Circumventing Magnetostatic Reciprocity: a Diode for Magnetic Fields*

Lorentz reciprocity establishes a stringent relation between electromagnetic fields and their sources. For static magnetic fields, a relation between magnetic sources and fields can be drawn in analogy to Green's reciprocity for electrostatics. So far, the magnetostatic reciprocity principle remains unchallenged and the magnetostatic interaction is assumed to be symmetric (reciprocal). Here we show that a linear and isotropic electrically conductive material moving with constant velocity is able to circumvent the magnetostatic reciprocity principle and realize a diode for magnetic fields. This result is demonstrated by measuring an extremely asymmetric magnetic coupling between two coils that are located near a moving conductor. The possibility to generate controlled unidirectional magnetic couplings implies that the mutual inductances between magnetic elements or circuits can be made extremely asymmetric. Other strategies to break magnetic reciprocity based on novel kinds of magnetic metamaterials will be also discussed.