Exchange field effect in the crystal-field ground state of CeMAl4Si2

The crystal-field ground-state wave functions of the tetragonal, magnetically ordering Kondo lattice materials $\mathrm{Ce}M{\mathrm{Al}}_{4}{\mathrm{Si}}_{2}$ $(M=\text{Rh}$, Ir, and Pt) are determined with low-temperature linearly polarized soft-x-ray absorption spectroscopy, and estimates for the crystal-field splittings are given from the temperature evolution of the linear dichroism. Values for the dominant exchange field in the magnetically ordered phases can be obtained from fitting the influence of magnetic order on the linear dichroism. The direction of the required exchange field is $\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}c$ for the antiferromagnetic Rh and Ir compounds, with the corresponding strength of the order of ${\ensuremath{\lambda}}_{\text{ex}}\ensuremath{\approx}6\phantom{\rule{4pt}{0ex}}\text{meV}$ (65 K). Furthermore, the presence of Kondo screening in the Rh and Ir compound is demonstrated on the basis of the absorption due to ${f}^{0}$ in the initial state.