Magnon polarons induced by a magnetic field gradient

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