Antiferromagnetism of γ Fe-Mn Alloys

The antiferromagnetism of polycrystalline and single crystal samples of disordered γ Fe-Mn alloys containing between 20 to 50 at% Mn was investigated by means of a magnetic balance, a sensitive torque magnetometer and neutron diffraction. The alloys with 20 to 27 at% Mn exhibited ε ↔γ trans-formation in the same temperature range as the magnetic transformation. The relation between the magnetic and the crystallographic transformations was investigated and a phase diagram of Fe-Mn alloys in this composition region was obtained. A neutron diffraction study of single crystal samples confirmed the generalized antiferromagnetic spin structure proposed by the powder neutron diffraction study of Kouvel and Kasper. The torque measurements, made on single crystals which had been cooled through the Néel point in a strong magnetic field, indicated that the spin structure is intrinsically cubic, the spins on the four sublattices being directed toward the different cube diagonals. The results are discussed from the viewpoint of a molecular field theory based on the localized electron model. However, the temperature dependence of the susceptibility below and above the Néel point cannot be interpreted on the basis of a localized electron model, although the latter can account for the proposed spin structure.