  A current challenge in the field of magnetoelectric multiferroics is to
identify systems that allow a controlled tuning of states displaying distinct
magnetoelectric responses. Here we show that the multiferroic ground state of
the archetypal multiferroic TbMnO3 is dramatically modified by epitaxial
strain. Neutron diffraction reveals that in highly strained films the magnetic
order changes from the bulk-like incommensurate bc-cycloidal structure to
commensurate magnetic order. Concomitant with the modification of the magnetic
ground state, optical second-harmonic generation (SHG) and electric
measurements show an enormous increase of the ferroelectric polarization, and a
change in its direction from along the c- to the a-axis. Our results suggest
that the drastic change of multiferroic properties results from a switch of the
spin-current magnetoelectric coupling in bulk TbMnO3 to symmetric
magnetostriction in epitaxially-strained TbMnO3. These findings experimentally
demonstrate that epitaxial strain can be used to control single-phase
spin-driven multiferroic states.
