On the Theory of Antiferromagnetism

J. H. Van Vleck

Harvard University, Cambridge, Massachusetts

(Received October 19, 1940)

Abstract

Expressions for the susceptibility of a medium with antiferromagnetic exchange coupling are derived by means of the Dirac vector model, with an accuracy comparable to that in the usual first approximation of the WeissHeisenb~rg theory of ferromagnetism. The °formulas yield a Curie temperature Te above which the internal field disappears and at which the susceptibility X is a maximum. The calculated value of X is ~ as large at T=O as at T= Te. This prediction accords nicely with experimental data on MnO, but not on MnSe. In an antiferromagnetic medium, the internal field should probably be treated as randomly directed, and our results may be regarded as generalizations and combinations of formulas previously obtained by . Neel and Hulthen and by Bitter for the cases of inner fields respectively perpendicular and parallel to the applied field. A slight accentuation of the perpendicular alignment by very powerful applied fields explains the dependence of the susceptibility on field strength observed by Bizette, Squire, and Tsai in MnO when T < T e. 

https://sci-hub.tw/https://doi.org/10.1063/1.1750830 

Cited as: J. H. Van Vleck, J . Chem. Phys. 9, 85 (1941).