A magnetic structure generating a uniform magnetic field within a region of interest using permanently magnetized prism-shaped blocks of magnetic material positioned around a cavity and which constitute the predominant generator of the uniform magnetic field in the desired region of interest. The magnetic structure has a plane surface comprising a high permeability ferromagnetic member interfacing with a first side of the cavity and forming a single pole piece of the magnetic structure. The magnetic structure produces within the cavity adjacent the single pole piece the uniform magnetic field and collects the magnetic flux flowing through the single pole piece and through the cavity, with the region of interest being located adjacent the single pole piece. A second side of the cavity generally opposite to the first side is open to a surrounding medium and forms an interface with the surrounding medium but not with a magnetic member. The magnetic structure is designed assuming a hypothetical high permeability membrane parallel to the single pole piece and located where the open second side of the cavity will be made, the high permeability membrane forming an equipotential surface at the interface with the surrounding medium. This minimizes the fringe field generated in the surrounding medium when the second side of the cavity is opened.