Fundamental Theory of Ferro- and Ferri-Magnetism

The presentation is centered around the four important forms of magnetic energyin ferromagnetic media: (I) the Zeeman energy of the elementary magnets in the applied magnetic field, (II) the classical electromagnetic interaction between dipoles, (III) the Heisenberg exchange energy, and (IV) the energy of anisotropy which is pseudodipolar or pseudoquadrupolar in structure but is attributable ultimately to rather recondite quantum-mechanical spin-orbit interaction. The primary cause of ferromagnetism is (III), and anisotropy arises from (IV). Domain structure and the concomitant hysteresis phenomena owe their existence to (II). The magnetic theory of the ferrites is in many ways simpler and less ambiguous than that for conducting ferromagnetic media, as there are no complications due to electron migrations. Néel's theory of ferrimagnetism explains the saturation magnetization and other properties of the ferrites.