  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 the Green's reciprocity
principle for electrostatics. Here we theoretically and experimentally show
that a linear and isotropic electrically conductive material moving with
constant velocity is able to circumvent the magnetic 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 breaks down one of the most deeply-established relations in
classical electromagnetism, namely that mutual inductances are symmetric. This
result might provide novel possibilities for applications and technologies
based on magnetically coupled elements.
