The magnetization
M(h) is used to identify three singlet quantum phases of the ladder with isotropic exchange interactions. The Dimer phase with frustrated F exchanges in rungs and legs has a first-order
M(h) transition at
0 K from singlet to ferromagnetic at the saturation field
hs. The Haldane-DAF phase with strong F exchange in rungs and net AF exchange between rungs has continuous
M(h) and is adiabatically connected to the
S=1 Heisenberg AF chain. The AF phase with strong F exchange in legs and net AF exchange between legs has continuous
M(h) and is adiabatically connected to the spin-1/2
J1−J2 model with
J1>0 and
J2<0. All three singlet phases have finite gaps to the lowest triplet state.
The quantum phases of two-leg spin-
21 ladders with skewed rungs are obtained using exact diagonalization of systems with up to 26 spins and by density-matrix renormalization-group calculations to 500 spins. The ladders have isotropic antiferromagnetic (AF) exchange
J2>0 between first neighbors in the legs, variable isotropic AF exchange
J1 between some first neighbors in different legs, and an unpaired spin per odd-membered ring when ${J}_{1}\ensuremath{\gg}{J}_{2}$. Ladders with skewed rungs and variable
J1 have frustrated AF interactions leading to multiple quantum phases: AF at small
J1, either ferromagnetic or AF at large
J1, as well as bond-order-wave phases or reentrant AF (singlet) phases at intermediate
J1.
We study an isotropic Heisenberg spin-
21 model on a trellis ladder which is composed of two ${J}_{1}\ensuremath{-}{J}_{2}$ zigzag ladders interacting through antiferromagnetic rung coupling
J3.
J1 and
J2 are ferromagnetic zigzag spin interaction between two legs and antiferromagnetic interaction along each leg of a zigzag ladder. A quantum phase diagram of this model is constructed using the density matrix renormalization group method and linearized spin-wave analysis. In the small
J2 limit, a short-range striped collinear phase is found in the presence of
J3, whereas, in the large
J2/J3 limit, a noncollinear quasi-long-range phase is found. The system shows a short-range noncollinear state in the large
J3 limit. The short-range-order phase is the dominant feature of this phase diagram. We also show that the results obtained by DMRG and linearized spin-wave analysis show a similar phase boundary between the collinear striped and noncollinear short-range phases, and the collinear phase region shrinks with increasing
J3. We apply this model to understand the magnetic properties of
CaV2O5 and also fit the experimental data of susceptibility and magnetization. We note that
J3 is a dominant interaction in this material, whereas
J1 and
J2 are approximately half of
J3. The variation of magnetic specific heat capacity as a function of temperature for various external magnetic fields is also predicted.