In this paper, a proposal for a field weakening method for a permanent-magnet synchronous motor is evaluated. The method consists in inserting giant magnetostrictive elements between permanent magnets. The magnetic permeability of these magnetostrictive parts is modified when the speed increases. The field leakages around the permanent magnets are consequently increased, thus reducing the electromotive forces in the armature windings. An original modeling approach, based on the concept of equivalent stress, is proposed in order to evaluate this field weakening solution.