Abstract
The coupling strain in nanoscale systems can achieve control of the physical properties in functional materials, such as ferromagnets, ferroelectrics, and superconductors. Here, we directly demonstrate the atomic-scale structure of super-tetragonal PbTiO3 nanocomposite epitaxial thin films, including the extraordinary coupling of strain transition and the existence of the oxygen vacancies. Large strain gradients, both longitudinal and transverse (similar to 3 x 10(7) m(-1)), have been observed. The original non-magnetic ferroelectric composites notably evoke ferromagnetic properties, derived from the combination of Ti3+ and oxygen vacancies. The saturation ferromagnetic moment can be controlled by the strain of both the interphase and substrate, optimized to a high value of similar to 55 emu/cc in 10-nm thick nanocomposite epitaxial thin films on the LaAlO3 substrate. Strain engineering provides a route to explore multiferroic systems in conventional non-magnetic ferroelectric oxides and to create functional data storage devices from both ferroelectrics and ferromagnetics.