Antiferromagnetic ordering in a mixed-valent cerium compound CeRuSn

Abstract

We report the results of extensive neutron powder diffraction experiments on a ternary compound CeRuSn in a temperature range from 2 to 520 K. At room temperature CeRuSn orders in a monoclinic structure that is related to the CeCoAl type by the doubling of the $c$-axis parameter caused by the splitting of the original single sites for each element into two sites. Upon cooling a very broad hysteretic structural transition has been observed in CeRuSn leading to the presence of higher-ordered variants having multiple $c$-axis parameters. Depending on the temperatures, one or the other variant seems to be more stable, but the sample as a whole becomes multiphase. We find also evidence for further symmetry lowering at low temperatures. CeRuSn is found to exhibit an antiferromagnetic order below ${T}_{N}=2.8(1)$ K. The order is described by two incommensurate propagation vectors ${q}_{1}=(0,0,\ensuremath{\approx}0.30)$ and ${q}_{2}=(0,0,\ensuremath{\approx}0.43)$ with respect to the doubled crystal structure variant. Although the details of the magnetic structure remain elusive, the best agreement has been found for a complicated distorted cycloidal magnetic structure with Ce moments of $\ensuremath{\approx}$0.7 ${\ensuremath{\mu}}_{B}$ being confined to the $ac$ plane.