Magnetic deformation of ferrogel bodies: Procrustes effect.

Deformation of spheroidal ferrogel bodies caused by a uniform magnetic field is investigated theoretically. The deformation is induced by two competitive mechanisms-magnetostatic and magnetostrictive. The former is due to the demagnetizing field of the sample and hence depends on its shape, while the latter originates from the magnetoelasticity of ferrogel and is shape independent. Both mechanisms are dipolar in nature and contribute-for a body of commensurate dimensions-oppositely to the effect. For an isotropic ferrogel sphere, the magnetostatic contribution still prevails and the magnetic field elongates the body. The two opposing mechanisms balance each other out for a prolate spheroidal sample with the axes aspect ratio a/b approximately 1.3 . It determines the so-called "Procrustes point" or "Procrustes size"-the magnetic field shrinks the body if a>1.3b and stretches it when a<1.3b .