  We study the T = 0 magnetization of frustrated two-leg spin ladders with
arbitrary value of the spin S. In the strong rung limit, we use degenerate
perturbation theory to prove that frustration leads to magnetization plateaux
at fractional values of the magnetization for all spins S, and to determine the
critical ratios of parallel to diagonal inter-rung couplings for the appearance
of these plateaux. These ratios depend both on the plateau and on the spin. To
confirm these results, and to investigate the properties of these ladders away
from the strong coupling limit, we have performed extensive density matrix
renormalization group (DMRG) calculations for S <= 2. For large enough
inter-rung couplings, all plateaux simply disappear, leading to a magnetization
curve typical of integer-spin chains in a magnetic field. The intermediate
region turns out to be surprisingly rich however, with, upon increasing the
inter-rung couplings, the development of magnetization jumps and, in some
cases, the appearance of one or more phase transitions inside a given plateau.

Strongly Correlated Electrons;Other Condensed Matter