DIMENSIONALITY CROSSOVERS IN THE MAGNETIZATION OF THE WEAKLY FERROMAGNETIC 2-DIMENSIONAL MANGANESE ALKYLPHOSPHONATE HYDRATES MNCNH2N+1PO3-CENTER-DOT-H2O, N=24

Critical exponents of magnetization beta below TN in the weakly ferromagnetic layer compounds MnCnH2n+1PO3.H2O have been measured by SQUID magnetometry for n = 2-4. In all three compounds crossovers are observed in beta as follows (beta(1), beta(2)): 0.21(2), 0.73(2) (n = 2); 0.18(1), 0.42(6) (n = 3), 0.18(1), similar to 0.6 (n = 4). The crossover occurs at values of the reduced temperature epsilon = (T-N - T)/T-N that become smaller as the separation between the magnetic layers increases.