MAGNETISM AND STRUCTURAL STABILITY IN COMN ALLOYS

The magnetic and structural properties of Co100-xMnx alloys have been studied in the range 0 less-than-or-equal-to x less-than-or-equal-to 50 at. %. The structural phase diagram, which was revised with the aid of resistivity and thermal-expansion measurements for 4 < T < 1200 K, shows that the fcc <-> hcp (gamma <-> epsilon) martensitic transformation temperature in Co-rich ferromagnetic CoMn alloys decreases progressively with increasing Mn content and vanishes at x congruent-to 32 at. %, rather than abruptly decreasing at a lower concentration as previously suggested. It is found that antiferromagnetic order, which occurs at x > 32 at. %, causes a large increase in the atomic volume, thereby preventing the structure from collapsing into the smaller volume hcp phase. The atomic volumes at 0 K of fcc Co and fcc Mn, which were determined by extrapolation, were found to be in good agreement with those obtained from band calculations.