Magnetostatic surface modes in a thin film with nonuniform in-plane fields

The experimental observation of magnetostatic surface waves (MSSW´s) in a rectangular YIG film placed between strips of permalloy and in the plane of the strips is reported. In such a configuration, the DC magnetic field between the permalloy strips is primarily in-plane with a minimum halfway between and increasing toward each strip. In the presence of such a gradient, a series of discrete modes appears near the low frequency end of the normally continuous MSSW band. The frequencies of these modes lie below the bottom of the conventional MSSW band as calculated from the field at the center of the sample where the field is minimum. However, reversing the bias field reduces the coupling to these modes indicating a nonreciprocal surface localization. Estimates of the group velocities of these modes based on the slope of phase vs. frequency data indicate that the discrete modes travel several times faster than conventional MSSW´s.