Antiferromagnetism and ferrimagnetism of non-stoichiometrie compounds

By an extension of the Heitler-London equivalent field methods, a treatment is given of the temperature-dependence of the magnetic properties of an antiferromagnetic substance containing vacancies. For the body-centred cubic lattice with nearest neighbour interactions the results may be conveniently summarized in terms of the magnetic Curie temperature, θ; the critical temperature, Tc, below which a superstructure of the vacancies begins to form; and a parameter x specifying the number of vacancies. For x = 0 the substance is a normal antiferromagnet. For x > 0 and such that θ > Tc the susceptibility-temperature curve is similar to that of a normal antiferromagnet (x = 0) for the range T ≳ Tc but, as T → T0 the susceptibility increases rapidly with an eventual transition to a ferrimagnetic state at T = Tc. For x > 0 and such that Tc> θ there is a discontinuity in slope of the susceptibility-temperature curve at T = Tc͘͘͘͘͘͘͘͘͘ followed by a rapid rise in susceptibility with an eventual transition to a ferrimagnetic state at T = θ. For other lattices, or other than nearest neighbour interactions only, the possible kinds of behaviour are more complicated and do not lend themselves to a brief summary. The theoretical predictions are discussed in relation to the experimental results for non-stoichiometric compounds such as the iron and chromium sulphides, selenides and tellurides.