Change of magnetic ground state by light electron doping in CeOs ²Al¹⁰

The effect of Ir substitution for Os in CeOs²Al¹⁰, with an unusually high Néel temperature of T^*∼28.5 K, has been studied by high-resolution neutron diffraction and magnetization measurements. A small amount of Ir (∼8%) results in a pronounced change of the magnetic structure of the Ce sublattice. The induced magnetic ground state is controlled by single ion anisotropy and implies an antiferromagnetic arrangement of the Ce moments along the a axis, as expected from the anisotropy of the paramagnetic susceptibility. The value of the ordered moments, 0.92(1)μ_B, is substantially bigger than in the undoped compound, whereas the transition temperature is reduced down to 21 K. A comparison of the observed phenomena with the recently studied CeRu_1.9Rh _0.1Al¹⁰ system, exhibiting similar behavior, strongly suggests the electron doping as the main origin of the ground state changes. This provides additional ways for exploring the anomalous magnetic properties of the Ce(Ru/Os)²Al¹⁰ compounds.