Control of the domain structures in multiferroic thin films is of crucial importance in order to gain access to their functional responses. Here we report on the evolution of the nanodomain structures observed in epitaxial thin films of multiferroic TbMnO3 grown on SrTiO3 substrates. Thin films with thickness ranging from 2 to 140 nm were grown at 0.25 and 0.9 mbar oxygen partial pressures. Transmission electron microscopy was employed to study and understand the domain evolution. A transition from a fully coherent, highly strained, tetragonal film to a partially coherent, distorted, orthorhombic structure occurs via a transformation twinning mechanism, giving rise to four different domain orientations. Then, at larger thicknesses, a transition to the fully relaxed orthorhombic structure occurs via changes in the domain-substrate orientation relationships leading to only two domain variants. Differences have been observed in the strain relaxation behavior for films grown at different oxygen pressures. All the observations have allowed us to understand the evolution of the domain structures and to accurately explain the measured orthorhombic distortion. This mechanism can be generalized for -oriented orthorhombic perovskites grown on cubic substrates with large misfit values.
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