Magnetization and the Magneto-Caloric Effect

Abstract

Definition of the effect.---The magneto-caloric effect is defined as the reversible evolution of heat accompanying a change in the magnetization of a ferromagnetic substance, in contradistinction to the irreversible evolution of heat referred to as hysteresis.Assumption regarding the mechanism of magnetization.---In two previous notes it was suggested that ferromagnetic substances are spontaneously magnetized in small regions containing roughly ${10}^{5}$ atoms. Here the following points are briefly discussed: (1) the formation of these units, called blocks; (2) the interactions of these blocks to form clusters, which probably give rise to the Barkhausen effect; (3) the changes that take place in the neighborhood of the Curie point; (4) the changes that take place during magnetization.The Magneto-caloric effect.---The essential result of the above discussion is that a sudden change in the applied field will produce changes in the energy of every block. The new energy distribution is not stable, and equilibrium is reached by means of an adiabatic diffusion process resulting from thermal agitation, which brings with it a reversible change in temperature. The mechanism is illustrated by means of a simple model.The effect under certain simple conditions.---If the strains due to magnetostriction, stray internal fields, and the effect of crystal orientation are negligible, the magneto-caloric effect is amenable to calculation. It is pointed out under what experimental conditions such neglects are justifiable.The interpretation of the experiments of Weiss and Forrer on Nickel.---A detailed comparison is undertaken between experimental observations of the magneto-caloric effect in nickel in high fields and the theoretical predictions based on the assumption that the effect is due to processes involving Weiss' molecular field. The agreement is satisfactory at temperatures not too near the Curie point. Too little is known about the nature of the magnetic transformation to permit a quanitative analysis to be made there.Magnetization and the magneto-caloric effect in single crystals.---Akulov's static theory of the magnetization curve of single crystals predicts the absence of a magneto-caloric effect due to rotation of the regions of spontaneous magnetization, i.e., the work done in rotating the magnetization of such a region is equal to its change in potential energy, and therefore there can be no change in heat content. Experimentally this relation is approximately satisfied for iron and nickel, and less well for cobalt. If, in the case of cobalt, the discrepancy is real, a magneto-caloric effect of the order of 0.03\ifmmode^\circ\else\textdegree\fi{}C is to be expected. The magnetization curves for crystals of nickel and cobalt on the basis of Akulov's theory are calculated. It is pointed out that experiments on the magneto-caloric effect of single crystals would be of great help in determining the mechanism of magnetization.