Les oxynitrures d'europium semiconducteurs ferromagnetiques. Proprietes magnetiques et electriques

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Resume At room pressure and temperature the system EuO 1− x N x has two solid-solubility ranges, each with the NaCl structure: for 0⩽ x ⩽ 0.30 the system is ferromagnetic and semiconducting above the Curie temperature; for 0.92 ⩽ x x for 0⩽ x ⩽ 0.30. Magnetic susceptibilities are consistent with 4 f 6 configurations at x europium ions per molecule and a ferromagnetic Curie temperature T c that increases with x . Low-temperature transport measurements were made only for 0.20⩽ x ⩽ 0.30: a minimum in the electrical conductivity, approximately 30 K above T c correlates well with the onset of an anomalous low-temperature crystal contraction and with deviations from a Curie-Weiss law typical of short-range magnetic order. Below T c there is a metal-to-semiconductor transition similar to that found in EuO 1−δ . The magnitude of the negative Seebeck coefficient increases with T between T c and 310 K as might be expected for an extrinsic conductor with a transport contribution similar to the kinetic contribution, as in a metal. These properties are interpreted in terms of a Eu: 5 d conduction band, a two-electron donor level at oxygen vacancies, and a (Eu 2+ :4 f 7 ) localized-electron level that lies 1.1 eV below the bottom of the 5 d conduction band in EuO and above it in the nitrogen-rich metallic phase. In the oxygen-rich semiconducting phase of EuO 1− x N x , an electron-configuration transition 4 f 7 5 d 0 → 4 f 6 5 d 1 occu atoms, the 5 d 1 electron simultaneously being donated to a covalent Eu-N bond. The configuration transition raises the 4 f 7 level above the bottom of the 5 d band producing a small polaron of anomalously large energy. No small polaron anomaly is found in metal deficient Eu 1− δ O ; the configuration transition is apparently induced by the more strongly covalent Eu-N bonding. The excited electron configuration gives a ferromagnetic direct-exchange Eu-Eu interaction that appears to dominate the ferromagnetic superexchange term.