Neutron-Diffraction Study of Magnetic Ordering in UP0.75S0.25

The magnetic ordering of U${\mathrm{P}}_{0.75}$ ${\mathrm{S}}_{0.25}$, which has the NaCl-type structure, has been determined by powder neutron-diffraction techniques. The initial ordering at about 100°K is ferromagnetic. At around 70°K, a ferrimagnetic structure appears in which ferromagnetic (001) sheets are directed along the $c$ axis in the sequence 5+, 4-, 5+, 4-,.... This corresponds to a ferrimagnetic antiphase type of structure in which the period of modulation is $4.5c$ and which closely resembles the low-temperature structure of thulium. The ordered moment in this structure is about $1.4{\mu }_B$ per uranium atom, and the net spontaneous moment is $0.16{\mu }_B$, which is in agreement with magnetization measurements. Below about 20°K, a further transition occurs, resulting in the type-$\mathrm{I}A$ antiferromagnetic structure, in which the sheets are stacked in the sequence 2+, 2-, 2+, 2-,... with an ordered moment of $1.7\pm 0.05{\mu }_B$. The magnetic ordering in this material, which is a good conductor of electricity, provides strong evidence that long-range oscillatory interactions of Ruderman-Kittel-Kasuya-Yosida (RKKY) type between localized moments are important in actinide materials.