Experimental phase diagram of Cu-2(C5H12N2)(2)Cl-4: A quasi-one-dimensional antiferromagnetic spin-1/2 Heisenberg ladder

Using magnetisation, susceptibility, and spin resonance techniques, three distinct magnetic phases of Cu-2(C5H12N2)(2)Cl-4, a spin-1/2 ladder, are identified at zero temperature. For magnetic fields below H-c1 = 7.5 T, the one-dimensional ground state is a valence-bond ordered singlet separated from a triplet band by an energy gap Delta(1)k(B) approximate to g mu(B)H(c1)1k(B)=10.8 K. For H-c1<H<H-c2 = 13.2 T, the ground state is probably an antiferromagnetic canted or incommensurate state, and above H,2 the ground state is ferromagnetic. The exchange parameters used to parametrize the magnetic Hamiltonian are determined.