  Tuning functional properties of thin caloric films by mechanical stress is
currently of high interest. In particular, a controllable magnetisation or
transition temperature is desired for improved usability in magnetocaloric
devices. Here, we present results of epitaxial magnetocaloric Ni-Mn-Ga-Co thin
films on ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)$_{0.72}$Ti$_{0.28}$O$_3$
(PMN-PT) substrates. Utilizing X-ray diffraction measurements, we demonstrate
that the strain induced in the substrate by application of an electric field
can be transferred to the thin film, resulting in a change of the lattice
parameters. We examined the consequences of this strain on the magnetic
properties of the thin film by temperature and electric field dependent
measurements. We did not observe a change of martensitic transformation
temperature but a reversible change of magnetisation within the austenitic
state, which we attribute to the intrinsic magnetic instability of this
metamagnetic Heusler alloy.
