  Ferroelectric domain nucleation and growth in multiferroic BiFeO3 films is
observed directly by applying a local electric field with a conductive tip
inside a scanning transmission electron microscope. The nucleation and growth
of a ferroelastic domain and its interaction with pre-existing 71^{\circ}
domain walls are observed and compared with the results of phase-field
modeling. In particular, a preferential nucleation site and direction-dependent
pinning of domain walls is observed due to slow kinetics of metastable
switching in the sample without a bottom electrode. These in-situ
spatially-resolved observations of a first-order bias-induced phase transition
reveal the mesoscopic mechanisms underpinning functionality of a wide range of
multiferroic materials.
