  We report on isofield curves of $\sqrt{-M}$ vs. T, where M is the reversible
magnetization, of YBa_2Cu_3O_{6.95}, YBa_2Cu_3O_{6.65}, and
Bi_2Sr_2CaCu_2O_{8+x} with the magnetic field, H, applied parallel to the
c-axis of the samples (and also parallel to the ab- planes for YBaCuO). For
temperatures close to the critical temperature, T_c, the quantity \sqrt{-M} is
proportional to the order parameter amplitude |\psi|. Curves of \sqrt{-M} vs. T
allowed to study the asymptotic behavior of the form (T_a-T)^m of |\psi| near
T_c, as a function of field. Results for the studied samples produced values of
T_a(H) lying above T_c, suggesting that the magnetic field gradually allows to
probe a region of temperatures where phase correlations persist above T_c. The
study performed here in YBaCuO samples allowed to study how phase correlations
evolve with doping in the pseudo-gap region of YBaCuO. \sqrt{-M} vs. T curves
for all samples show a rather large amplitude fluctuation with no phase
correlation extending well above T_a(H) which is interpreted in terms of a
Gaussian Ginzburg-Landau approach with a {total-energy} cutoff in the
fluctuation spectrum. Resulting values for the exponent m found for all
samples, 0.5 < m < 0.7, are interpreted as due to phase fluctuations of the
d-wave pairing symmetry of the order parameter in the ab- planes.
