Localized high-Q ferromagnetic resonance in nonuniform magnetic fields

We report the observation of highly localized, unusually sharp resonances in microwave coupling between closely spaced antennas on the surface of a bulk single crystal YIG slab. The dc magnetic field is applied perpendicular to the slab. In a non-optimized experimental configuration, typical half-power bandwidth is 1.8 MHz at 2.5 GHz with insertion loss less than 10 dB. The resonances are magnetically tunable and therefore may prove useful in a variety of microwave filter applications. We hypothesize that the resonances result from the confinement of the mode energies to certain regions of high dc field gradient within the crystal. This selective confinement is made possible, in part, by the use of highly localized microwave excitation and sensing antennas. The resulting "gradient-bound" modes propagate along apparent internal "surfaces" which are effectively created by naturally occurring or artificially induced internal magnetic field gradients. Modes propagate along such surfaces without experiencing the loss mechanisms encountered at true surfaces. Consequently, the Q of the resulting resonance is primarily a result of the intrinsic linewidth of the bulk crystal along with normal microwave circuit loading considerations.