The interaction of a body with electromagnetic radiation may be modified by controlling the structure of its surfaces. When the surface has a periodic structure with unit cell size smaller than the wavelength, the body may be described as an effective medium with modified permittivity ε and permeability μ. For example, when both ε and μ are negative the material is said to be left-handed and can exhibit new properties such as super-lensing. At a microscopic level, the unit cell can be thought of as a LC resonator. Much work has been done to manipulate the inductance L of the cell via the geometrical structure. However much larger changes of inductance may be obtained through the inclusion of magnetic materials into the surface. This project will explore whether magnetic insulators may be used to construct planar cavity resonator structures with size much smaller than the free space wavelength. Such structures may find immediate application as inductive anti-theft tags operating in the low MHz frequency range and may provide a solution to the long-standing problem of tagging metallic objects, on which we are collaborating with Crown Technology, a leading manufacturer of metal packaging. In the longer term, arrays of resonators may be used to construct microwave cloaks and lenses.