Intrinsic magnetic properties of uqaternary Sm_(1-x)R_xMn_2Ge_2 compounds with R=Y, x=0.25 and R=Gd, x=0.25, 0.5, and 0.6 have been studied at temperatures ranging 4.2 K to 350 K and in applied field strengths up to 60 kOe. Partial substitution of Sm by Gd or Y atoms raises the transition temperature T_(AFM/FM) of antiferromagnetic (AFM) to ferromagnetic phase (FM) to vicinity of room temperature. A magnetic phase diagram for Sm_(1-x)Gd_xMn_2Ge_2 as a function of Gd concentration has been determined. It was found that replacement of 50
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