Experimental phase diagra: A quasi-one-dimensional antiferromagnetic spin- Heisenberg ladder

Using magnetization, susceptibility, and spin resonance techniques, three distinct magnetic phases of (Formula presented)((Formula presented)(Formula presented)(Formula presented)(Formula presented)(Formula presented), a spin- ladder, are identified at zero temperature. For magnetic fields below (Formula presented)=7.5 T, the one-dimensional ground state is a valence-bond ordered singlet separated from a triplet band by an energy gap (Formula presented)/(Formula presented)≈g(Formula presented)(Formula presented)/(Formula presented)=10.8 K. For (Formula presented)<H<(Formula presented)=13.2 T, the ground state is probably an antiferromagnetic canted or incommensurate state, and above (Formula presented) the ground state is ferromagnetic. The exchange parameters used to parametrize the magnetic Hamiltonian are determined.