Resonant photoemission studies of (Eu,Gd)Te/Te thin layers

Substituting Eu2+ by Gd3+ ions in antiferromagnetic EuTe semiconductor crystal we can control electrical and magnetic properties of this material. Since Gd3+ ions are donors in (Eu,Gd)Te the electrical conductivity becomes metallic. The free electron concentration is enough high so the Eu ions ferromagnetically interact via free electrons (RKKY mechanism). The protection of (Eu,Gd)Te layers from oxidation is the key task because of the change of the Eu ion's charge state from +2 to +3. As a result of oxidation the free electron concentration dramatically decreases and the oxidized (Eu,Gd)Te material returns to the antiferromagnetic state. As a protecting layer we propose to use the amorphous tellurium. The thermal treatment conditions are investigated in this work. It is observed that the acceptable way to evaporate the protecting Te layer and improve the crystal quality of (Eu,Gd)Te layer is the annealing in UHV chamber at 320 oC during long time (above 40 h). We carried out the photoemission spectroscopy measurements for study of the valence band electronic structure of (Eu,Gd)Te layers. Based on the Fano-type resonance photoemission spectra of Eu 4d-4f transition the resonance and antiresonance energies were determined for Eu2+ and Eu3+ ions. We defined that Eu ions are in +2 charge state in as grown (Eu,Gd)Te layer and amount of Eu3+ ions is negligibly low. (author)