Low-Temperature Magnetization of Cu(NO3)2·2.5H2O

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Abstract
We have measured magnetization isotherms for single crystals of monoclinic Cu(NO3)2·2.5H2O between 1.2° and 4.2°K by a ballistic method. Fields up to 43 kOe were applied along the b axis, i.e., the long axis of the needlelike specimens. The character of the M vs H curves changes rapidly in this temperature interval. They are described in first approximation by a model of this salt in which Cu+ + ions are coupled by isotropic antiferromagnetic exchange, −JS1·S2, in isolated pairs. The best fit of the data with the isolated cluster model yields J = −5.23k and g = 2.23, in reasonable agreement with earlier susceptibility, specific heat, and proton resonance results. Several modifications of the model were considered in order to remove small systematic discrepancies. The addition of a very weak intercluster coupling in a molecular field approximation yields a quantitative fit of all the data. If H = H0+nM, we find n = −1.2 mole/cgs while J = −5.14k and g = 2.35, in even closer agreement with other observations. This weak antiferromagnetic coupling is large enough to explain the absence of hyperfine structure on paramagnetic resonances seen in the excited pair triplet state.