Experimental observation of temperature and magnetic-field evolution of the 4f states in CeFe2 revealed by soft x-ray magnetic circular dichroism

We revisit the delocalized character of the $4f$ states of ${\mathrm{CeFe}}_{2}$ in the ferromagnetically ordered phase by x-ray magnetic circular dichroism (XMCD) in x-ray absorption spectroscopy (XAS) with improved data quality using single crystals. Surprisingly, the Ce ${M}_{4,5}$ XMCD spectral shape changes significantly as a function of temperature and applied magnetic field, with no concomitant changes in the spectral shape of the Ce ${M}_{4,5}$ XAS as well as the Fe ${L}_{2,3}$ XAS and XMCD. This unusual behavior is characterized by the $J=7/2$ states in a $4{f}^{1}$ configuration mixed into the $J=5/2$ ground state. Such extreme sensitivity of the Ce $4f$ states to the external perturbations can be related to the magnetic instability toward an antiferromagnetic phase in ${\mathrm{CeFe}}_{2}$. Our experimental data presented here provide valuable insights into the underlying physics in strongly hybridized ferromagnetic Ce compounds.