PHYSICAL PROPERTIES OF EuO VERSUS ELECTRONIC CONCENTRATIONS

Homogeneous single crystals of EuO, Eu rich EuO and Gd doped EuO have been obtained and characterized by X-ray, microprobe analysis;Mossbauer, infrared spectroscopy and resistivity measurements versus temperature. Photoconductivity, Hall effect, optical and magnetic measurements are reported. They are interpreted within a simple model taking into account Coulomb, kinetic and exchange energies and considering the degree of compensation of the donor level due to oxygen vacancies or Gd atoms. Introduction.Europium oxide EuO is a magnetic semiconductor which was the subject of many studies due to the following reasons. Its simple crystalline structure (NaCl) simplifies the study of magnetic interactions in this ferromagnetic compound. In addition, by departure from stoechiometry or by doping, a drastic change of the carrier concentration can be achieved resulting in noticeable variations of physical properties; then it is needed to perform the measurements of physical properties on single crystals. For example, with a moderate concentration of oxygen vacancies a Metal Insulator Transition (MIT) appears at about 50 K' . Numerous theoretical studies of this transition involving the concept of /2/ Bound Magnetic Polaron (BMP) showed that the magnetic interactions have a strong effect on transport properties. Reciprocally, the electronic properties of the material affect the nature of the exchange interaction induced by free carriers. Optical spectra of Eu rich EuO evidence structures which also were correlated to the magnetic interactions. Because of this interdependence between various physical properties it seemed to us that it was interesting to investigate them as a whole with a restricted number of samples. The growing technics of crystals build the first section. The results of characterization measurements not only direct (X-rays, microprobe analysis) but also indirect (infrared absorption spectra, Mossbauer effect, resistivity measurements) are the subject of section 2 . We report transport measurements including photoconductivity data in section 3, reflectivity measurements in section 4, and magnetization measurements in section 5. In the last section we present a simple model taking into account a) the contributions of exchange, Coulomb and kinetic energies to the binding energy of the donor state ; b) the statistics of free carriers. This model accounts for the experimental pro-