Alternating-spin ladders in a magnetic field:: Formation of magnetization plateaux

We study numerically the formation of magnetization plateaux with the Lanczos method in two-leg ladders with mixed spins of magnitudes (S-1,S-2)=(1,1/2) located at alternating positions along the ladder and with dimerization gamma, For interchain coupling J' > 0 and gamma= 0, we find normalized plateaux at m = 1/3 starting at zero field and in = 1 (saturation), while when gamma not equal 0 is columnar, another extra plateau at I,I = 2/3 shows up. For J'<0, when <gamma>< <gamma>(c)(J') we find no plateau while for gamma> gamma (c)(J') we find four plateaux at m = 0,1/3,2/3,1. We also apply several approximate analytical methods (spin-wave theory, low-energy effective Hamiltonians, and bosonization) to understand these findings and to conjecture the behavior of ferrimagnetic ladders with a larger number of legs.