  We report magnetization measurements of As-deficient LaO_0.9F_0.1FeAs_1-delta
(delta about 0.06) samples with improved superconducting properties as compared
with As-stoichiometric optimally doped La-1111 samples. In this As-deficient
system with almost homogeneously distributed As-vacancies (AV), as suggested by
the (75)As-nuclear quadrupole resonance (NQR) measurements,we observe a strong
enhancement of the spin-susceptibility by a factor of 3-7. This observation is
attributed to the presence of an electronically localized state around each AV,
carrying a magnetic moment of about 3.2 mu_Bohr per AV or 0.8 mu_Bohr/Fe atom.
From theoretical considerations we find that the formation of a local moment on
neighboring iron sites of an AV sets in when the local Coulomb interaction
exceeds a critical value of about 1.0 eV in the dilute limit. Its estimated
value amounts to ~ 2.5 eV and implies an upper bound of ~ 2 eV for the Coulomb
repulsion at Fe sites beyond the first neighbor-shell of an AV. Electronic
correlations are thus moderate/weak in doped La-1111. The strongly enhanced
spin susceptibility is responsible for the Pauli limiting behavior of the
superconductivity that we observe in As-deficient LaO_0.9F_0.1FeAs_1-delta. In
contrast, no Pauli limiting behavior is found for the optimally doped,
As-stoichiometric LaO_0.9F_0.1FeAs superconductor in accord with its low spin
susceptibility.
