Proposal for Obtaining Ultrasmall Ferromagnetic-Resonance Linewidths

It is shown that it should be possible to reduce the dominant two-magnon linewidth well below the smallest value obtained to date by choosing the radius $R$ and thickness $S$ of a thin film of ferromagnetic insulator such that there are no modes degenerate with the main-resonance mode. For a sufficiently thin sample ($S\lesssim 1\mu$) the frequencies of the exchange modes are well above that of the main-resonance mode, and for a sufficiently small aspect ratio ($\frac{2R}{S}\lesssim 3\times {10}^3$) the low-order magnetostatic modes are also well resolved. In the limit of no two-magnon scattering, the resonance linewidth of yttrium iron garnet at 9.3 GHz approaches 0.2 Oe at 300°K, or a few mOe at 4.2°K.