Described embodiments include a system, method, and apparatus. The apparatus includes a magnetic substrate at least partially covered by a first negative-permittivity layer comprising a first plasmonic outer surface. The apparatus includes a plasmonic nanoparticle having a magnetic element at least partially covered by a second negative-permittivity layer comprising a second plasmonic outer surface. The apparatus includes a dielectric-filled gap between the first plasmonic outer surface and the second outer surface. The first plasmonic outer surface, the dielectric-filled gap, and the second plasmonic outer surface are configured to support one or more mutually coupled plasmonic excitations.