Tuesday, March 6, 2012, 9:30 am — Seminar Room, Bldg. 725
In thin films, the biaxial strain imposed by heteroepitaxial growth can cause the
appearance of novel properties wholly different from those of the bulk material. Two
interesting examples are lanthanum cobaltite (LaCoO3) and europium titanate (EuTiO3).
In bulk form, LaCoO3 has a non-magnetic ground state. When synthesized as a thin film,
the ground state of LaCoO3 becomes ferromagnetic with a Curie temperature near 85 K.
EuTiO3 is a paraelectric-antiferromagnet that becomes a ferroelectric-ferromagnet under
biaxial tensile strain. In order to examine the origins of these phenomena, we have
examined the atomic and electronic structures of epitaxial LaCoO3 and EuTiO3 thin films
using x-ray absorption fine structure (XAFS) spectroscopy and x-ray diffraction (XRD).
Highly strained LaCoO3 deposited on SrTiO3 has a pseudomonoclinic structure with a
large difference between in-plane and out-of-plane Co-O bond lengths, which strongly
affects the Co-O orbital hybridization in the material. However, increased hybridization is
not the cause of ferromagnetism in LaCoO3. Instead, strain induced distortions of the
oxygen octahedra increase the population of unpaired spins beyond a stabilization
threshold for ferromagnetic order. In EuTiO3 films deposited on DyScO3, a strong
coupling between ordering of the oxygen octahedral rotations and the orientation of the
ferroelectric polarization is observed.
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