Strained thin films


Epitaxial strain can induce displacive ferroelectric phase transitions in incipient ferroelectrics, which can be recognized by observation of anomalies in the frequencies of polar soft phonons. For that reason, our group developed a unique method of investigating phonons in ultrathin films using IR reflectance and THz transmission spectroscopy. In the past we demonstrated it in the tensile strained SrTiO3, Srn+1TinO3n+1, EuTiO3 and EuO. In superlattices of EuO/BaO we achieved 6.3 % tensile strain in EuO which has induced the ferroelectric phase transition near 100 K. In this way we prepared a new ferroelectric ferromagnet with possible high magnetoelectric coupling. We also investigated compressively strained MnO and NiO thin films, where a strain-dependent phonon splitting was observed in the antiferromagnetic phase. The effect was successfully explained by the strain dependence of the magnetic exchange coupling coefficient J1. In progress are studies of the strained Sr1-xBaxTiO3 with perovskite and Ruddlesden-Popper structure, where we observed an anomalously low microwave losses and high electric field tunability of the permittivity. Our experiments confirmed that the soft phonon behavior is responsible for the unique microwave properties.

Strained thin films

(a) Schematic picture of crystal structure of the perovskite thin film of EuTiO3  under biaxial tensile strain generated by DyScO3 substrate,
(b) Ruddlesden-Popper crystal structure of Srn+1TinO3n+1 under tensile strain.

  1. D. Nuzhnyy et al., Soft mode behavior in SrTiO3/DyScO3 thin films: Evidence of ferroelectric and antiferrodistortive phase transitions, Appl. Phys. Lett. 95, 232902 (2009).
  2. J.H. Lee et al., A strong ferroelectric ferromagnet created via spin-lattice coupling, Nature 466, 954-959 (2010).
  3. S. Kamba et al., Magnetodielectric effect and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT, Phys. Rev. B 85, 094435 (2012).
  4.  C.-H. Lee et al., Exploiting dimensionality and defect mitigation to create tunable microwave dielectricsNature 502, 532-536 (2013).
  5.  J. Petzelt and S. Kamba, Far infrared and terahertz spectroscopy of ferroelectric soft modes in thin films: A review, Ferroelectrics 503, 19-44 (2016).
  6. S. Skiadopoulou et al., Spin and lattice excitations of a BiFeO3 thin film and ceramics, Phys. Rev. B 91, 174108 (2015).
  7. N. M. Dawley et al., Targeted chemical pressure yields tuneable millimetre-wave dielectric, Nature Materials 19, 176-181 (2020).
  8. A. Kashir, et al., Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films, J. Phys.: Condens. Matter 32, 175402 (2020).
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