doi:10.1038/nature13918

Progress in nanotechnology requires new approaches to materials synthesis thatmake it possible to control material functionality down to the smallest scales. An objective of materials research is to achieve enhanced control over the physical properties of materials such as ferromagnets, ferroelectrics and superconductors. In this context, complex oxides andinorganicperovskites are attractivebecause slight adjustments of their atomic structures can produce large physical responses and result inmultiple functionalities. In addition, these materials often contain ferroelasticdomains.The intrinsic symmetry breaking that takes place at the domain walls can induce properties absent fromthe domains themselves, such asmagnetic or ferroelectric orderand other functionalities, aswell as coupling between them.

Moreover, large domainwall densities create intense strain gradients, which can also affect the material's properties. Here we show that, owing to large local stresses, domain walls can promote the formation of unusual phases. In this sense, the domain walls can function as nanoscale chemical reactors.We synthesize a two-dimensional ferromagnetic phase at the domain walls of the orthorhombic perovskite terbiummanganite (TbMnO3), which was grown in thin layers under epitaxial strain on strontiumtitanate (SrTiO3) substrates.This phase is yet to be created by standard chemical routes. The density of the two-dimensional sheets can be tuned by changing the film thickness or the substrate lattice parameter (that is, the epitaxial strain), and it can be made at least as large as one sheet every 5 nanometres in ultrathin films, such that it represents up to 25 per cent of the filmvolume. The general concept of using domain walls of epitaxial oxides topromote the formation of unusual phasesmaybe applicable to othermaterials systems, thus giving access to new classes of nanoscale materials for applications in nanoelectronics and spintronics.