HEISENBERG-CHAINS WITH ALTERNATING SPIN QUANTUM NUMBERS

In linear magnetic Heisenberg chains, alternation effects can be caused by alternation of the spin quantum numbers. These effects are found to be different from those caused by alternation of the interaction constant. When one of the spin quantum numbers is infinite (i.e. can be treated classically) and the other arbitrary, the zero field partition function and thermodynamical quantities can be obtained analytically. The susceptibility of the infinite spin sublattice can also be derived. Spin-wave theory predicts that the dispersion relation is split into an acoustic and an optic branch, of which only the latter has a gap. The acoustic branch behaves essentially ferromagnetically, except in the uniform antiferromagnetic limit.