Magnetic structure of the antiferromagnetic Kondo lattice compounds CeRhAl₄Si₂ and CeIrAl₄Si₂

We have investigated the magnetic ground state of the antiferromagnetic Kondo-lattice compounds CeMAl₄Si₂(M = Rh, Ir) using neutron powder diffraction. Although both of these compounds show two magnetic transitions T_N1 and T_N2 in the bulk properties measurements, evidence for magnetic long-range order was only found below the lower transition T_N2. Analysis of the diffraction profiles reveals a commensurate antiferromagnetic structure with a propagation vector k = (0, 0, 1/2). The magnetic moment in the ordered state of CeRhAl₄Si₂ and CeIrAl₄Si₂ were determined to be 1.14(2) and 1.41(3) μ_B Ce^-1, respectively, and are parallel to the crystallographic c-axis in agreement with magnetic susceptibility measurements.