Long-range Magnetic Interactions (RKKY-type) in the UP-US Solid Solutions

UP, US, and the UP-US solid solutions have the NaCl-type structure and are good conductors of electricity. UP is anti-ferromagnetic of type I; US is ferromagnetic. Assuming U
⁴⁺
cations, P
³⁻
and S
²⁻
anions, and RKKY interactions between the uranium localized 5f-electrons via the conduction electrons (≈1 in UP and ≈2 in US), the observed magnetic structures are accounted for, although these structures are also predicted by molecular field theory. A neutron diffraction study of the UP-US solid solutions has revealed two new magnetic structures, the type IA (2+, 2=) antiferromagnetic structure and the antiphase (5+, 4=) ferrimagnetic structure, which are of long range (several lattice parameters) and are typical of long-range RKKY-type interactions, as assumed in the simple model for these compounds. Further experimental evidence is given for the long-range magnetic interactions in uranium monopnictides and monochalcogenides and their solid solutions. The situation in these uranium compounds is compared with the corresponding lanthanide compounds, and the role of covalency and superexchange in the case of heavier anions is discussed.