Magnetic Transitions and Structural Changes in Hexagonal Manganese Compounds

The lattice parameters of the isomorphous compounds MnAs, MnSb and MnBi have been measured over a temperature range which includes the temperatures at which certain magnetic transitions occur. Characteristic changes develop near the magnetic transition temperatures and are of two kinds: (a) discontinuous changes of lattice dimensions (MnAs at 40°c, MnBi at 320°c), which are associated with magnetic transitions of the first order, (b) discontinuous changes of the temperature derivatives of the lattice parameters (MnAs at 130°c, MnSb at 320°c), associated with second-order magnetic transitions. Correlation of the results on MnAs and MnSb with those obtained by Greenwald on MnTe indicates that the magnetic exchange energy for manganese atoms at a distance r apart is a maximum for r = 2.94 °. The magnetic change which takes place in MnBi near 440°c is caused by decomposition of the material and is not a Curie point phenomenon.