Weak Ferromagnetism

the existence of a small spontaneous magnetic moment in certain classes of antiferromagnetic materials. The magnitude of the magnetic moment is approximately 0.1–10 centimeter-gram-second electromagnetic units per mole, or 10²— 10⁴ amperes/m-mole. In what is known as transverse weak ferromagnetism, the cause of the magnetic moment is that the magnetization vectors of the antiferromagnet are not strictly antiparallel. A magnetic moment also arises when the magnetizations of two antiparallel sublattices of an antiferromagnet are not equal (seeANTIFERROMAGNETISM).

Weak ferromagnetism has been studied in greatest detail in NiF₂, in the orthoferrites RFeO₃ (where R is a trivalent ion of a rare-earth element), and in rhombohedral antiferromagnets, such as α-Fe₂ O₃, MnCO₃, NiCO₃, CoCO₃, and FeBO₃. It should be noted that weak ferromagnetism is observed in chemically pure antiferromagnetic materials and is not associated with ferromagnetic impurities. This fact was established for NiF₂ by L. Mataresse and J. Stout of the USA in 1954 and for MnCO₃ and CoCO₃ by A. S. Borovik-Romanov and M. P. Orlova of the USSR in 1956. Transverse weak ferromagnetism has been detected in all known antiferromagnetic materials with weak ferromagnetism.

A theoretical explanation for weak ferromagnetism was provided by I. E. Dzialoshinskii of the USSR in 1957. He showed that the existence of weak ferromagnetism follows from general concepts of the magnetic symmetry of crystals. Dzialoshinskii’s theory explains, in particular, why weak ferromagnetism is observed in uniaxial crystals when the magnetization of the sub-lattices is directed perpendicularly to the principal symmetry axis of the crystal and why weak ferromagnetism is absent when the magnetization is parallel to the axis. The effective magnetic field resulting in weak ferromagnetism is sometimes called the Dzialoshinskii field. It is weaker by a factor of 10^2–10⁴ than the effective field of the exchange interaction that is responsible for the magnetization of crystal sublattices.