ENHANCEMENT OF ROOM-TEMPERATURE MAGNETORESISTANCE IN La_(0.5)Sm_(0.2)Sr_(0.3)MnO_3/Ag_x

La0.5Sm0.2Sr0.3MnO3/(Ag2O)x/2(x=0.00,0.04,0.08,0.25,0.30) samples were prepared by the solid-state reaction method,and their magnetic proverties ,transport behavior,transport mechanism and magnetoresistance effect were studied through M～T curves measuring ,ρ～T curves measuring and fitting. The results showed that: Ag may take part in reaction when the doping amount is small. Ag is mainly distributed at the grain boundary of the mother body and is in metallic state when the doping amount is relatively large,then the system becomes a two-phase composite. A small amount of Ag doping can apparently increase grain-boundary magnetoresistance induced by spin-dependent scattering. The resistivity of the sample doped with 30mol.% Ag is one order of magnitude smaller than that of low-doped samples,and its magnetoresistance in the magnetic field of 0.5T and at 300K strengthens apparently and reach 9.4%,which is connected not only with the improvement of grain-boundary structure of the mother body but also with the decrease of material resistivity.