  Systematic P-NMR studies on LaFe(As_{1-x}P_x)(O_{1-y}F_y) with y=0.05 and 0.1
have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low
energies are markedly enhanced around x=0.6 and 0.4, respectively, and as a
result, Tc exhibits respective peaks at 24 K and 27 K against the
P-substitution for As. This result demonstrates that the AFMSFs are responsible
for the increase in Tc for LaFe(As_{1-x}P_x)(O_{1-y}F_y) as a primary mediator
of the Cooper pairing. From a systematic comparison of AFMSFs with a series of
(La_{1-z}Y_z)FeAsO_{\delta} compounds in which Tc reaches 50 K for z=0.95, we
remark that a moderate development of AFMSFs causes the Tc to increase up to 50
K under the condition that the local lattice parameters of FeAs tetrahedron
approaches those of the regular tetrahedron. We propose that the T_c of
Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the
AFMSFs and other factors originating from the optimization of the local
structure.
