The Gopalan Group

Research of Photonic Materials, Probes, and Devices

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Ferroelectric: Strained Films

Influence of anisotropic strain on the dielectric and ferroelectric properties of SrTiO3thin films on DyScO3 substrates

M. D. Biegalski, E. Vlahos,G. Sheng, Y. L. Li, L. Q. Chen, V. Gopalan, S. Trolier-Mckinstry, D. G. Schlom, S. K. Streiffer, M. Bernhagen, P. Reiche and R. Uecker, “,” Phys. Rev. B,. 79, 224117 (2009). PDF

Abstract

The in-plane dielectric and ferroelectric properties of coherent anisotropically strained SrTiO3 thin films grown on orthorhombic (101) DyScO3 substrates were examined as a function of the angle between the applied electric field and the principal directions of the substrate. The dielectric permittivity revealed two distinct maxima as a function of temperature along the [100]p and [010]p SrTiO3 pseudocubic directions. These data, in conjunction with optical second-harmonic generation, show that the switchable ferroelectric polarization develops first predominantly along the in-plane axis with the larger tensile strain before developing a polarization component along the perpendicular direction with smaller strain as well, leading to domain twinning at the lower temperature. Finally, weak signatures in the dielectric and second-harmonic generation response were detected at the SrTiO3 tilt transition close to 165 K. These studies indicate that anisotropic biaxial strain can lead to new ferroelectric domain reorientation transitions that are not observed in isotropically strained films.

Sample Figure

FIG. 10. (Color online) Representative domain structures from phase-field simulations following each of the three transitions for (a) ?12=5.5x10?12 cm6 dyn/esu4 and (b) ?12=1.7x10?12 cm6 dyn/esu4. Different colors are used to identify the four kinds of domain variants for both (100) and (110) polar states and the directions of the vectors p and q are shown by the arrows.


Stripe Vertical Domain Walls of Epitaxial (001) BiFeO3 Thin Films on Orthorhombic TbScO3 Substrate

 

C. M. Folkman, S.H. Baek, H.W. Jang, C. B. Eom, C. T. Nelson, X.Q. Pan, Y. L. Li, L. Q. Chen, A. Kumar, V. Gopalan, and S. K. Streiffer, Appl. Phys. Lett., 94 , 251911 (2009). PDF

Abstract

We have analyzed the ferroelastic and ferroelectric domain structure of high crystalline quality (001) BiFeO3 films on orthorhombic (110) TbScO3 substrates. Two domains were present in stripes separated by (010) vertical boundaries, with spontaneous polarizations in adjacent domains rotated by 109°. The striped morphology was caused by nucleation of only two ferroelastic domains on the low symmetry GdFeO3-type substrate. Domain engineering through substrate symmetry is an important finding for rhombohedral ferroelectric epitaxial thin films. The stripe pattern with vertical walls may be useful for extracting domain wall contributions to magnetism and electrical transport properties of BiFeO3 materials.

Sample Figure

FIG. 2. (Color online) AFM topography (a) of a BiFeO3 film on TbScO3 substrate and a dark field TEM cross section (b) of the BiFeO3 film on TbScO3 substrate. A high resolution image is shown of a vertical domain boundary (c) (marked with arrows).


Magnetic Color Symmetry of Lattice Rotations in a Diamagnetic Material

 

S. Denev, A. Kumar, M. Biegalski, H. W. Wang, C. M. Folkman, A. Vasudevarao, Y. Han, I. M. Reaney, S. T. Mckinstry, C. B.- Eom, D. G. Schlom and V. Gopalan, Phys. Rev. Lett., 100, 257601 (2008). PDF

Abstract

Oxygen octahedral rotations are the most common phase transitions in perovskite crystal structures. Here we show that the color symmetry of such pure elastic distortions is isomorphic to magnetic point groups, which allows their probing through distinguishing polar versus magnetic symmetry. We demonstrate this isomorphism using nonlinear optical probing of the octahedral rotational transition in a compressively strained SrTiO3 thin film that exhibits ferroelectric (4mm) and antiferrodistortive (4'mm') phases evolving through independent phase transitions. The approach has broader applicability for probing materials with lattice rotations that can be mapped to color groups.

Sample Image

FIG. 1 (color online). AFD transition in SrTiO3 manifested by rotation of the neighboring oxygen octahedra in opposite direction by angle ? resulting in an enlarged unit cell with lattice parameters ?2ap x ?2ap x 2ap where ap is the pseudocubic
lattice parameter. The ferroelectric polarization vector p and the AFD vector q are pointing out of the plane. Also shown in green are the resulting symmetry elements of the 4'mm' point group.