Experimental observation of temperature and magnetic-field evolution of the $4f$ states in ${\mathrm{CeFe}}_2$ 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 $4f^1$ 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.

Figure 1

Experimental results for ${\mathrm{CeFe}}_2$ measured at $T=10$ K with $H=10$ T (red curves) and at 100 K with 1 T (blue curves) compared with those for ${\mathrm{CeB}}_6$ at 10 K with 10 T (black curves). (a),(b) Ce $M_{4,5}$ XAS and (c),(d) XMCD spectra. (d) The spectra are normalized to the maximum Ce $M_4$ peak intensity. (e) Normalized XMCD integration curves. (f) Ce $M_5$ XMCD in (d) together with those obtained at 10 K with 1 T and 100 K with 10 T, all normalized to the maximum Ce $M_4$ peak intensity. (g) Fe $L_{2,3}$ XAS and XMCD spectra of ${\mathrm{CeFe}}_2$.

Figure 2

(a) Fe $L_3$ XMCD peak intensity as a function of applied magnetic field at 10 and 100 K. Inset: temperature dependence of the bulk magnetization $M\left(T\right)$ under 1 and 7 T. (b) Field scans of the $\mathrm{Ce}{M}_{4,5}$ XMCD signals at peak A and peak B as marked in Fig. 1 for 10 and 100 K normalized at 10 K with 10 T along with the XMCD-derived $\mathrm{Ce}4f$ orbital moments and $M\left(H\right)$ data at 2 and 100 K.

Figure 3

(a) Schematic illustration of the different effects of applied ($H$) and exchange ($H_{\mathrm{exch}}$) fields on the magnetic moments originated from a ${\mathrm{Ce}}^{3+} 4f^1$ configuration. The spin ($m_S$) and orbital ($m_L$) moments are coupled antiparallel and parallel to each other within the $J=5/2$ and $7/2$ states, respectively, following Hund's rule. The total moments, $m_{\mathrm{total}}=m_S+m_L$, of the $J=5/2$ and $7/2$ states are magnetically aligned parallel by $H$, while aligned antiparallel by $H_{\mathrm{exch}}$. (b) Brillouin functions for $J=5/2$ and $J=7/2$ plotted as a function of $T/H_{\mathrm{Ce}}$.