  In this work, we report the theoretical possibility of generating magnon
polaron excitations through a space-varying magnetic field. The spatial
dependence of the magnetic field in the Zeeman interaction gives rise to a
magnon-phonon coupling when a magnetic field gradient is applied, and such a
coupling depends directly on the strength of the gradient. It is also predicted
that the direction of the magnetic field gradient allows control over which
phonon polarization couples to the magnons in the material. Here we develop the
calculations of the magnon-phonon coupling for an arbitrary (anti)ferromagnet,
which are later used to numerically study its consequences. These results are
compared to the ones obtained with the phenomenological magnetoelastic coupling
in YIG, where we show that the magnon polaron bandgap seen in YIG can be also
obtained with a magnetic field gradient of $\sim 0.1$T/m which can be achieved
with the current experimental techniques. Our results propose a new way of
controlling the magnetoelastic coupling in an arbitrary material and open a new
route to exploit the magnon-phonon interaction in magnonic and spintronic
devices.
