Magnetic structure and crystal-field levels in PrAg

Neutron scattering experiments have been performed on polycrystalline samples of PrAg (CsCl structure) to determine the magnetic structure at low temperature and the energy separation of the crystal-field levels. At 14°K PrAg becomes antiferromagnetic with the ($\pi \pi 0$) structure. This consists of ferromagnetic (110) planes coupled antiferromagnetically. Ordered moments of (2.1 ± 0.1)${\mathrm{\mu }}_{\mathrm{B}}$/(Pr atom) lie in the (001) planes. The crystal-field energy levels in PrAg have been measured directly with neutron spectroscopy at 5, 78, and 297°K. The measurements indicate that the ${\Gamma }_5$ level is the ground state. The over-all splitting is ∼ 20 meV and the crystal-field parameters $W$ and $x$ are —0.37 ± 0.02 meV and —1.00 ± 0.01, respectively. A calculation based on a first- and second-neighbor point-charge model with neutral Ag ions and tripositive Pr ions gives reasonable agreement with the experimental parameters. The linewidths of the crystal-field transitions are observed to be very broad (full widths at half-maximum are ∼ 11 meV at 297°K). A simple Ruderman-Kittel-Kasuya-Yosida (RKKY) model has been used to explain these widths, which are at least a factor of 2 larger than observed, for example, in the praseodymium monopnictides.