Spin fluctuation and the transport mechanism in vanadium oxide spinels with a metal-insulator transition

Spin fluctuation and the transport mechanism in the spinel systems Li_xMg_1-xV₂O₄ and Li_xZn_1-xV₂O₄ with 0<~x<~1 have been studied through measurements of x-ray diffraction, electrical resistivity, thermoelectric power, magnetization, and nuclear magnetic resonance. These compounds range from being antiferromagnetic and insulating for MgV₂O₄ (Mott type) accompanied with a structural transition to the metallic state of LiV₂O₄ with no magnetic order. The metal-insulator transition may be of Anderson type and occurs in the vicinity of x_c=0.4. The coherence length of the wave function of hole carriers in the variable-range-hopping regime has a critical exponent -1.3 against |x-x_c|. The metallic phase above x_c may have two kinds of carriers from dynamic mixed valence state of V³⁺ and V⁴⁺. Based on the magnetic susceptibility and relaxation analyses, metallic compounds may be considered to be highly correlated electron systems with a low degeneracy temperature or large mass enhancement. On the other hand, insulators have short-range ordered spin correlation and/or superparamagnetic effects. At low temperatures, an antiferromagnetic phase is realized for x<~0.05 and a spin-glass phase originating from the frustration inherent in the spinel B lattice appears in the region of 0.07<~x<~0.7. The latter phase is enhanced for concentrations slightly less than x_c.