Spin-singlet formation in the geometrically frustrated spinel oxide AlV₂O₄: ⁵¹V and ²⁷A l NMR measurements

We report V 51 and A 27 l NMR studies of the spin-frustrated spinel oxide AlV2 O4 in a temperature range from 850 to 2 K across a structural phase transition at 700 K. Knight shifts and nuclear spin-lattice relaxation rates at the V and Al sites show that the local spin susceptibility at two V sites diminishes at low temperatures in the insulating state, giving direct evidence for the spin-singlet formation. In addition to the singlet spins, one of the two Al sites probes paramagnetic spins with the Curie-Weiss behavior and the spin-glass transition. The results demonstrate coexistence of the spin-singlet cluster and the nearly free spins, anticipated from a theoretical model and the bulk magnetic-susceptibility measurements.