Optical spectroscopy - Interface and Strain Effects in Epitaxial Perovskite Structures

Perovskite-type metal oxides exhibit variety of electronic phases and ferroic orderings that makes these materials multifunctional and enables numerous applications. Conceptually new devices can be created using advanced oxides, in which composition-dependent or epitaxy-controlled nanoscale phenomena lead to creation of novel or improved response functions. One of the fundamental problems here is a detection of the phase states in ultra–thin perovskite epitaxial films (couple of nm thick). Our team succeeded in the analysis revealing the phase states of such films using the temperature-dependent ellipsometric studies. For example, in the work [1] we demonstrated the existence of the predicted strain- induced ferroelectric ordering in quantum paraelectric KTaO₃ epitaxial films by combining experimental study and density functional theory calculations. The most important proof of the existence of the ferroelectric state was obtained from ellipsometry experiment.

               

SQUID measured sample magnetization as a function of external magnetic field determined at room temperature in as-deposited films of KTaO₃/SrTiO₃ (red curve), KNbO₃/SrTiO₃ (blue curve), and NaNbO₃/SrTiO₃ (black curve).

Our team designed the optical setup for high temperature measurements and performed the optical experiments. All partners contributed to the data analysis and manuscript preparation. For our team this work established the basement for optical study of ultra-thin perovskite films and helped us to develop new experimental strategy. The next fundamental problem in study of these perovskite ultra-thin systems is knowledge of electronic energies and excitations. Through combining optical ellipsometric studies, examination of the ferroelectric states, and materials characterization, we intend to explore connections between electronic transitions and ordering behavior that contribute to fundamental understanding of perovskite-type oxides and nanoscale phenomena therein. In this direction we recently found d0 ferromagnetism at a charge-imbalanced interface between two non-magnetic perovskites. Epitaxial d0 SrTiO₃/KTaO₃, SrTiO₃/KNbO₃, and SrTiO₃/NaNbO₃ interfaces were observed by transmission electron microscopy and their optical properties were determined from spectroscopic ellipsometry measurements [2]. Magnetic measurements of these samples exhibited the ferromagnetic hysteresis at room temperature. Even though the observed phenomenon is mainly important for the basic research, its application potential is also very significant. Use of such interface-induced magnetism allows the development of new materials combining ferroelectric and ferromagnetic ordering (magnetoelectric multiferroics) without any conventional magnetic materials.
The next our achievement in the research of thin films under various strain conditions is the complex optical study of epitaxial PbSc_0.5Nb_0.5O₃ (PSN) films. In particular, it was shown that a biaxial epitaxial compression can favor the relaxor state appearance over ferroelectricity. The absence of the ferroelectric transition and the existence of the low temperature relaxor state are evidenced by a combination of dielectric, polarization, and optical studies of epitaxial PSN films [3]. This finding is beyond existing models of polarization in perovskite structure epitaxial films and beyond the established ability to induce ferroelectricity by an epitaxial strain. Moreover, the dramatic changes in the spectra of the dielectric functions and the absorption coefficient are found under various strain conditions. A frustration of the ferroelectric phase transition is evidenced by thermooptical studies. A complex relationship between strain, polarization, and optical properties is discussed in terms of possible ionic displacements in metrically tetragonal PSN films [4].

References
<sub>1. M. Tyunina, J. Narkilahti, M. Plekh, R. Oja, R. M. Nieminen, A. Dejneka, V. Trepakov, Evidence for Strain-Induced Ferroelectric Order in Epitaxial Thin-Film KTaO3. Phys. Rev. Lett. (2010) 104: 227601.
2. R. Oja, M. Tyunina, L. Yao, T. Pinomaa, T. Kocourek, A. Dejneka, O. Stupakov, M. Jelinek, V. Trepakov, S. van Dijken, R. M. Nieminen d0 Ferromagnetic Interface between Nonmagnetic Perovskites. Phys. Rev. Lett. (2012) 109: 127207.
3. Tyunina, Marina ; Levoska, J. ; Janolin, P.E. ; Dejneka, Alexandr. Low temperature relaxor state induced by epitaxial compression in PbSc0.5Nb0.5O3 films. Phys. Rev. B (2013) 87: 2241071.
4. Lynnyk, Anna ; Chvostová, Dagmar ; Pacherová, Oliva ; Kocourek, Tomáš ; Jelínek, Miroslav ; Dejneka, Alexandr ; Tyunina, Marina. Optical properties of epitaxial relaxor ferroelectric PbSc0.5Nb0.5O3 films. Appl. Phys. Lett. (2013) 103: 132901.</sub>