A. Langari et Ma. Martin-delgado, Alternating-spin ladders in a magnetic field: Formation of magnetization plateaux, PHYS REV B, 62(17), 2000, pp. 11725-11730
We study numerically the formation of magnetization plateaux with the Lancz
os 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 plat
eaux 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 seve
ral approximate analytical methods (spin-wave theory, low-energy effective
Hamiltonians, and bosonization) to understand these findings and to conject
ure the behavior of ferrimagnetic ladders with a larger number of legs.