Thermodynamic model of ideal magnetostriction

A self-consistent model of ideal magnetostriction is presented on a thermodynamic basis for the coupling between magnetostriction of ferromagnets and the behaviour of magnetization or induction curves (an anhysteretic approach is developed). The model is based on a rigorous adherence to the energy conservation law which is written in a characteristic differential form. The author employed an additional assumption concerning the presence of saturation effects to obtain a magnetic equation of state for some ferromagnetic materials. The coefficient kappa is introduced for the coupling between magnetic and mechanical processes. A mechanical equation of state (the expression for magnetostriction) is derived using the thermodynamic Maxwell reciprocity relations. The separation of strains into magnetic and purely mechanical parts comes about as a logical consequence of the self-consistent approach. The conclusion is made that the well-known Villari and Joule effects can be treated as a coupling between magnetic and mechanical processes within the framework of an invariant coupling coefficient. The behaviour of the magnetostrictional modulus of elasticity for ferromagnetic materials is discussed and, as is shown it takes on negative values within a certain range of the magnetic field and mechanical stress. The last Section of the paper concentrates briefly on the strength problems. Modeling results are in a good agreement with experiments.