In the case of a single stress  acting upon a single magnetic domain, the magnetic strain energy density  can be expressed as:
where  is the magnetostrictive expansion at saturation, and  is the angle between the saturation magnetization and the stress's direction. 
When  and  are both positive (like in iron under tension), the energy is minimum for  = 0, i.e. when tension is aligned with the saturation magnetization. Consequently, the magnetization is increased by tension.