  Multiferroics offer an elegant means to implement voltage-control and on the
fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic
materials. These properties are particularly interesting for the field of
magnonics, where spin waves are used to perform advanced logical or analogue
functions. Recently, the emergence of nano-magnonics {\color{black} is expected
to} eventually lead to the large-scale integration of magnonic devices.
However, a compact voltage-controlled, on demand reconfigurable magnonic system
has yet to be shown. Here, we introduce the combination of multiferroics with
ferromagnets in a fully epitaxial heterostructure to achieve such
voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant
electrical polarization in thin multiferroic $\mathrm{BiFeO_3}$ with a
periodicity of $500\,\mathrm{nm}$ yields a modulation of the effective magnetic
field in the micron-scale, ferromagnetic $\mathrm{La_{2/3}Sr_{1/3}MnO_3}$
magnonic waveguide. We evidence the magneto-electrical coupling by
characterizing the spin wave propagation spectrum in this artificial, voltage
induced, magnonic crystal and demonstrate the occurrence of a robust magnonic
bandgap with $>20 \,\mathrm{dB}$ rejection.
