A. S. Anokhin

Strain engineering of perovskite thin films using a single substrate

Combining temperature-dependent x-ray diffraction, Raman spectroscopy and first-principles-based effective Hamiltonian calculations, we show that varying the thickness of (Ba0.8Sr0.2)TiO3 (BST) thin films deposited on the same single substrate (namely, MgO) enables us to change not only the magnitude but also the sign of the misfit strain. Such previously overlooked control of the strain allows several properties of these films (e.g. Curie temperature, symmetry of ferroelectric phases, dielectric response) to be tuned and even optimized. Surprisingly, such desired control of the strain (and of the resulting properties) originates from an effect that is commonly believed to be detrimental to functionalities of films, namely the existence of misfit dislocations. The present study therefore provides a novel route to strain engineering, as well as leading us to revisit common beliefs.

Phonon and magnon excitations in Raman spectra of an epitaxial bismuth ferrite film

An epitaxial film of bismuth ferrite BiFeO3 on a MgO(001) single-crystal substrate has been prepared by pulsed laser deposition using SrTiO3 and SrRuO3 buffer layers. At room temperature, the polarization characteristics of the Raman spectra of the BiFeO3 film under study suggest a monoclinic symmetry. The high-temperature (295–1100 K) investigations of the Raman spectra have been performed in the frequency range 20 cm−1 < ν < 1600 cm−1. Particular attention has been paid to the high-frequency region with a band observed at 610 cm−1, which corresponds to the maximum density of states of the magnon branch at the Brillouin zone boundary, and an intense band in the second-order Raman spectra with the maximum at ∼1250 cm−1, which corresponds to the density of states of two-magnon excitations. It has been found that the intensity of the band at ∼1250 cm−1 decreases linearly with an increase in the temperature and, above 650 K, this band is absent. The extrapolation of the temperature dependence of the integrated intensity of the band at 1250 cm−1 suggests that this film undergoes an antiferromagnetic phase transition at a temperature of ∼670 K.

Effect of growth mechanisms on the deformation of a unit cell and polarization reversal in barium–strontium titanate heterostructures on magnesium oxide

The effect of a growth mechanism on the unit cell strain and the related change in the properties of single-crystal Ba0.8Sr0.2TiO3 films grown on MgO substrates according to the Frank–van der Merwe and Volmer–Weber growth mechanisms is studied. The unit cell strain is shown to depend substantially on the film thickness and the growth mechanism. It is found that the same film–substrate pair can be used to vary stresses in the film from two-dimensional tensile to compressive stresses due to a change in the growth mechanism and the film thickness.

Phase transitions in barium–strontium titanate films on MgO substrates with various orientations

A comparative study of the lattice dynamic upon phase transitions in a polycrystalline Ba0.8Sr0.2TiO3 (BST) film on a Pt substrate and in epitaxial BST films grown on various sections of an MgO substrate has been performed by Raman spectroscopy. It has been found that different sequences of phase transitions take place in these films. The BST/Pt films demonstrate the same sequence of phase transitions that is observed in the bulk ceramics. The hardening of a soft mode in BST/(001)MgO and BST/Pt films shows that the transition from the tetragonal ferroelectric phase to the paraelectric phase has features of the displacement-type phase transition and also the order–disorder phase transition. When approaching the ferroelectric transition temperature, the soft mode in the BST/(111)MgO film is softened, following the Cockran law, which indicates the displacement-type phase transition.

Dynamic Spectral Response of Solid Solutions of the Bismuth-Strontium Ferrite Bi 1- x Sr x FeO 3- δ in the Frequency Range 0.3-200 THz

Ceramic samples of Bi1 − xSrxFeO3 − δ have been investigated using X-ray diffraction analysis, Raman spectroscopy, and infrared spectroscopy. It has been shown that the spectra of these solid solutions should be considered as a superposition of resonant phonon responses of the cubic perovskite and a relaxation response. No relaxators are observed in the pure compounds BiFeO3 and SrFeO3 − δ. It has been found that two concentration phase transitions occurs with the symmetry changes

Structure and lattice dynamics of solid solutions (1 − x)BiFeO3-xANbO3 (A = K, Na)

R3c \leftrightarrow Pm\bar 3m

Structure and lattice dynamics of heterostructures based on bismuth ferrite and barium strontium titanate

in the range 0.1 < x < 0.2 and