The Nature of the Mn Moment in Laves Phase Compounds: Evolution of the Magnetic Order in Ho1–xYxMn2

Neutron powder diffraction has been used to study the evolution of long range magnetic order in the pseudobinary CI 5 Laves phase system Ho1-xYxMn2. Particular attention has been paid to the nature of the Mn moment. At Y-rich compositions (x > 0.9) an incommensurate antiferromagnetic structure, similar to that of YMn2 is observed. Transition to the ordered state, as in YMn2, is accompanied by a 5% expansion of the unit cell, identifying the Mn moments, of 2.7 μB, as intrinsic. The magnetic structure of compositions with x<0.7 resembles that of HoMn2 and DyMn2, with only one quarter of the chemically equivalent Mn sites possessing a moment of 0.6 μB induced by the local symmetry of the antiferromagnetically ordered Ho sublattice. Transition to the ordered state is not accompanied by a cell expansion. Between x = 0.7 and x = 0.9 there is no longe range magnetic order, nor is there an expansion of the unit cell, suggesting the total absence of either induced or intrinsic Mn moments. The results indicate that a critical Mn-Mn near neighbour distance of 2.663 Å is necessary to sustain an intrinsic Mn moment in these compounds.