Antiferromagnetic mean-field model with unusual precritical exponents

The antiferromagnetic transition in a system of dipoles aligned along the c ube axes [100] in a cubic crystal is treated in the mean-field approximatio n. The critical exponents are found to be such as usually occur at tricriti cal points: alpha = alpha' = 1/2, beta = 1/4, gamma = gamma' = 1, delta = 5 , eta = 0, rather than the expected set alpha = alpha' = 0, beta = 1/2, gam ma = gamma' = 1, delta = 3, eta = 0. The reason is the vanishing of the thi rd-order term in the expansion of the magnetization. The approximation stay s valid even when the symmetry is lowered to tetragonal or monoclinic. Some layered structures such as RbFeF4 undergo antiferromagnetic transitions wi th a wide range of validity for these anomalous exponents. Even though even tually a crossover to the "genuine" critical exponents does occur it is rem arkable that this extremely simple model accounts for an important range of precritical behavior. [S0163-1829(99)02738-1].