${\mathrm{CeMnNi}}_4$ exhibits an unusually large spin polarization, but its origin has baffled researchers for more than a decade. We use bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory based on the Green's function technique to demonstrate the importance of electron-electron correlations of both the Ni $3d$ ($U_{\text{Ni}}$) and Mn $3d$ ($U_{\text{Mn}}$) electrons in explaining the valence band of this multiply correlated material. We show that Mn-Ni antisite disorder as well as $U_{\text{Ni}}$ play a crucial role in enhancing its spin polarization: Antisite disorder broadens a Ni $3d$ minority-spin peak close to the Fermi level ($E_F$), while an increase in $U_{\text{Ni}}$ shifts it toward $E_F$, both leading to a significant increase of minority-spin states at $E_F$. Furthermore, the rare occurrence of a valence state transition between the bulk and the surface is demonstrated highlighting the importance of HAXPES in resolving the electronic structure of materials unhindered by surface effects.

• Received 2 June 2018
• Revised 13 November 2018

DOI:https://doi.org/10.1103/PhysRevB.99.035102