  The experimental realization of various spin ladder systems has prompted
their detailed theoretical investigations. Here we study the evolution of
ground state magnetization with an external magnetic field for two different
antiferromagnetic systems: a three-legged spin-1/2 ladder, and a two-legged
spin-1/2 ladder with an additional diagonal interaction. The finite system
density-matrix renormalization group method is employed for numerical studies
of the three-chain system, and an effective low-energy Hamiltonian is used in
the limit of strong interchain coupling to study the two- and three-chain
systems. The three-chain system has a magnetization plateau at one-third of the
saturation magnetization. The two-chain system has a plateau at zero
magnetization due to a gap above the singlet ground state. It also has a
plateau at half of the saturation magnetization for a certain range of values
of the couplings. We study the regions of transitions between plateaus
numerically and analytically, and find that they are described, at first order
in a strong-coupling expansion, by an XXZ spin-1/2 chain in a magnetic field;
the second order terms give corrections to the XXZ model. We also study
numerically some low-temperature properties of the three-chain system, such as
the magnetization, magnetic susceptibility and specific heat.
