Spin populations in one-dimensional alternant spin systems: a theoretical approach

Abstract

Spin-density maps, deduced from polarized neutron diffraction experiments, for both the pair and chain compounds of the system Mn²⁺Cu²⁺ have been reported recently. These results have motivated us to investigate theoretically the spin populations in such alternant mixed-spin systems. In this paper, we report our studies on the one-dimensional ferrimagnetic systems (S_A,S_B)_N where N is the number of AB pairs. We have considered all cases in which the spin S_A takes on allowed values in the range 1 to 7/2 while the spin S_B is held fixed at ½. The theoretical studies have been carried out on the isotropic Heisenberg model, using the density matrix renormalization group method. The effect of the magnitude of the larger spin S_A on the quantum fluctuations in both A and B sublattices has been studied as a function of the system size N. We have investigated systems with both periodic and open boundary conditions, the latter with a view to understanding end-of-chain effects. The spin populations have been followed as a function of temperature as well as an applied magnetic field. High-magnetic fields are found to lead to interesting re-entrant behavior. The ratio of spin populations |PA/PB| is not sensitive to temperature at low temperatures.