Exchange-anisotropy field in disordered nickel-manganese alloys

Magnetization data for disordered and weakly ordered Ni-Mn alloys from 22.1 to 32.1 at.% Mn have been obtained upon warming from 4.2 to 300 K, for measuring fields up to 100 kOe, after first cooling from 300 to 4.2 K in zero field or in 100 kOe. The measurements show that there is a minimum measuring field for which the data for zero-field cooling match the data for field cooling. This field, for the disordered alloy, increases from 10 kOe at 22.1 at.% Mn to 90 kOe at 32.1 at.% Mn. Partial atomic ordering reduces this coupling field. The coupling field is smaller than expected from previous studies. The data are qualitatively explained by a recently proposed local-environment model which considers explicitly the nearest-neighbor environment of Mn atoms in the alloy.