A rotor (1) comprising a plurality of alternating magnetic poles formed by permanent magnets (3) distributed regularly between a circumferential portion (5) and a central portion (6) of the magnetic body (2) of the rotor and defining circumferential pole sections (7). The circumferential portion comprises axial grooves (15) which are arranged between the pole sections facing the magnets and which form openings in recesses (4) containing the magnets. The openings have a pre-determined first width (a) such as to minimize a leakage magnetic flux (PhiL) flowing through the circumferential portion (5) and to maximize a useful magnetic flux (PhiU) flowing radially through each of the pole sections (7).