DIMENSIONALITY CROSSOVERS IN THE MAGNETIZATION OF THE CANTED ANTIFERROMAGNETS NH4MNPO4.H2O AND ND4MNPO4.D2O

Critical exponents of the magnetization below TN measured by SQUID mag netometry and neutron diffraction are reported for the two-dimensional canted antiferromagnets NH4MnPO4.H2O and ND4MnPO4.D2O. Both compounds exhibit a crossover in the power law dependence of magnetization on t emperature. In the former we find beta1 0.21(3), beta2 0.40(7), T(N) 1 7.5(1) K; in the latter beta1 0.20(1), beta2 0.39(4), T(N) 17.5(3) K. The crossover between the two exponents occurs at a reduced temperatur e of 0.07(1) in the hydrogenated compound and 0.030(5) in the deuterat ed one. The values of beta1 are compared with those predicted by two d imensional (2D) models of exchange and the change from beta1 to beta2 is attributed to a crossover in lattice dimensionality from 2d-3d.