Magnetic effects on the optical absorption of EuO and EuS

Abstract

The relationship between optical absorption and ferromagnetic order has been investigated on single crystals and thin films of EuO and EuS. The 4f 7-4f65d transition can be identified in evaporated films as an absorption peak of ∼105 cm-1 at 2.1 and 2.4 eV, respectively, which tails out exponentially to lower energies. The onset of the ferromagnetic order is accompanied by an unusually large red shift of the absorption peak which superimposes on the normal magneto optical dichroism. The detailed temperature dependance of this shift does not relate exponentially to the variation in magnetization as found earlier by Busch et al. for powdered samples,1 but is approximately proportional to the nearest neighbor spin correlation function 〈S1S2〉/S2. This is in agreement with an exciton like model for the excited state in which the electron is localized in 5d-orbitals at the excited Eu-ion and its 12 nearest Eu neighbors. The exchange interaction of the excited electron with the neighboring 4f7-spins causes the red shift which is much larger (EuO:0.23 eV; EuS:0.11 eV) and extends to higher temperatures than one expects for the effect of a molecular field on excited electrons in a conduction band. © 1967 The American Institute of Physics.