  Epitaxial strain plays an important role in determining physical properties
of perovskite ferroelectric oxide thin films. However, it is very challenging
to directly measure properties such as polarization in ultrathin strained films
using traditional sandwich capacitor devices, because of high leakage current.
We employed a planar electrode device with different crystallographical
orientations between electrodes along different electric field orientation to
directly measure the in-plane polarization-electric field (P-E) hysteresis
loops in fully strained thin films. At high misfit strains such as -4.4%, the
pure Tetrogonal-like phase is obtained and its polarization vector is
constrained to lie in the (010) plane with a significantly large in-plane
component, ~44 {\mu}C/cm2. First-principle calculations are carried out in
parallel, and provide a good agreement with the experimental results. Our
results pave the way to design in-plane devices based on T-like BFO and the
strategy proposed here can be expanded to study all other similar strained
multiferroic ultrathin films.
