CHAPTER V - FERROMAGNETISM AND ANTIFERROMAGNETISM

Abstract

This chapter discusses ferromagnetism and antiferromagnetism. There is a close analogy between the magnetic properties of ferromagnets and the electric properties of ferroelectrics. Both exhibit spontaneous polarization, magnetic or electric, in macroscopic volumes. In each case, this polarization vanishes at a temperature corresponding to a second-order phase transition. In ferroelectrics, the interaction between the molecules in the crystal lattice is essentially anisotropic and consequently, the spontaneous polarization vector is fairly closely related to certain directions in the crystal. The formation of a magnetic structure is mainly because of the exchange interaction of the atoms. The anisotropy of the magnetic properties of ferromagnets is due to the relativistic interactions among their atoms and these interactions are comparatively weak. The anisotropy constant of a ferromagnet is a function of temperature. The direction of the spontaneous magnetization changes and so does the symmetry of the magnetic structure. The resulting transitions among different phases of a magnetic substance are called orientational transition. Like ferromagnetism, an antiferromagnetic structure is also established by the isotropic exchange interaction of electrons; the weaker relativistic interactions determine the crystallographic orientation of the magnetizations of the sub-lattices. The phenomena of piezomagnetism and the magnetoelectric effect in antiferromagnets are closely related to the magnetic symmetry. Piezomagnetism is the occurrence of magnetization when elastic stresses are applied to the crystal; it is analogous to piezoelectricity and is represented by the presence in the thermodynamic potential of the crystal of a term linear both in the field and in the elastic stress tensor: Like piezomagnetism, the magnetoelectric effect is possible only for certain magnetic symmetry classes; the magnetoelectric tensor is odd under time reversal and zero in bodies without a magnetic structure.