Revelation of the Role of Impurities and Conduction Electron Density in the High Resolution Photoemission Study of Ferromagnetic Hexaborides

We investigate the temperature evolution of the electronic structure of ferromagnetic ${\mathrm{CaB}}_6$ using ultrahigh resolution photoemission spectroscopy; the electronic structure of paramagnetic ${\mathrm{LaB}}_6$ is used as a reference. High resolution spectra of ${\mathrm{CaB}}_6$ reveal a finite density of states at the Fermi level ${ϵ}_F$ at all the temperatures and evidence of impurity induced localized features in the vicinity of ${ϵ}_F$, which are absent in the spectra of ${\mathrm{LaB}}_6$. Analysis of the high resolution spectra suggests that disorder in the $B$ sublattice inducing partial localization in the mobile electrons and low electron density at ${ϵ}_F$ is important to achieve ferromagnetism in these systems.

Figure 1

Magnetic hysteresis curves of ${\mathrm{CaB}}_6$, ${\mathrm{LaB}}_6$, and B powder ($\mathrm{\text{purity}}=99.7\%$) used for sample preparation.

Figure 2

(a) Valence band spectrum of ${\mathrm{CaB}}_6$ at 10 K. Calculated (b) TDOS, (c) B $2s$ and B $2p$ PDOS, and (d) Ca $3d$ PDOS.

Figure 3

He I spectra of (a) Ag, (b) ${\mathrm{LaB}}_6$, and (c) ${\mathrm{CaB}}_6$ in the vicinity of ${ϵ}_F$. The right panels exhibit SDOS obtained by dividing the spectra in the left panel by the Fermi-Dirac distribution function.

Figure 4

(a) He I spectra of ${\mathrm{CaB}}_6$ at different temperatures. The lines represent the low temperature spectra derived from SDOS at 150 K. (b) The temperature induced spectral weight transfer calculated by subtracting 150 K spectrum from all the other spectra. (c) Difference spectra at 30 K and 10 K obtained by subtracting the lines in (a) from the corresponding raw data.