Spin-resolved electronic structure of EuO across the Curie Temperature.

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy and density functional theory. Valence bands of EuO exhibit unique properties of hosting both the fully exchange split Eu $4f$, as well as O $2p$ levels. Our spin-resolved data directly demonstrates the exchange splitting in O $2p$ and reveals that, while the macroscopic magnetization of the sample vanishes at the Curie temperature $T_C$, the experimentally-determined exchange splitting of the O 2$p$ band persists up to $T_{C}$ if the picture of fluctuating spin-blocks is assumed. We discuss possible temperature-related spectral changes by analyzing ferromagnetic and antiferromagnetic phases, directional effect due to spin-orbit coupling, as well as effects due to sample aging, unavoidable for this highly reactive material. Our calculations with a Hubbard $U$ term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands, shining a new light on the interpretation of previous optical and photoemission spectroscopic results.