Temperature dependence of the spin lattice relaxation time of proton nmr in the mixed antiferromagnets A1−xBxCl2·2H2O

The mixed transition metal dichloride dihydrates A 1− x B x Cl 2 ·2H 2 O (A, B=Co, Mn, Ni) have been prepared in order to understand the specificity of the Mn spins in the mixtures. The temperature dependences of the spin lattice relaxation times T 1 of proton nuclear magnetic resonance (NMR) on these mixed crystals have been measured. In Ni 1− x B x Cl 2 ·2H 2 O (B=Co, Mn), Mn substitution increased the relaxation rates 1/ T 1 more than Co substitution, even when the amounts of substitution were almost the same. In Co 1− x B x Cl 2 ·2H 2 O (B=Mn, Ni), Mn has a significant impact on the relaxation rates in comparison with Ni. In Mn 1− x B x Cl 2 ·2H 2 O (B=Ni, Co), the relaxation rates are much increased by a slight substitution of Co and exhibit a minimum in the temperature range of observation. This appearance of a minimum in the relaxation rates at low temperatures is believed to reflect an instability due to the occurrence of a reentrant spin-glass transition. A similar trend is seen at low temperatures in the case of Ni substitution, too. A Co 1− x Fe x Cl 2 ·2H 2 O ( x = 0.1) sample has been prepared, too. In this sample, a minimum of the relaxation rate is seen in the temperature range of observation. This may reflect an instability due to the occurrence of an oblique antiferromagnetic transition.