  We consider the spin-1/2 two-leg ladders with ferromagnetic (FM) interactions
along legs and rungs. Using the stochastic series expansion QMC method, we
study the low-temperature magnetic behavior of the system. An isolated spin-1/2
FM two-leg ladder is in the gapped saturated FM phase at zero temperature. As
soon as the spin-1/2 FM two-leg ladders are connected with antiferromagnetic
(AFM) inter-ladder interaction, a first-order commensurate-incommensurate
quantum phase transition occurs in the ground state magnetic phase diagram. In
fact a jump in the magnetization curve is observed. We found that, coupled
spin-1/2 FM two-leg ladders are in a nonmagnetic phase at zero temperature.
Applying a magnetic field, the ground state of coupled spin-1/2 FM two-leg
ladders remains in the nonmagnetic phase up to a quantum saturate critical
field.
