grown oxid films can be engineered via strain, e.g. in case of SrTiO3 tensile strain leads to an increase of Tc by more than 250K, whereas compressive strain reduces the transition temperature of NaNbO3 by more than 300K.
Relaxor Ferro- and Paraelectricity in Anisotropically Strained SrTiO3 Films
R. Wördenweber, J. Schubert, T. Ehlig, and E. Hollmann
Peter Grünberg Institute (PGI) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany
JOURNAL OF APPLIED PHYSICS Volume: 113 Issue: 16 Article Number: 164103
Published: APR 28 2013
Abstract: The ferroelectric properties of anisotropically strained SrTiO3 films are analyzed by detailed measurements of the complex dielectric constant as function of temperature, frequency, bias voltage and electric field direction. At low temperatures, strain induces a relaxor-ferroelectric phase that persists up to room temperature. The transition temperature and characteristic parameters (e.g. Curie temperature, static freezing temperature, degree of diffuseness of the phase transition, activation energy) of the relaxor phase depend strongly on the orientation of the electric field and therefore on the amount of structural strain in the given electric field direction. Also above the ferroelectric transition temperature a relaxation of the permittivity is visible, i.e. the strain causes a relaxor-paraelectric behavior. Only at high enough temperatures the relaxation time constant tends to zero and the ‘classical’ dielectric state is obtained. Frequency and time dependent relaxation experiments demonstrate an extremely large distribution of the relaxation rates in both relaxor states (ferroelectric and paraelectric) which is indicative for the large distribution in the mobility of polar SrTiO3 regions with randomly distributed directions of dipole moments in the film. The large distribution might be taken as an indication for a large distribution in size and orientation of nanosize domains in the anisotropically strained SrTiO3 film.
Impact of compressive in-plane strain on the ferroelectric properties of epitaxial NaNbO3 films on (110) NdGaO3
R. Wördenweber1, J. Schwarzkopf2, E. Hollmann1, A. Duk2, B. Cai1, M. Schmidbauer2
1Peter Grünberg Institute (PGI) and JARA-Fundamentals of Future Information Technology, Forschungszentrum Jülich, D-52425 Jülich, Germany ; 2Leibniz-Institute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, Germany
APPLIED PHYSICS LETTERS Volume: 103 Issue: 13 Article Number: 132908
Published: SEP 23 2013
Abstract: Epitaxial a-axis oriented NaNbO3 films are grown on (110) oriented NdGaO3 substrates. The lattice mismatch between substrate and film leads to compressive strain of ~0.7% in the a-c plane. As a consequence the in-plane permittivity and tunability are strongly enhanced compared to bulk NaNbO3 and a pronounced maximum in the temperature dependence of the permittivity occurs. Below the maximum at Tmax»250K ferroelectric behavior is observed that seems to vanish above Tmax. The pristine phase of the film at T<Tmax is antiferroelectric and is easily suppressed by small applied electric fields. The ferroelectric phase shows a relaxor type behavior.