  Multiferroicity can be induced in strontium titanate by applying biaxial
strain, resulting in the coexistence of both ferroelectric and
antiferrodistortive domains. The magnitude and sign of the strain imposed on
the lattice by design can be used to tune the phase transitions and
interactions between these two phenomena. Using optical second harmonic
generation, we report a transition from centrosymmetric 4/mmm phase to
ferroelectric mm2, followed by an antiferrodistortive transition to a coupled
ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film strained
in biaxial tension by 0.94%. The results agree well with theoretical first
principles and phase-field predictions. Direct imaging of domains arising from
the ferroelectric phase transition, and its switching under electric fields is
demonstrated using piezoelectric force microscopy. Nonlinear optics combined
with phase-field modeling is used to show that the dominant multiferroic domain
switching mechanism is through coupled 90 degree ferroelectric-ferroelastic
domain wall motion. More broadly, these studies of coexisting ferroelectric
(polar) and antiferrodistortive rotation (axial) phenomena could have relevance
to multiferroics with coexisting ferroelectric (polar) and magnetic (axial)
phenomena.
