Abstract: A process is disclosed for fabricating narrow line-width yttrium iron garnet (YIG) disks suitable for microwave applications. The process comprises forming an epitaxial thin film of yttrium iron garnet, containing from about 0.5 to 1.5 atom percent trivalent lanthanum ions on the dodecahedral sites, on a substrate such as gadolinium gallium garnet (GGG), forming a thin layer of SiO.sub.2 on the YIG film, forming a photoresist mask layer on the SiO.sub.2 layer, removing portions of the photoresist mask layer to expose portions of the underlying SiO.sub.2 layer, removing portions of the SiO.sub.2 layer to expose portions of the underlying YIG layer and removing the exposed portions of the YIG layers to form isolated La:YIG disks supported on the GGG substrate. The substrate is then further processed, as by dicing, to provide individual La:YIG disks for fabrication into microwave devices. Linewidths of about 0.45 Oe are obtained by the process.