Magnetic ordering in $\mathrm{Ce}{M}_2{\mathrm{Si}}_2$ ($M=\mathrm{A}\mathrm{g},\mathrm{A}\mathrm{u},\mathrm{P}\mathrm{d},\mathrm{R}\mathrm{h}$) compounds as studied by neutron diffraction

We report the results of neutron-diffraction experiments on $\mathrm{Ce}{M}_2{\mathrm{Si}}_2$ ($M=\mathrm{A}\mathrm{g},\mathrm{A}\mathrm{u},\mathrm{P}\mathrm{d},\mathrm{R}\mathrm{h}$) which were performed to explore the role of valence fluctuations and $4f$ hybridization in the magnetic ordering of cerium compounds. All four order antiferromagnetically, the first three exhibiting structures consisting of ferromagnetic layers with moments perpendicular to the layers, which are believed to be characteristic of $4f-4f$ interactions mediated through hybridization with conduction electrons. Ce${\mathrm{Pd}}_2$ ${\mathrm{Si}}_2$ has an anomalously small moment ($0.62{\mu }_B$) in the ordered state. Ce${\mathrm{Ag}}_2$ ${\mathrm{Si}}_2$ exhibits an incommensurate longitudinal, static magnetization wave with moment and propagation direction along the $a$ axis. The fourth compound, Ce${\mathrm{Rh}}_2$ ${\mathrm{Si}}_2$, has the highest known transition temperature (39 K) reported for cerium ordering; it exhibits another second-order transition at 27 K to a complex commensurate structure with modulated moments. The results are discussed in terms of the effects that $4f$ hybridization can have on ordering.