The magnetic properties of rare earth compounds are usually well captured by assuming a fully localized $f$ shell and only considering the Hund's rule ground state multiplet split by a crystal electric field (CEF). Currently, the standard technique for probing CEF excitations in lanthanides is inelastic neutron scattering. Here we show that with the recent leap in energy resolution, resonant inelastic soft x-ray scattering (RIXS) has become an attractive alternative for looking at CEF excitations. This has been used for studying the CEF scheme in ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$, a system that has been investigated intensely for more than two decades now but for which no consensus has been reached yet as to its CEF scheme. Using high energy resolution of about 30 meV as well as polarization analysis in the scattered beam, both features that have become available only very recently in RIXS, allowed us to find a unique CEF description for ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$. The result agrees well with previous inelastic neutron scattering and magnetic susceptibility studies. Due to its strong resonant character, RIXS is applicable to very small samples, presents very high cross sections for all lanthanides, and further benefits from the very weak coupling to phonon excitations. The foreseeable further progress in energy resolution will make this technique increasingly attractive for the investigation of the CEF scheme in lanthanides.

• Received 29 March 2018
• Revised 2 June 2018

DOI:https://doi.org/10.1103/PhysRevB.97.245130