Ferromagnetic Domains and the Magnetization Curve

The magnetization curve and hysteresis loop of a ferromagnetic material used to be explained qualitatively through the interaction of ``molecular magnets.'' It is now known that interaction of the atoms produces, directly, only magnetic saturation, and that the ``magnets'' whose interaction is responsible for the details of the hysteresis curve are much larger things-magnetically saturated regions big enough to be seen under a microscope. These ``spontaneously'' magnetized regions are known as ``domains,'' and with them progress is being made toward a quantitative theory of the magnetization process. The basic ideas of the domain theory are described in this article.