High-Pressure Suppression of the Magnetic State of V2O3: 51V Nuclear Resonance at 4.2°K and 65 kbar

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Abstract
The metal to insulator transition of V2O3 proceeds from a low‐temperature antiferromagnetically ordered insulating state1 to a high‐temperature metallic state. Pressures greater than 26 kbar completely suppress the metal‐insulator transition,2 leaving V2O3 metallic to the lowest temperatures. This allows study of the metallic‐phase magnetic properties at low temperature. Here we report high‐pressure 51V nuclear magnetic resonance (NMR) measurements in V2O3 which show that the metallic phase, unlike the insulating phase, does not order magnetically down to 4.2 K. The measurements were made with spin echo and free induction decay techniques at a frequency of 6.9 MHz in a cryogenic press with a nonmagnetic high‐pressure die, and are the highest pressure resonance measurements which have been made at cryogenic temperatures. The 51V NMR was observed at pressures greater than 26 kbar with a linewidth of 300 G and a Knight shift of (− 1.0±0.2) % + (0.01±0.005) P% (P in kbar). At lower pressures the resonance intensity decreased rapidly, evidently as antiferromagnetic ordering produced larger frequency shifts and moved the resonance out of the observable range. The linewidth value implies that no static configuration of vanadium antiferromagnetism with moments larger than 10−3 μB exists in the metallic phase, while the Knightshift value shows that no space‐average spin magnetization greater than 0.5×10−3 μB per vanadium atom exists under the experimental conditions. It is concluded that there are no static localized moments in metallic V2O3. Any fluctuating moments would have to fluctuate at frequencies ≥1013 sec−1 to account for the 4.2 K relaxation rate of ≤200 sec−1. This is nearly a band frequency, indicating that metallic V2O3 is best described by an exchange‐enhanced band picture. The pressure dependence of the Knight shift implies that the d‐spin component of the susceptibility is unusually strongly dependent on volume, with dlnχd/dln V = 8±5. This strong volume dependence, together with a correspondingly anomalous volume dependence of the resistivity,2 suggests that metallic V2O3 is very nearly critical with respect to formation of an insulating paramagnetic state. Indeed, alloying of ≳1% of Cr with V2O3 produces a first‐order transition at T>170 K to a paramagnetic insulating phase.3 This transition upon Cr alloying involves neither a lattice symmetry change nor magnetic ordering and has all the features of a Mott metal‐insulator transition.

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