Microscopic observation of the spin-Singlet cluster in the frustrated spinel AlV₂O₄

The spinel oxide AlV₂O₄ exhibits a structural phase transition from cubic to rhombohedral lattice at T_c 700 K, accompanied by V valence skipping. We have investigated the microscopic magnetic properties by the ⁵¹V and ²⁷Al NMR measurements across T_c in a temperature range from 2 K to 850 K. The Knight shifts at the ⁵¹V and ²⁷Al sites selectively monitor the local spin susceptibilities with different temperature dependence. We reveal that there are two V sites having a nonmagnetic ground state and a frustrated paramagnetic spin. The results are consistent with the spin-singlet heptamer formation consisting of seven vanadium atoms with 18 electrons. This transition uniquely releases the spin and charge frustration on the original V pyrochlore lattice with an aid of the orbital order and the structural distortion. The spin frustration is also highlighted in the anomalous magnetic correlations detected by the nuclear spin-lattice relaxation rate at the V and Al sites.