Metal-Insulator Transitions ofV2O3: Magnetic Susceptibility and Nuclear-Magnetic-Resonance Studies

Abstract

Magnetic susceptibilities have been measured and $^{51}\mathrm{V}$ nuclear magnetic resonances have been observed in a series of compounds ${({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x})}_{2}{\mathrm{O}}_{3}$ and ${({\mathrm{V}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x})}_{2}{\mathrm{O}}_{3}$ ($0\ensuremath{\le}x\ensuremath{\le}0.04$). The compounds are of interest because of their metal-insulator transition in the composition and temperature (4.2-900 \ifmmode^\circ\else\textdegree\fi{}K) range studied. The paramagnetic insulating phase is found to possess a higher susceptibility than the metallic phase, a more positive $^{51}\mathrm{V}$ nuclear-resonance frequency shift, and a reduced $d$-spin hyperfine coupling constant. The nuclear-resonance relaxation and linewidth are understood in terms of a metallic state with a high band susceptibility and an insulating state with local paramagnetic moments. Anomalies observed previously in $^{51}\mathrm{V}$ frequency-shift-susceptibility relationships for pure ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$ are shown to result from the gradual supercritical change from metalliclike to insulatinglike behavior of pure ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$. A high degree of covalency in the insulating phase is implied by the frequency shifts of $^{27}\mathrm{Al}$ nuclear resonances also observed in the samples containing Al.