  The magnon propagator is evaluated in the AF (\pi,\pi,\pi) and the F-AF
(0,\pi,\pi) states at the RPA level, and the spin-fluctuation corrections are
compared. Transverse spin fluctuations are sharply enhanced by the frustration-
inducing NNN hopping t', reducing the zero-temperature AF order in two
dimensions, and the Neel temperature in three dimensions. The phase boundary
between the insulating AF and F-AF states is obtained in the full range of
interaction strength, indicating that the AF state is interestingly stabilized
with decreasing U.
