Apparatus for nuclear magnetic resonance includes a sample container, a receiver coil adjacent to and surrounding the sample container and a flux return yoke for directing magnetic flux from the same falling outside the receiver coil back thereto, the yoke comprising a material having a variable magnetic permeability tuned for a particular frequency range. The material of the return yoke with a variable magnetic permeability includes an array of capacitive/inductive elements. Each capacitive/inductive element has a low resistance conducting path and is configured such that a magnetic component (H) of electromagnetic radiation lying within a predetermined frequency band induces an electrical current (j(x)) to flow around said path and through the associated element. The size of the elements and their spacing apart are selected to provide a predetermined permeability in response to the received electromagnetic radiation.