The heat capacity CP of powdered Cu(NO3)2·2.5H2O has been measured between 0.5° and 4.2°K. In zero applied field, CP exhibits a Schottky anomaly having its maximum value of 0.51R at 1.82°K. The data are quantitatively described by assuming that Cu2+ ions exist in this salt in isolated pairs (binary clusters) coupled by isotropic antiferromagnetic exchange with J/k = −5.18°K. The same model was previously found to give a good fit of the observed magnetic susceptibility. The present results, however, clearly exclude the possibility that Cu2+ ions in this material are coupled in extended linear chains. CP measured in an applied field of 8.7 kG was found to be as predicted by the pair model. As a result of the isotropy of the pair interaction and the small single-triplet separation, it has been possible to cool this salt by adiabatic magnetization of the powder in relatively small fields.
The heat capacity C p of powdered specimens of Mn(CH3COO)2'4H 2 0 has been measured between 0.4 and 20 °K in two vacuum calorimeters, one of which was cooled with liquid He 3 . A sharp X-like anomaly occurs at 3.18°K, the magnetic ordering temperature. The behavior of this peak upon application of a magnetic field supports the conclusion that the ordered phase is ferrimagnetic. Near 0.7°K, C p exhibits a rounded anomalous maximum of the Schottky type. This probably reflects the inequivalence of Mn + + ions within the unit cell. It is suggested that one group of Mn + + ions may interact only weakly with another group which are comparatively strongly coupled to one another.
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