Coexistence of double alternating antiferromagnetic chains in (VO)₂P₂O₇: NMR study

Nuclear magnetic resonance (NMR) of ³¹P and ⁵¹V nuclei has been measured in a spin-1/2 alternating-chain compound (VO)₂P₂O₇. By analyzing the temperature variation of the ³¹P NMR spectra, we have found that (VO)₂P₂O₇ has two independent spin components with different spin-gap energies. The spin gaps are determined from the temperature dependence of the shifts at ³¹P and ⁵¹V sites to be 35 K and 68 K, which are in excellent agreement with those observed in the recent inelastic neutron scattering experiments [A.W. Garrett et al., Phys. Rev. Lett. 79, 745 (1997)]. This suggests that (VO)₂P₂O₇ is composed of two magnetic subsystems showing distinct magnetic excitations, which are associated with the two crystallographically inequivalent V chains running along the b axis. The difference of the spin-gap energies between the chains is attributed to the small differences in the V-V distances, which may result in the different exchange alternation in each magnetic chain. The exchange interactions in each alternating chain are estimated and are discussed based on the empirical relation between the exchange interaction and the interatomic distance.