Hard x-ray photoemission spectroscopy of GdNi and HoNi

We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie temperature ${T}_c=69$ and ${T}_c=36\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, respectively. These materials are useful for magnetic refrigerator applications at low temperature as they exhibit a large magnetocaloric effect near ${T}_c$. We have used hard x-ray photoemission spectroscopy (HAXPES) to investigate the core-level and valence-band electronic states of GdNi and HoNi. HAXPES measurements of the Gd and Ho $3d, 4d, 4p, 5p$, and $4s$ core-level spectra have been compared with atomic multiplet calculations of ${\mathrm{Gd}}³⁺$ and ${\mathrm{Ho}}³⁺$ ionic configurations. The good match between the experimental and calculated spectra clarify the important role of spin-orbit coupling, as well as Coulomb and exchange interactions in the intermediate-coupling scheme. The core-level spectra also show plasmons in addition to the atomic multiplets. The Gd and Ho $4s$ spectra show clear evidence of exchange splitting. The Ni $2p$ and $3s$ spectra of GdNi and HoNi show a correlation satellite at a binding energy of 7 eV above their main peaks. The Ni $2p$ and Ni $3s$ spectra could be reproduced using charge transfer multiplet calculations. Valence-band HAXPES of GdNi and HoNi shows that the Gd $4f$ and Ho $4f$ features are also consistent with atomic multiplets and occur at high binding energies away from the Fermi level. The Ni $3d$ density of states are spread from the Fermi level to about 3 eV binding energy. The results indicate a partially filled Ni $3d$ band and show that the charge transfer model is not valid for describing the electronic structure of GdNi and HoNi.