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

We propose a general treatment for solving the case of ferrimagnetic chains made up of two sublattices (S,s) and characterized by isotropic couplings between nearest neighbors, as well as two exchange parameters. Therefore, two cases of physical interest are examined: i) the chains are exclusively composed of ferromagnetic or antiferromagnetic couplings; ii) the chains show a regular alternation of these two types of couplings. Closed-form expressions of the zero-field partition function, the spin-spin correlations, as well as the susceptibility, are given. The low-temperature behavior of the susceptibility is studied by means of the correlation length. In particular, in the magnetic moment compensation, we show that this behavior is mainly described by the competition between the divergence of the correlation length and the evanescence of the magnetic moment per unit cell. Finally we recall an experimental test which has initiated this theoretical model: it concerns the compound MnCu(obp)(H2O)(3).H2O [where obp=oxamidobis(propionato)].