Anomalously large spin-dependent electron correlation in the nearly half-metallic ferromagnet CoS2

The spin-dependent band structure of CoS2, which is a candidate for a half-metallic ferromagnet, was investigated by both spin- and angle-resolved photoemission spectroscopy and theoretical calculations 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 CoS2 is not a half-metal but very close. Band calculations using density functional theory with generalized gradient approximation showed good agreement with the observed majority spin eg 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 eg bands. Our 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.