Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O

Magnetic excitations have been investigated in the spin dimer material VODPO 4 · 1 / 2D 2 O using inelastic neutron scattering. A dispersionless magnetic mode was observed at an energy of 7.81(4) meV. The wavevector dependence of the scattering intensity from this mode is consistent with the excitation of isolated V 4+ 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 PO 4 group. This surprising result casts doubt on the interpretation of (VO) 2 P 2 O 7 as a spin ladder.