Probing the magnetic exciton in EuS during the ferromagnetic transition with valence-to-core RIXS at the Eu L3 edge

EuX (X = O, S, Se, Te) are rare examples of intrinsic magnetic semiconductors. They crystallize in the rock salt structure and the Eu2+ ions have a half-filled 4f shell carrying a purely spin magnetic moment, making them prototypical Heisenberg magnets. The underlying mechanism behind the magnetic behaviour is still discussed. In the ‘70s the debate focused on a double peak structure observed in the UV-visible spectrum. Kasuya's group ascribed these features to magnetic excitons, i.e. localized many-body states which mediate the magnetisation. More recently, Matsubara and collaborators demonstrated that the ferromagnetism can be tuned optically by changing the carrier density. We recently provided the first experimental proof that the excitonic interpretation from Kasuya is correct by using valence-to-core RIXS at the Eu L3 edge on EuS. With this proposal we will extend our RIXS investigation to characterize the magnetic exciton under the ferromagnetic transition.