CNMS User Research
Method to Reversibly and Uniformly Strain Epitaxial Oxide Thin Films
Using a Piezoelectric Substrate
Dörr,1 M.D. Biegalski,2 D.H. Kim,3-4 and
H. M. Christen2
1CNMS USER Institute for Metallic Materials, IFW Dresden,
2Center for Nanophase Materials Sciences, Oak Ridge National
Laboratory, Oak Ridge, Tennessee
3Materials Science and Technology Division, Oak Ridge National
Laboratory, Oak Ridge, Tennessee
4Department of Physics, Tulane University, New Orleans,
A method to vary reversibly the strain in a single epitaxial film,
while keeping all other parameters constant, is crucial to understanding
the properties of nanoscale thin film materials. We have shown that
the strain exerted from a piezoelectric PMN-PT (0.72Pb(Mg1/3,Nb2/3)O3-0.28
PbTiO3) substrate is reversible, and is uniform within
the plane and through the thickness of an epitaxial thin film stack
pulsed laser deposition. The uniformity of strain in this method
makes it singularly useful for investigations of elastic properties
in epitaxial thin films. This novel method gives a direct window
into the effects of strain, eliminating the effects of changing defect
structure and composition inherent in earlier methods based on comparing
sets of films with different lattice-mismatch or thicknesses.
In this study, piezoelectric substrates of pseudocubic PMN-PT were
utilized to exert uniform and reversible strain to single-crystal epitaxial
perovskite films. An electric field applied across the crystal significantly
changes the lattice parameter of the PMN-PT crystal, as measured using
4-circle x-ray diffraction. This work shows that the biaxial strain
is fully transferred to epitaxial films, regardless of defects and
buffer layers. Using this method, strains in excess of 0.15% are readily
achieved, and were remarkably linear with the applied voltage. Additionally,
the strain achieved for a given voltage is essentially temperature-independent
(from 80K to room temperature), enabling temperature-dependent strain-effect
measurements. In-plane and out-of-plane lattice parameters can be determined
for different films, from which the Poisson ratio of these materials
can be determined directly without the need for additional information.
Results for SrTiO3, BiFeO3, LaScO3 and MgO are shown in Table 1. In
addition to the measurement of mechanical properties, the technique
enables measurement of the effects of strain on ferroelectric behavior
(e.g. BiFeO3) with the results showing a remarkable agreement with
This straightforward method provides a general and valuable technique
to study the effects of strain on a great variety of material properties.
This technique is applicable not only for epitaxial oxide thin films,
but for a wide range of materials including metastable materials
and nanostructured films. The distinct advantage of this technique
over others is that inherent trends in material properties can be
extracted directly from single samples, eliminating sample-to-sample
artifacts and variations.
D. Biegalski, K. Dörr, D. H. Kim, and H. M. Christen, “Reversible
Uniform Strain in Epitaxial Oxide Films,” manuscript
Thiele, K. Dörr, O. Bilani, J. Rödel and L.
of Strain on the Magnetization and Magnetoelectric Effect in
La0.7A0.3MnO3/PMN-PT(001) (A=Sr,Ca),” Phys.
Rev. B 75, 054408 (2007).
Bilani-Zeneli, A. D. Rata, A. Herklotz, O. Mieth, L. M. Eng, L. Schultz,
M. D. Biegalski, H. M. Christen, and K. Dörr, “SrTiO3 on
Piezoelectric PMN-PT(001) for Application of Variable Strain,” J.
Appl. Phys. 104, 054108
||0.242 ± 0.015
||0.34 ± 0.021
||0.235 ± 0.018
1: Table of measured Poisson ratios for several oxide thin films
and their bulk values. The values of Poisson’s ratio
are similar to bulk values for the SrTiO3 films and similar to the
for BiFeO3 thin films. LaScO3 has never been measured; these results
show the flexibility of the technique. The MgO film shows a deviation
from the bulk properties because of residual strain in film illustrating
the importance of measuring the elastic properties of thin films.
Figure 1: Schematic of the induced strain in the film stack due to
an applied voltage across the sample. The applied voltage, due to the
piezoelectric effect, causes the substrate to elongate along the direction
of the applied voltage and shrink perpendicular to applied field direction.
This shrinking is transferred to the film being examined and causes
a biaxial strain.
Figure 2: Lattice constant of (a) PMN-PT Substrate, (b) SrTiO3 and
(c) BiFeO3 as a function of applied voltage to the substrate with at
maximum field of ~15 kV/cm. The results show that the change in lattice
parameter is linear as a function of the applied voltage. This indicated
the changes in the unit cell of film as a function of strain.
Figure 3: Ferroelectric properties of a BiFeO3 thin film grown
on a PMN-PT substrate with a (La0.80,Sr0.20)MnO3 bottom electrode.
polarization is aligned with the pseudocubic (111) direction in
bulk BiFeO3 and is only weakly dependent on strain. However, the
projection of the remanent polarization along the (001) direction
changes slightly, as predicted theoretically, but the coercive
field has a strong and previously unknown dependence on strain.