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 ${\left({\mathrm{V}}_{1-x}{\mathrm{Cr}}_x\right)}_2{\mathrm{O}}_3$ and ${\left({\mathrm{V}}_{1-x}{\mathrm{Al}}_x\right)}_2{\mathrm{O}}_3$ ($0\le x\le 0.04$). The compounds are of interest because of their metal-insulator transition in the composition and temperature (4.2-900 °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.