Magnetostriction and Elastic Properties of Ferromagnetic Substances at High Magnetic Fields

The manner in which the anisotropic forced magnetostriction contributes to the magnetostriction of ferromagnetic substances at high magnetic fields is discussed from a localized model of ferromagnetism. With this assumption, the free energy of a ferromagnetic substance can be conveniently divided into several terms and it can be concluded quite naturally that a free-energy term corresponding to that of ferromagnetic anisotropy gives rise to an anisotropic forced magnetostriction. In the past, the isotropic forced volume magnetostriction has been considered to be the only cause of the forced magnetostriction. This is probably one reason for the discrepancies between the experimental value of the volume magnetostriction and the theoretical value expected from the pressure dependence of the Curie point, etc. From a similar point of view, anomalous behavior of elastic properties of a ferromagnetic substance, such as the change of the temperature coefficient of the elastic constants at the Curie point, the dependence of the elastic constants upon the direction of the magnetic field, etc., are explained as an intrinsic change in the elastic constants with spontaneous magnetization. In the past, these effects have been explained only as a result of the deformation of the crystal lattice due to the spontaneous magnetostriction (volume effect, morphic effect).