The series of compounds Sm2−xGdxCo17−yMnywas studied with regard to its suitability for employment as permanent magnet material in microwave/millimeter‐wave traveling‐wave tubes (TWT′s). The pertinent properties investigated are saturation magnetization, 4&pgr;Ms, temperature coefficient of 4&pgr;Ms, &agr;, and anisotropy field, HA. The composition ranges investigated were from x =0 to 0.6 and y =0 to 2. Values of 4&pgr;Msat 25 C ranged from 12.8 kG for x = 0, y = 1 to 9.4 kG for x = 0.6, y = 2. For the compounds studied, maximum HAoccurs at y = 1 for x = 0. The compounds, whose formulas contain a single atom of manganese, were found to yield excellent temperature compensation via Gd substitution in the range +20 to +150 C with the best coefficient occurring in Sm1.6Gd0.4Co16Mn for which &agr; = −0.008&percent;/C (4&pgr;Ms=10.5 kG).