Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO4·(1)/(2)D2O

Magnetic excitations have been investigated in the spin dimer material VODPO4 ·(1)/(2)D2O using inelastic neutron scattering. A dispersionless magnetic mode was observed at an energy of 7.81(4)meV. The wave vector dependence of the scattering intensity from this mode is consistent with the excitation of isolated V4+ spin dimers with a V-V separation of 4.43(7) A. This result is unexpected since the V-V pair previously thought to constitute the magnetic dimer has a separation of 3.09 A. We identify an alternative V-V pair as the likely magnetic dimer, which involves superexchange pathways through a covalently bonded PO4 group. This surprising result casts doubt on the interpretation of (VO)2P2O7 as a spin ladder. copyright 1997 The American Physical Society