Magnetic fluctuations and possible formation of a spin-singlet cluster under pressure in the heavy-fermion spinelLiV2O4probed byLi7andV51NMR

$^{7}\mathrm{Li}$ and $^{51}\mathrm{V}$ NMR measurements up to 9.8 GPa have been made to elucidate local magnetic properties of a heavy-fermion spinel oxide ${\mathrm{LiV}}_{2}{\mathrm{O}}_{4}$ which undergoes a metal-insulator transition above $\ensuremath{\sim}7$ GPa. The temperature $T$ and pressure $P$ dependences of the $^{7}\mathrm{Li}$ and $^{51}\mathrm{V}$ Knight shifts and the nuclear spin-lattice relaxation rates $1/{T}_{1}$ show that in the metallic phase, there is a crossover from a high-$T$ region with weak ferromagnetic fluctuations to a low-$T$ one with antiferromagnetic (AFM) fluctuations. The AFM fluctuations are enhanced below 20 K and 1.5 GPa, where a heavy Fermi-liquid state with the modified Korringa relation is formed. The evolution of the magnetic fluctuations is discussed from the aspect of the competition among several magnetic interactions. Above ${P}_{\mathrm{MI}}\ensuremath{\sim}6.7$ GPa, we find the coexistence of metallic and insulating phases due to the first-order metal-insulator transition. The $^{7}\mathrm{Li}$ and $^{51}\mathrm{V}$ NMR spectra coming from the insulating phase have $T$-independent small Knight shifts and ${}^{7}(1/{T}_{1})$ with the thermally activated $T$ dependence, indicating the formation of a spin-singlet cluster. We propose a model of a spin-singlet tetramer as discussed in geometrically frustrated materials.