  We report single crystal growth and magnetic susceptibility and neutron
diffraction studies of the S=1/2 quasi-1D antiferromagnet CaCu2O3. The
structure of this material is similar to that of the prototype two-leg
spin-ladder compound SrCu2O3. However, the Cu-O-Cu bond angle in the ladder
rungs in CaCu2O3 is equal to 123 deg, and therefore the magnetic interaction
along the rungs is expected to be much weaker in this material. At high
temperatures, the magnetic susceptibility of CaCu2O3 can be decomposed into a
contribution from 1D antiferromagnetic chains of finite-size chain segments
together with a weak Curie contribution. The intrachain magnetic exchange
constant, determined from the magnetic susceptibility measurements, is 2000 K.
CaCu2O3 undergoes a Neel transition at T_N=25 K with ordering wavevector of
(0.429(5), 0.5, 0.5). The magnetic structure is incommensurate in the direction
of the frustrated interchain interaction. Weak commensurate (0.5, 0.5, 0.5)
magnetic peaks are also observed below T_N. Application of a magnetic field
induces a metamagnetic transition at which the incommensurability of the
magnetic structure is substantially reduced. The material possesses only
short-range magnetic order above the transition field.
