A miniature magnetic core includes at least one bar magnet having, in the demagnetized state, a plurality of magnetic domains separated by magnetic walls, the bar magnet having permanent discontinuities placed at least approximately at the probable locations of at least some of these magnetic walls in the absence of these permanent discontinuities. A miniature sensor includes a miniature core that cooperates with at least one excitation coil and at least one detection coil. A method for manufacturing the magnetic core includes identifying the probable location of the magnetic walls, depositing at least a thin film of magnetic material on a support in order to form the core and producing, in said core, discontinuities approximately at the identified locations of the walls.