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

doi:10.1016/0022-3697(78)90033-1

Journal of Physics and Chemistry of Solids

Volume 39, Issue 5, 1978, Pages 539-549

https://doi.org/10.1016/0022-3697(78)90033-1 Get rights and content

Résumé

At room pressure and temperature the system EuO_1−xN_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 < 1 it is metallic. Conductivity and Seebeck voltages indicate intrinsic behaviour above 310K with an energy gap that decreases with increasing x for 0⩽x ⩽ 0.30. Magnetic susceptibilities are consistent with 4f⁶ 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: 5d conduction band, a two-electron donor level at oxygen vacancies, and a (Eu²⁺:4f⁷) localized-electron level that lies 1.1 eV below the bottom of the 5d conduction band in EuO and above it in the nitrogen-rich metallic phase. In the oxygen-rich semiconducting phase of EuO_1−xN_x, an electron-configuration transition 4f⁷5d⁰ → 4f⁶5d¹ occu atoms, the 5d¹ electron simultaneously being donated to a covalent Eu-N bond. The configuration transition raises the 4f⁷level above the bottom of the 5d band producing a small polaron of anomalously large energy. No small polaron anomaly is found in metal deficient Eu_1−δO; 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.

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Citation Excerpt :
GdN1−xCx was reported to be ferromagnetic with a TC value of 190 K, and material with an additional deficiency of N was reported with a TC as high as 340 K [186,187]. In addition, replacing up to 30% of O in EuO with N augments TC from 69.5 K to as much as 77 K [188]. This latter result was attributed to a decreasing RE-RE distance with an increasing N concentration, resulting in a stronger ferromagnetic exchange interaction between the RE cations.
When the rare earth mononitrides (RENs) first burst onto the scientific scene in the middle of last century, there were feverish dreams that their strong magnetic moment would afford a wide range of applications. For decades research was frustrated by poor stoichiometry and the ready reaction of the materials in ambient conditions, and only recently have these impediments finally been overcome by advances in thin film fabrication with ultra-high vacuum based growth technology. Currently, the field of research into the RENs is growing rapidly, motivated by the materials demands of proposed electronic and spintronic devices. Both semiconducting and ferromagnetic properties have been established in some of the RENs which thus attract interest for the potential to exploit the spin of charge carriers in semiconductor technologies for both fundamental and applied science. In this review, we take stock of where progress has occurred within the last decade in both theoretical and experimental fields, and which has led to the point where a proof-of-concept spintronic device based on RENs has already been demonstrated. The article is organized into three major parts. First, we describe the epitaxial growth of REN thin films and their structural properties, with an emphasis on their prospective spintronic applications. Then, we conduct a critical review of the different advanced theoretical calculations utilized to determine both the electronic structure and the origins of the magnetism in these compounds. The rest of the review is devoted to the recent experimental results on optical, electrical and magnetic properties and their relation to current theoretical descriptions. These results are discussed particularly with regard to the controversy about the exact nature of the magnetic state and conduction processes in the RENs.
Chapter 166 Ternary and higher order nitride materials
1998, Handbook on the Physics and Chemistry of Rare Earths
This chapter reviews ternary and higher order nitride materials. Such multinary nitride materials result from association to rare-earth metal and nitrogen of other elements, which can belong either to the cationic or anionic subnetwork. Considering R–M–N ternary nitrides, where M is a cationic element, different categories of compounds are found depending on M, more precisely on the nature of the M–N chemical bond. It is well-known that nitrogen, which is located in the Periodic Table between carbon and oxygen, gives rise to two main classes of binary nitrides having similarities either with carbides or with oxides. Transition metal nitrides, thus, are closely related to transition metal carbides and are essentially metallic in character, because they can be generally described as insertion compounds of nitrogen in the metal network. Iono-covalent nitrides, with more or less significant ionic or covalent character, form the second class of compounds—close to oxides—that includes the rare-earth nitrides.
Niobates et tantalates d'europium de type "Bronze de Tungstene Quadratique"-I. Proprietes electriques et magnetiques
1980, Journal of Physics and Chemistry of Solids
Les propriétés électriques et magnétiques de quelques niobates et tantalates d'europium divalent de type bronze de tungstène quadratique de formule générale Eu_xNbO₃(0.5⩽x⩽0.6), Eu_0.5TaO₃ et .EuBCM₅O₁₅ (M = Ta, Nb; B = Eu¹¹, Sr, Ba; C : Na, K) ont été étudiées. A côté de l'europium divalent, les mesures de susceptibilité magnétique ont mis en évidence de notables proportions d'europium trivalent qui ont été confirmées par spectroscopie Mössbauer: les spectes Mössbauer ont également confirmé l'absence d'ordre magnétique même à la température de l'hélium liquide et l'absence de tout phénomène de transfert de charge même à température ambiante. Les mesures de conductivité et de coefficient Seebeck entre 77 et 800 K ont mis en évidence un comportement semiconducteur pour tous les composés. Cependant, l'interprétation des courbes de variation de l'effet Seebeck en fonction de la température permet de distinguer une conductivité par sauts à basse température et une conductivité caractéristique d'un semiconducteur extrinsèque à houte température. Un modèle de niveaux d'énergie dérivé de celui de Goodenough pour la structure bronze nous a permis d'expliquer qualitativement les résultats expérimentaux.
The magnetic and electrical properties of some TTB type niobates and tantalates of Eu^II with general formula Eu_xNbO₃ (0.5⩽ x⩽ 0.6), Eu_0.5TaO₃ and EuBCM₅O₁₅ (M = Ta, Nb; B = Eu^II, Sr, Ba; C = Na, K) have been investigated. Suceptibility measurements showed a large proportion of Eu³⁺ ions among the Eu²⁺ ions, and this was confirmed by Mössbauer spectroscopy. Mössbauer spectra also showed the absence of any magnetic ordering of the europium ions down to 4.2K, and the absence of any charge hopping phenomena up to room temperature. Conductivity and Seebeck coefficient measurements revealed typical semiconductor behaviour from 77 to 800 K. The Seebeck coefficient vs temperature curves showed a hopping conductivity regime at low temperature and an extrinsic semiconductor conductivity at high temperature. A Goodenough type of derived schematic energy level model for the tungsten bronze type structure was found to be qualitatively in agreement with the experimental results.
Niobates et tantalates d'europium de type "Bronze de Tungsténe Quadratique"-II. Evalution Des Niveaux D'Enerige Dans La Structure
1980, Journal of Physics and Chemistry of Solids
Des propriétés électriques et magnétiques de quelques niobates et tantalates d'europium divalent de formule Eu_xNbO₃ (0.5 $$ ̌$ x $$ ̌$ 0.6), Eu_0.5TaO₃, et EuBCM₅O₁₅ (M = Ta, Nb; B = Eu^II, Sr, Ba; C = Na, K), nous avons déduit un modèle des niveaux d'énergie dans la structure bronze. Ce modèle, en accord avec les principaux résultats expérimentaux rend compte de la présence d'europium trivalent dans ces composés, mais n'explique pas les taux d'europium trivalent mesurés par spectrométrie Mössbauer. Des calculs de potentiels électrostatiques dans la structure bronze ont permis de retrouver le diagramme des niveaux d'énergie pour Eu_0.6NbO₃. Mais, pour Eu_0.5NbO₃ et EuBCM₅O₁₅, l'accord entre les niveaux théoriques et expérimentaux n'est obtenu que par une mise en ordre des ions Eu³⁺ dans les cages pérovskites le long de l'axe c de la structure; avec trois arrangements des files de cages pérovskite ordonnées, nous avons pu retrouver les taux d'europium trivalent pour la plupart des composés.
From the magnetic and electrical properties of some TTB type niobates and tantalates of Eu¹¹ with general formula Eu_xNbO₃ (0.5 $\ ̌$ x $\ ̌$ 0.6), Eu_0.5TaO₃ and EuBCM₅O₁₅ (M = Ta, Nb; B = Eu¹¹, Sr, Ba; C = Na, K) we have derived a schematic energy level model of Goodenough type for tungsten bronze type structure. This was found to be qualitatively in agreement with experimental results but could not account for large amounts of Eu³⁺ calculated on the basis of Mössbauer spectra. Computed electrostatic potentials in the TTB structure allowed verification of the proposed energy level diagram for Eu_0.6NbO₃. However, for Eu_0.5NbO₃ and EuBCM₅O₁₅, agreement between theoretical potentials and experimental results is obtained by ordering the Eu³⁺ ions in perovskite cages along the c axis of the structure. With three arrangements of the ordered perovskite units, it was possible to account for the amounts of Eu³⁺ found experimentally in most of the compounds.
Blocking of valence fluctuations in europium oxynitrides
1979, Solid State Communications
Pressure Dependence of the Resistivity of the Rare Earth Oxide Nitrides Eu<inf>1−x</inf><sup>2+</sup>Ln<inf>x</inf><sup>3+</sup>N<inf>x</inf>O<inf>1−x</inf> (Ln = Nd, Eu)
1979, physica status solidi (b)

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

Résumé

Chem. Phys. Lett.

Sol. State Commun.

Solid State Commun.

Phys. Rev. Lett.

Colloque International du CNRS sur les Eléments des Terres Rares

J. Appl. Phys.

J. Appl. Phys.

Phys. Rev.

J. Appl. Phys.

IBM, J. Res. Develop.

J. Appl. Phys.

J. Appl. Phys.

Brevet d'Invention No. 668.289

J. Appl. Phys.

Soviet Phys. Sol. State

Soviet Phys. Sol. State

Phys. Rev.

C.R. Acad. Sci.

Fiz. Met. Metall

Gen. El. Res. Lab. Rep. No. 57 RL

J. Phys. E: Sci. Instrum.

Physik Kondens. Materie