The Analysis of Magnetization Curves

By an analysis of a magnetization curve is meant the determination of the character and extent of the various elementary changes occurring over any part of the curve. Even if the quantitative characteristics of the various elementary processes are known, an analysis cannot in general be made simply on the basis of the information provided by a given magnetization curve, owing to the varied and complicated way in which the different processes are superposed with an ordinary polycrystalline specimen. Further information is obtainable from measurements such as those of Bates on the thermal changes accompanying adiabatic magnetization, but little progress can be made in the analysis of curves in the hysteresis range unless the magnetic and thermal effects due to the reversible and irreversible changes separately can be unambiguously determined.

A broad survey of the general problem is given with particular reference to recent theoretical and experimental work at Leeds. The theoretical approach and the methods of analysis are along the same lines as in the treatment of the magnetocaloric effect by Stoner and Rhodes (1949). The experimental work reviewed includes studies of the reversible susceptibility, the Barkhausen effect, and the reversible change of magnetization with temperature at constant field (Tebble, with Corner, Newhouse, Skidmore, Wood, and others, 1948 onwards).

Through this work, it is now possible to determine not only the relative contributions from reversible and irreversible processes to the change of magnetization over any range but also the thermal changes associated with each. The part of the reversible change due to change in intrinsic magnetization can be determined, and in many cases analysis will show whether the remainder of the reversible change is mainly due to rotation of the magnetization against the crystalline and strain anisotropy forces, or to other processes such as boundary movements. Much detailed information about irreversible processes is obtained from determinations of the number-size relations of Barkhausen discontinuities, which in general correspond not to spontaneous movements of primary domain walls as wholes, but to smaller scale sectional irregularities in the motion. The thermal effect associated with these irreversible processes may be a heating or a cooling.

Partial analyses are presented for annealed nickel, hard-drawn iron, and other materials. These show clearly the potentiality of the general method for obtaining detailed quantitative information about the elementary processes contributing to the change of magnetization along a magnetization curve.