Temperature-dependent spin-resolved electronic structure of EuO thin films

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly dispersive nearly fully polarized Eu 4 f bands, as well as O 2p levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrate the exchange splitting in O 2p and its vanishing at the Curie temperature. Our calculations with a Hubbard U term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of indirect exchange in the O 2p levels by analyzing the orbital resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu 4 f band manifold. The analysis of the shape of spin-resolved spectra in the Eu 4 f spectral region reveals signatures of hybridization with O 2p states, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature, and demonstrate that they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material.