  We investigate the influence of a well-defined reversible biaxial strain
<=0.12 % on the magnetization (M) of epitaxial ferromagnetic manganite films. M
has been recorded depending on temperature, strain and magnetic field in 20 -
50 nm thick films. This is accomplished by reversibly compressing the isotropic
in-plane lattice parameter of the rhombohedral piezoelectric 0.72PMN-0.28PT
(001) substrates by application of an electric field E <= 12 kV cm-1. The
magnitude of the total variable in-plane strain has been derived.
Strain-induced shifts of the ferromagnetic Curie temperature (Tc) of up to 19 K
were found in La0.7Sr0.3MnO3 (LSMO) and La0.7Ca0.3MnO3 films and are
quantitatively analysed for LSMO within a cubic model. The observed large
magnetoelectric coupling coefficient alpha=mu0 dM/dE <= 6 10-8 s m-1 at ambient
temperature results from the strain-induced M change in the
magnetic-film-ferroelectric-substrate system. It corresponds to an enhancement
of mu0 DeltaM <= 19 mT upon biaxial compression of 0.1 %. The extraordinary
large alpha originates from the combination of three crucial properties: (i)
the strong strain dependence of M in the ferromagnetic manganites, (ii) large
piezo-strain of the PMN-PT substrates and (iii) effective elastic coupling at
the film-substrate interface.
