Disorder, frustration and lattice volume effects in YMnIn, Th0.8R0.2MnAl (R=Sc, Lu and Y), ThMn1.3Al0.7 and ThMnAl1−xInx alloys

The antiferromagnetic exchange interaction between the Mn ions in the cubic Laves-phase alloys Th0.8R0.2MnAl, ThMnAl1−xInx (x≤0.2), ThMn1.3Al0.7 and the hexagonal Laves-phase YMnIn is inherently geometrically frustrated due to the tetrahedral co-ordination of the Mn ions. Together with disorder on the Mn-sublattice due to the partial replacement of Mn by Al and/or In, it leads to spin-glass-type freezing of the Mn magnetic moments in these alloys when the average Mn–Mn bond length is 2.850 Å or less. For alloys with greater bond lengths (x>0.2), the data corroborate the suggestion made earlier [J. Magn. Magn. Mater. 231 (2000) 404] that the exchange becomes ferromagnetic.