Interacting spin-wave dispersion relations of ferrimagnetic Heisenberg chains with crystal-field anisotropy

We study the effect of crystal-field anisotropy on the dispersion relations of mixed-spin ( S , s ) alternating chains by using the interacting spin-wave theory and the density-matrix renormalization group algorithm. For the easy-plane anisotropy case we find that the spin-wave results fail to describe the ground-state properties of the systems under consideration, whereas for the easy-axis anisotropy regime the method demonstrates a surprising efficiency showing, for example for the system ( S , s ) = ( 3 / 2 , 1 / 2 ) , a discrepancy from the density-matrix renormalization group of about 0.0006% for the ground-state energy and of 2% for the sublattice magnetizations.