"In rock magnetism our interest in very low temperature properties is restricted to two effects, the processes of demagnetization which accompany phase changes in magnetite and hematite, and the low-temperature evidence for cation distribution between non-equivalent lattice sites in ferrites. However, s themselves are important. They provide the mechanism whereby thermal excitation affects the magnetic properties of ferromagnetic materials ... The structure of spin waves is closely related to that of domain walls ..."