A magnet structure comprising a plurality of individual magnets in the form of an elongate block (4) having a length (4a) extending beyond the thickness of the magnet structure. The elongate block (4) is cylindrical or polyhedral in shape with at least one flat longitudinal face (4b) orientated towards a working surface of the magnet structure, the elongate block (4) having a line of magnetisation extending along its length. The individual magnets (4) being positioned at a distance from each other in the magnet structure in order to be electrically isolated from each other, the length (4a) of each block (4) being greater than the diameter of the flat longitudinal face (4b) for a cylindrical block (4) or with a larger diagonal (4c) connecting two apexes of said longitudinal face (4b) for a block (4) in the form of a polyhedron.