Magnetic plasmon resonance. 
It is demonstrated that metallic horseshoe-shaped (also referred to as u-shaped) nanostructures can exhibit a magnetic resonance in the optical spectral range.  This magnetic plasmon resonance is distinct from the purely geometric LC resonance occurring in perfectly conducting split rings because the plasmonic nature of the metal plays the dominant role.  Similarly to the electrical surface plasmon resonance, the magnetic plasmon resonance is determined primarily by the metal properties and nanostructure geometry rather than by the ratio of the wavelength and the structure's size.  Magnetic plasmon resonance occurs in nanostructures much smaller in size than the optical wavelength.  Electromagnetic properties of periodically assembled horseshoe-shaped nanostructures are investigated, and the close proximity of the electrical and magnetic plasmon resonances is exploited in designing a negative index metamaterial.  Close to the magnetic plasmon resonance frequency both magnetic permeability mu and electric permittivity epsilon can become negative, paving the way for the development of subwavelength negative index materials in the optical range.