Thermal expansion anomaly and Invar effect of Mn 1- xCo xB

We present the results of the thermal expansion of Mn _{1- x}Co _xB with 0.1 ⩽ x ⩽ 0.9. An Invar-type thermal expansion anomaly is observed for 0.1 ⩽ x ⩽ 0.5. The spontaneous volume mangetostriction at 0 K. ω_s(0), is proportional to the Mn content, 1 - x, for x ⩽ 0.5. These results suggest that the Mn moment remains constant for x ⩽ 0.5 and that ω_s(0) is attributable to the collapse of the Mn moments at T_C.