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.
Current interest in the behavior of Heisenberg alternating antiferromagnetic quantum chains has been stimulated by the discovery of an unusual class of magnetoelastic spinPeierls systems. Copper nitrate, Cu(N03}i·2.5H20, does not display a spin-Peierls transition, but its dominant magnetic behavior is that of a strongly alternating antiferromagnetic chain with temperature-independent alternation. A remarkable, simultaneous fit is demonstrated between theoretical studies and a wide variety of zero-(low-) field experimental measurements, including susceptibility, magnetization, and specific heat. The fitting parameters are a (degree of alternation)=0.27, J 1/k=2.58 K, gb =2.31, and g1 =2.11. Slight systematic discrepancies are attributed to weak interchain coupling. Theoretical studies also predict a rich variety of behavior in high fields, particularly in the region involving the lower and upper critical fields, Hc 1 =28 kOe and Hc 2 =44 kOe. Experimental specific-heat measurements at H=28.2 and 35.7 kOe show quantitative agreement with theory in this interesting parameter region. The fitting parameters are the same as for zero field and, again, small discrepancies between theory and experiment may be attributed to interchain coupling. The exceptional magnetic characterization of copper nitrate suggests its use for further experimental study in the vicinity of the high-field ordering region.
The magnetic susceptibilities of orthorhombic single crystals of CsMnCl3*2H20 have been measured along the a, b, and c axes between 0.35 and 77°K. Above ~9°K, the susceptibility is essentially that of a system of extended weakly interacting chains of spins (S=i, g=2.00) antiferromagnetically coupled by nearestneighbor Heisenberg exchange interaction. The magnitude of the intrachain interaction has been deduced by comparison with Fisher's classical limiting form of the theory of the Heisenberg chain, with and without corrections for the finite spin of the Mn + + ion. With such corrections one finds /= -3.00k. A small uniaxial anisotropy evident at hydrogen temperatures is attributed primarily to single-ion effects. Below ~5°K, interchain interaction apparently causes long-range antiferromagnetic ordering, with the a axis the preferred direction of spin orientation. At 0.35°K an anomalous temperature dependence of uncertain origin is seen.
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