Permanent magnets

In Section 1 some theorems of magnetostatics are briefly dealt with to the extent required for the understanding of the demagnetization curve of a permanent magnet. Coercivity in regard to magnetization I and induction B respectively is defined. The ?fullness factor? and its upper and lower limits are discussed along a theory outlined by K. Hoselitz and the figure of merit for a permanent magnet defined. In Section 2 it is pointed out that ferromagnetism is a particular manifestation of paramagnetism, and accordingly first deals with the theory of paramagnetism. A brief account is given of Langevin´s classical theory of paramagnetism as balancing of magnetic potential energy against the thermal motion. Weiss´s theory of spontaneous magnetization (ferromagnetism) and of domains follows. The wave mechanical aspect of Weiss´s intramolecular field is presented in non-mathematical language. Spontaneous magnetization is given as a function of atomic magnetic moment and Curie temperature and accordingly, two sections are concerned with atomic moment and Curie temperature respectively. The importance of the number of Valency electrons per atom is emphasized and it is shown how in principle the magnetic properties of ferromagnetic alloys could be predicted. The change of shape and volume due to magnetization, that is magnetostriction, are also discussed. In Section 3 the author shows that the magnetic power (BH)max of a permanent magnet is expressed as the product of four terms: (BH)max = a(Ir/Is) Is.Hc. the first two of which constitute the geometrical factor, and the last two the magnetic hardness factor. For the isotropic magnets, the geometrical factor is nearly constant and the progress has been achieved mainly by increasing hardness. The progress achieved by anisotropic magnets is mainly due to an improved geometrical factor. The magnetic anisotropy of a single iron crystal is discussed and it is shown how the polycrystalline structure involves magnetic hysteresis. - Damping of torsional oscillations of an iron wire due to hysteresis is mentioned. Natural mechanical hardness of diamond and technical hardness of metals are compared. Easy slip planes and easy magnetization directions are considered as analogous and the effect of silicon and carbon additions is discussed. Dispersion hardening alloys and their mechanical and magnetic hardness are dealt with. Transition zones and Bloch´s theory of coercivity are explained by means of a model. It is shown how cooling in a magnetic field raises the remanence-to-saturation ratio in the principal direction at the expense of directions at right angles. Improvement of the fullness factor by the magnetothermal treatment is discussed, but no theory is given.