We investigate a two-leg spin-ladder system composed of alternating-spin chains with two different kinds of spins. The fixed-point properties are discussed by using spin-wave analysis and nonlinear sigma model techniques. The model contains various massive phases, reflecting the interplay between the bond alternation and the spin alternation.

• Received 7 May 1997

DOI:https://doi.org/10.1103/PhysRevB.57.398