  We have performed the powder neutron diffraction measurements on the solid
solutions of SrRu_{1-x}Mn_xO_3, and found that the itinerant ferromagnetic
order observed in pure SrRuO_3 changes into the C-type antiferromagnetic (AF)
order with nearly localized d electrons in the intermediate Mn concentration
between x=0.4 and 0.6. With increasing x, the AF moment is strongly enhanced
from 1.1 mB (x=0.4) to 2.6 mB (x=0.6), which is accompanied by the elongation
of the tetragonal c/a ratio. These results suggest that the substitution of Mn
for Ru suppresses the itinerant character of the d electrons, and induces the
superexchange interaction through the compression in the c plane. We have also
found that the magnetic and transport properties observed in our tetragonal
samples are quite similar to those of recently reported orthorhombic ones.
