Antiferromagnetic mean-field model with unusual precritical exponents

The antiferromagnetic transition in a system of dipoles aligned along the cube axes 〈100〉 in a cubic crystal is treated in the mean-field approximation. The critical exponents are found to be such as usually occur at tricritical points: (Formula presented) (Formula presented) (Formula presented) (Formula presented) (Formula presented) rather than the expected set (Formula presented) (Formula presented) (Formula presented) (Formula presented) (Formula presented) The reason is the vanishing of the third-order term in the expansion of the magnetization. The approximation stays valid even when the symmetry is lowered to tetragonal or monoclinic. Some layered structures such as (Formula presented) undergo antiferromagnetic transitions with a wide range of validity for these anomalous exponents. Even though eventually a crossover to the “genuine” critical exponents does occur it is remarkable that this extremely simple model accounts for an important range of precritical behavior.