Electronic structure investigation of GdNi using x-ray absorption, magnetic circular dichroism, and hard x-ray photoemission spectroscopy

GdNi is a ferrimagnetic material with a Curie temperature ${T}_C=69$ K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at $T=25$ K in the ferrimagnetic phase. We analyze the Gd ${M}_4,5$-edge ($3d\ensuremath{-}4f$) and Ni ${L}_2,3$-edge ($2p\ensuremath{-}3d$) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd ${M}_4,5$-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized $4f$ states. On the other hand, a model cluster calculation for Ni ${L}_2,3$-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest-neighbor Gd states, resulting in a $d$-electron count of 8.57. The Gd ${M}_4,5$-edge XMCD spectrum is consistent with a ground-state configuration of $S=7/2$ and $L=0$. The Ni ${L}_2,3$-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite total magnetic moment (${m}_tot\ensuremath{\sim}0.12{\ensuremath{\mu}}_B$) with the ratio ${m}_o/{m}_s\ensuremath{\sim}0.11$. Valence band hard x-ray photoemission spectroscopy shows Ni $3d$ features at the Fermi level, confirming a partially filled $3d$ band, while the Gd $4f$ states are at high binding energies away from the Fermi level. The results indicate that the Ni $3d$ band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge-transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni $3d$ and Gd $4f$ states.