Temperature dependence of the spin lattice relaxation time of proton nmr in the mixed antiferromagnets A1−x B x Cl2·2H2O

Abstract

The mixed transition metal dichloride dihydrates A1−xBxCl2·2H2O (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 T1 of proton nuclear magnetic resonance (NMR) on these mixed crystals have been measured. In Ni1−xBxCl2·2H2O (B=Co, Mn), Mn substitution increased the relaxation rates 1/T1 more than Co substitution, even when the amounts of substitution were almost the same. In Co1−xBxCl2·2H2O (B=Mn, Ni), Mn has a significant impact on the relaxation rates in comparison with Ni. In Mn1−xBxCl2·2H2O (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 Co1−xFexCl2·2H2O (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.