MAGNETOELASTIC TRANSITION AND ANTIFERRO-FERROMAGNETIC ORDERING IN THE SYSTEM MNFEP1-YASY

Neutron diffraction experiments have been performed on a range of hexagonal structures in the MnFeP1-yAsy system which exhibit a magnetoelastic transformation. Local transformations accompanying both the hexagonal crystal structure and the magnetic ordering have been measured. The anisotropic changes observed in the unit cell parameters arise primarily from the expansion of the shortest Fe-Fe distance when the compounds transform from the antiferro- to the ferromagnetic state. Substitution of As for P favours such a transition via the preferential expansion of selected basal interatomic bond lengths. The incommensurate type of antiferromagnetic ordering indicates some competing interatomic exchange interactions. The net result of these phenomena is that the magnetic moment of iron increases markedly and adopts a more localized weak ferromagnetic state.