Onset of Magnetism in Vanadium Oxides: 51V NMR Studies of VO

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Abstract
Vanadium ``monoxide'' VOx exists over a wide homogeneity range (VO0.79–VO1.30) in the cubic NaCl structure. With increasing oxygen content or V‐vacancy content, its properties progress from weak temperature‐independent paramagnetism to a stronger temperature‐dependent paramagnetism, and from metallic to semiconducting conductivity. In the present work we have observed the nuclear resonance of 51V in small single‐crystal pieces of pressure‐annealed VO0.86, VO1.02, and VO1.23, using cw and spin‐echo techniques in fields of 9–50 kOe at temperatures from 1.4°–300°K. In contrast with previously reported measurements, in no compound was there evidence of a sharp metal‐to‐insulator transition, and nuclear resonance was observable at all temperatures in all the compounds. A Knight shift of 0.4%, independent of composition and temperature, was observed, while the resonance linewidths increased with increasing susceptibility to a value of 5% of the applied field at 1.4°K in VO1.23. These results indicate that the bulk of the temperature‐dependent magnetization comes from local moments on a minority of sites whose nuclear resonances are unobservable. These moments, however, cause the broadening of the observed majority‐site resonance. A maximum in the transverse relaxation rates observed between 1.4° and 77°K in VO1.23 in a 47‐kOe field apparently results from the onset of spin‐spin correlations in this range.