Long-range antiferromagnetism in disordered FeNiMn alloys

Abstract

Neutron diffraction measurements at 77 and 298 °K reveal long-range antiferromagnetic ordering in the atomically-disordered face-centered cubic alloys with more than about 50 at. per cent Fe in the ternary series ( Ni, Fe) 3 Mn. Consistent with these results, the Néel temperatures of these alloys, deduced from the temperature dependence of their electrical resistivities, rise from 210 to 425°K as the iron concentration increases from 50 to 75 at. per cent. An antiferromagnetic structure that conforms to the neutron diffraction polycrystal data is the collinear, tetragonal configuration previously reported for the Mn-rich Mn-Cu alloys. However, quite unlike Mn-Cu, the Fe-Ni-Mn alloys give no detectable X-ray diffraction evidence for a tetragonal (or any other) distortion of their cubic atomic arrangement. Hence, an alternative structure proposed for these alloys is one that is cubic though not collinear, the moments on four equivalent sublattices being oriented along different 〈111〉 directions such that their vector sum is zero. Both these magnetic structures are then shown to be special cases of a general set of non-collinear, tetragonal configurations which produce the same neutron diffraction pattern for a given magnitude of the average atomic moment \\ ̄ gm For the ( Ni, Fe) 3 Mn series it is found that \\ ̄ gm increases from 1.1 to 1.7μ B with increasing iron concentration from 50 to 75 at. per cent, which is shown to be at least in part due to an improved degree of long-range magnetic order. The results of the present work are compared and related to the unusual ferromagnetic-antiferromagnetic behavior of the more Ni-rich alloys in this ternary series and to the low-temperature antiferromagnetic structure recently reported for γ-iron.