Abstract
Proton NMR in single crystals of Mn(HCOO)2·2H2O has been observed at the temperature region from 2.5°K to 300°K. The shift of the resonance lines is explained by a dipole sum carried out by the use of atomic parameters presumed from those of the isomorphous magnesium formate. The results give a good agreement with the experimental data obtained above 4.2°K, if the magnetic moments of two kinds of manganese ions in the crystal are assumed to be ciH⁄(T+Θ) for those of the one kind and to be ciH⁄T for the other, where ci is g2β2S(S+1)⁄3k, g=2.00, S=5⁄2, and Θ is 9.4±0.7°K.
Since an abrupt shift of the NMR lines occurs at 3.70±0.05°K, a magnetic transition is thought to exist at this temperature. It is presumably due to a sort of magnetic ordering in the one set of ions which is found to have an antiferromagnetic temperature dependence. At this temperature, a change in ESR spectrum and a sharp peak in the susceptibility are also found. Below this temperature, one of the NMR lines has a zero field shift and shows an angular dependence remarkably distorted from a sinusoidal variation.
