Neutron scattering studies of spin-1/2 twofold-period (alternating) and threefold-period quantum antiferromagnetic chains

We studied magnetism of two cuprates using inelastic neutron scattering and magnetization measurements on powder samples. A spin gap of 6.2  meV (72  K) was observed in -AgCuPO₄. The spin system was determined as a spin-1/2 twofold-period (alternating) antiferromagnetic (AF) chain with two exchange interaction values of 80 and 14  K. Contrary to common knowledge, the strongest exchange interaction exists in a Cu–Cu bond with the distance of 5.20  Å in spite of the existence of short Cu–Cu bonds. Cu₃(P₂O₆OD)₂ is known to have a spin-1/2 threefold-period chain with J₁-J₂-J₂ interactions, where J₁ and J₂ denote two AF exchange interaction parameters, showing a 1/3 magnetization plateau. A spin gap of 9.8  meV (114  K) was found, which generated the magnetization plateau. The gap corresponds to a singlet-triplet-like excitation of an AF dimer formed by the dominant J₁ interaction. We determined that J₁=111  K and J₂=30  K. ©2008 American Institute of Physics