SPIN POPULATIONS IN ONE-DIMENSIONAL ALTERNANT SPIN SYSTEMS - A THEORETICAL APPROACH

Spin-density maps, deduced from polarized neutron diffraction experime nts, for both the pair and chain compounds of the system Mn2+Cu2+ have been reported recently. These results have motivated us to investigat e theoretically the spin populations in such alternant mixed-spin syst ems. In this paper, we report our studies on the one-dimensional ferri magnetic systems (S-A,S-B)(N) where hi is the number of AB pairs. We h ave considered all cases in which the spin Sri takes on allowed values in the range I to 7/2 while the spin S-B is held fixed at 1/2. The th eoretical studies have been carried out on the isotropic Heisenberg mo del, using the density matrix renormalization group method. The effect of the magnitude of the larger spin SA 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 boun dary conditions, the latter with a view to understanding end-of-chain effects. The spin populations have been followed as a function of temp erature as well as an applied magnetic held. High-magnetic fields are found to lead to interesting re-entrant behavior. The ratio of spin po pulations \P-A/P-B\ is not sensitive to temperature at low temperature s. (C) 1998 American Institute as Physics.