  The electronic spin susceptibility of YBa_2Cu_3O_{6.95} has been measured
with high precision up to 24 Tesla with 17O nuclear magnetic resonance. Its
temperature dependence can be accounted for by superconducting fluctuations
that result in a smooth crossover from the normal to the vortex liquid state. A
magnetic field-temperature phase diagram for this crossover has been
established having strong upward curvature.
