Optical absorption spectra of ferromagnetic Eu 1+xO single crystals at the insulator-metal transition

Abstract

Optical absorption spectra of Eu-rich EuO single crystals have been studied in the spectral range between 1.1 and 3 μm. The measurements were performed in the paramagnetic and ferromagnetic ( T c = 69 K) temperature region (4.2 K ⩽ T ⩽ 300 K) and in external fields up to 6 T. The two characteristics absorption lines below the band edge for the interband transitions have previously been attributed to an exchange resonance of electrons weakly bound at oxygen vacancies in a bound magnetic polaron-like state. They are also thought to be responsible for the insulator-metal transition at low temperatures. The temperature- and magnetic field-dependence of the two lines, however, shows that such an interpretation leads to serious difficulties. Other possible kinds of transitions are discussed.