THERMODYNAMICS OF ALTERNATING QUANTUM-CLASSICAL SPIN CHAINS SHOWING ISOTROPIC NEAREST-NEIGHBOR COUPLINGS

We propose a general treatment for solving the case of ferrimagnetic c hains made up of two sublattices (S,s) and characterized by isotropic couplings between nearest neighbors, as well as two exchange parameter s. Therefore, two cases of physical interest are examined: i) the chai ns are exclusively composed of ferromagnetic or antiferromagnetic coup lings; ii) the chains show a regular alternation of these two types of couplings. Closed-form expressions of the zero-field partition functi on, the spin-spin correlations, as well as the susceptibility, are giv en. The low-temperature behavior of the susceptibility is studied by m eans of the correlation length. In particular, in the magnetic moment compensation, we show that this behavior is mainly described by the co mpetition between the divergence of the correlation length and the eva nescence of the magnetic moment per unit cell. Finally we recall an ex perimental test which has initiated this theoretical model: it concern s the compound MnCu(obp)(H2O)(3).H2O [where obp=oxamidobis(propionato) ].