Anomalously large spin-dependent electron correlation in nearly half-metallic ferromagnet CoS$_2$

TL;DR

This study investigates the electronic structure of CoS$_2$, revealing that it is nearly half-metallic with unusually strong spin-dependent electron correlations, challenging existing theoretical models and advancing understanding of half-metallic ferromagnets.

Contribution

The paper provides spectroscopic evidence of near half-metallicity in CoS$_2$ and highlights the presence of anomalously large spin-dependent electron correlations not captured by standard calculations.

Findings

CoS$_2$ is very close to half-metallic but not fully half-metallic.

Strong mass renormalization observed in minority-spin bands.

Dynamical mean field theory better explains the correlation effects.

Abstract

The spin-dependent band structure of CoS$_2$ which is a candidate for a half-metallic ferromagnet was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations, in order to reappraise the half-metallicity and electronic correlations. We determined the three-dimensional Fermi surface and the spin-dependent band structure. As a result, we found that a part of the minority spin bands is on the occupied side in the vicinity of the Fermi level, providing spectroscopic evidence that CoS$_2$ is not but very close to a half-metal. Band calculations using density functional theory with generalized gradient approximation showed a good agreement with the observed majority spin $e_g$ bands, while it could not explain the observed band width of the minority-spin eg bands. On the other hand, theoretical calculations using dynamical mean field theory could better reproduce the strong mass renormalization in the minority-spin $e_g$ bands. All those results strongly suggest the presence of anomalously enhanced spin-dependent electron correlation effects on the electronic structure in the vicinity of the half-metallic state. We also report the temperature dependence of the electronic structure across the Curie temperature and discuss the mechanism of the thermal demagnetization. Our discovery of the anomalously large spin-dependent electronic correlations not only demonstrates a key factor in understanding the electronic structure of half-metals but also provides a motivation to improve theoretical calculations on spin-polarized strongly correlated systems.