A current challenge in the field of magnetoelectric multiferroics is toidentify systems that allow a controlled tuning of states displaying distinctmagnetoelectric responses. Here we show that the multiferroic ground state ofthe archetypal multiferroic TbMnO3 is dramatically modified by epitaxialstrain. Neutron diffraction reveals that in highly strained films the magneticorder changes from the bulk-like incommensurate bc-cycloidal structure tocommensurate magnetic order. Concomitant with the modification of the magneticground state, optical second-harmonic generation (SHG) and electricmeasurements show an enormous increase of the ferroelectric polarization, and achange in its direction from along the c- to the a-axis. Our results suggestthat the drastic change of multiferroic properties results from a switch of thespin-current magnetoelectric coupling in bulk TbMnO3 to symmetricmagnetostriction in epitaxially-strained TbMnO3. These findings experimentallydemonstrate that epitaxial strain can be used to control single-phasespin-driven multiferroic states.
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