A material having magnetic permeability at r.f. frequency, for example a microstructured magnetic material has a magnetic permeability of negative value but unity magnitude over a particular r.f. frequency range. The singularity in the flux pattern has the result that magnetic resonant disturbances in a plane C,E normal to the line C,D are focussed into a plane D,F also normal to the line C,D and vice versa. This is particularly applicable to magnetic resonance apparatus, since the material can be used to transfer the r.f. magnetic flux distribution in a target region in a patient, for example at C,E to D,F where the flux may be directly measured by receive coils. Equally, transmit coils may generate flux to be focussed into the target region by the material. Magnetic resonance apparatus may be constructed which does not require gradient coils, and r.f. hypothermia may be carried out in a focussed way, minimising damage to surrounding tissue.