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.
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