AbstractIn Eu‐rich EuO (EuS), the resistivity versus temperature shows an insulator‐metal transition, and in some cases a shoulder and a peak situated above the Curie temperature when T increases. Especially the shoulder temperature range is studied. It is shown that this region has a De Gennes‐Friedel‐type behavior whereas the peak region exhibits a Fisher‐Langer‐type behavior. The long‐range magnetic interactions in the shoulder temperature range are due to the existence of bound magnetic polarons. These interactions are emphasized either by magnetic field or by increasing of the Eu excess, i.e. bound magnetic polaron concentration.
The magnetization of bulk samples of Eu, Gd 0, characterized by transport measurements has been measured as a function of x in the range 0 < * <f%. A metal insulator transition is observed for x # 1.3% correlated with a discontinuous jump of the Curie temperature T . '
Des monocristaux homogènes d'EuO, EuO riche en Eu, et EuO dopé au gadoliniura ont été obtenus et caractérisés par des analyses : aux rayons X, avec des microsondes et par des mesures, en fonction de la température, d'effet Mossbauer, de spectroscopie infra-rouge et de résistivité.Des résultats de photoconductivité^ effet Hall, mesures optiques et magnétiques sont apportés.Ils sont interprétés dans le cadre d'un modèle simple tenant compte des énergies de Coulomb, cinétique et d'échange et du degré de compensation d'un niveau donneur, ce niveau étant dû à l'existence de lacunes d'oxygène ou d'atomes de Gd.Abstract.-Homogeneoussingle 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.
Single crystals of Gd doped EuO have been grown with an initial Gd concen tration of 1.5%. The lattice parameter has been determined by X-ray analysis and is smaller that of pure EuO. Measurements of resistivity under hydrostatic pressure have heen performed up to 5 Kbar in the temperature range 50 K - 200 K. The carrier concentration has been determined at high and low temperature by Hall effect measu rements. The Curie temperature has been determined by magnetization measurements. The main points of interest of this work are : i) the resistivity decreases under pressure while it increases in samples of Eu doped EuO ; ii) the resistivity peak is shi fted by pressure towards high temperature at a rate of 0.8 K/kbar while the inflexion point of the p-vs-T curve is shifted at a rate of 0.7 K/kbar. The negative pressure coefficient of resistivity measured for this sample has been explained by the mixing of 6 s and 5 d bands ; so that the variations of resistivity under pressure are due to mobility variations because the sample remains degenerated at high and low tem perature.
Résumé. 2014 Nous avons étudié l’influence de la concentration de porteurs libres sur les propriétés magnétiques de monocristaux de EuO. L’accroissement des concentrations électroniques à partir de EuO quasi st0153chiométrique a été obtenu par écart à la st0153chiométrie (EuO riche en Eu) et par dopage (EuO dopé à 2% en Gd). L’aimantation des échantillons massifs caractérisés par des mesures de transport a été mesurée avec un magnétomètre vibrant. Pour les faibles dopages, les courbes d’aimantation ne diffèrent pas de celle donnée par la loi de Brillouin, avec une température de Curie de 69,4 K. Au contraire, pour l’échantillon très dopé au Gd, Tc atteint 143 K et l’aimantation réduite est plus petite que celle prévue par la loi de Brillouin. Nous proposons un modèle prenant en consi- dération à la fois l’interaction de super-échange responsable des propriétés magnétiques de EuO isolant, et l’échange indirect dû aux électrons de conduction pour les échantillons semiconducteurs et métalliques. Le point essentiel est que, contrairement aux modèles Abstract. 2014 The influence of free carriers concentrations on magnetic properties of EuO single crystals has been studied. Increasing electronic concentrations from quasi stoichiometric EuO have been achieved by departure from stoichiometry (Eu rich EuO) and by doping (2 % Gd doped EuO). The magnetization of bulk samples, characterized by transport measurements, has been measured with a vibrating sample magnetometer. For low doping, magnetization curves do not depart from the Brillouin law, with a Curie temperature Tc = 69.4 K. On the contrary, for heavily Gd doped sample, Tc increases up to 143 K and the reduced magnetization is smaller than that expected from the Brillouin law. We propose a model taking into account both the superexchange interaction which explains the magnetic behaviour of insulating EuO and an indirect exchange due to the conduction electrons in semiconducting and metallic samples. The main feature is that, contrary to usual models we take into account the carrier statistics, i.e. splitting of spin subbands and spread out of the Fermi level. There is a good quantitative agreement between theory and experiment for heavy doping (n 1.5 x 1020
The critical behavior of the pressure coefficient of the resistivity has been studied in a large temperature range (6-300 K) under hydrostatic pressure up to 5 kbar. Near the critical temperature ${T}_{C}$, this pressure coefficient reverses its sign. This is explained by the pressure effect on the magnetic order which induces a shift of ${T}_{C}$ towards high temperature. A positive value of the pressure coefficient of the resistivity has been found at low temperatures and at high temperatures. This behavior is observed for the first time in EuO and is explained by transfer of electrons from a higher-mobility band to a lower-mobility band. Below 60 K, the large decrease in the resistivity with increasing pressure has been explained by pressure effect on the self-trapping phenomenon of the conduction electrons.
Resistivity vs hydrostatic pressure has been measured at room temperature for several EuO samples which span a large range of resistivity. We show that the pressure coefficient of activation energy of resistivity β = KT (d log ϱ/d p) varies continuously with room temperature resistivity. The samples which present an insulator-metal Transition have unusual values of β. We relate these values to the presence of bound magnetic polaron or molecular bound magnetic polaron. Nous avons mesuréla résistivitéd'échantillons d'EuO en fonction de la pression hydrostatiqueàtempérature ambiante, dans une large gamme de résistivité. Nous montrons que le coefficient de pression de l'énergie d'activation de la résistivité β=dLogρdp varie continuement avec la résistivitéà300° K. Pour leséchantillons présentant la transition Isolant-Métal, β a un comportement particulier en fonction de ϱ que nous avons reliéàla présence de “Bound Magnetic Polaron ou de Molecular Bound Magnetic Polarons”.
Optical absorption coefficient of EuO shows two humps at 0.55 and 0.65 eV. The energy position of these humps remains constant when the temperature is lowered from 300 to 50 K. This data, in agreement with the assumption of the spin flip of an electron bound on an oxygen vacancy, gives a new evidence for the bound magnetic polaron.
Stoechiometric EuO samples do not present insulator metal transition below Curie temperature (Tc = 70 K). Experimental data presented in this paper show that when these samples are illuminated, it appears: an insulator metal transition below Tc, and, at room temperature, a decrease of activation energy, as well as a small increase in conductivity. Oxygen deficient samples exhibit usually this insulator metal transition. When they are illuminated room temperature activation energy remains unchanged, and the insulator metal transition is more important. These results cannot be explained by means of a free electron model. Electrons are photoexcitated from 4ƒ levels, or donor levels, to conduction band. Then, these electrons polarise Eu2+ spins and make magnetic polarons. These polarons cannot be “Bound Magnetic Polarons” because samples do not contain oxygen vacancies. In addition these polarons would be stable until room temperature. Thus, they are expected to be “Molecular Magnetic Polarons”. Our results confirm then this model proposed by Kusuya.
EuO single crystals have been obtained from different runs of growing. Conductivity, photoconductivity, and magnetoresistance measurements were made versus temperature and for magnetic fields up to 7 kOe. The study of the results was correlated to the growing parameters. In order to explain the two types of conductivities observed, a model of conduction is proposed with acceptor levels and two types of donor levels. Des monocristaux d'EuO ont été obtenus avec différents paramètres de croissance. Des mesures de conductivité, photoconductivité et magnétorésistance ont été faites sur ces échantillons, en fonction de la température et pour des champs magnétiques allant jusqu'à 7 kOe. L'étude des résultats a été faite en relation avec les paramètres de croissance. Afin d'expliquer les deux types de comportement observés, nous proposons un modèle de conduction avec des niveaux accepteurs et deux types de niveaux donneurs.
The influence of the phonon–phonon interaction on the variation of the attenuation coefficient of longitudinal and transversal ultrasonic waves for the frequency of 750 MHz propagating in p-type single crystals of pure GaSb has been studied. Single crystal samples were obtained by the Czochralsky method and had a charge carrier concentration of 1.2 × × 1017 cm−3. The investigation was made at temperatures from 4 to 77 °K along the [111], [110], and [100] crystal axis, using the pulse echo method. The results are compared with Maris' quantum theory. From these results the coupling coefficients were deduced along the various crystal axis studied, and the only kind of thermal phonons that is responsible for the ultrasonic wave attenuation is pointed out. Nous avons étudié l'influence de l'interaction phonon dans la variation du coefficient d'atténuation d'ondes ultrasonores longitudinales et transversales pour la fréquence de 750 MHz se propageant dans du GaSb naturel monocristallin de type p (les échantillons monocristallins sont obtenus par la méthode de Czochralsky et ont une concentration de 1,2 × 1017 porteurs par cm3) ceci pour des températures allant de 4 à 77 °K et suivant les axes cristallographiques [111], [110] et [100]. Les mesures sont faites en utilisant la méthode du „pulse echo”︁. Les résultats sont comparés à la théorie quantique de Maris. On en a déduit les coefficients de couplage suivant les différents axes cristallographiques étudiés et mis en évidence le seul type de phonons thermiques responsable de l'atténuation de l'onde ultrasonore.
AbstractAn investigation is made of the variation of the attenuation coefficient for shear and longitudinal ultrasonic waves which propagate through monocrystalline p‐type Gasb. The study was carried out for frequencies ranging from 0.5 to 1 GHz and in a range of temperatures from 5 to 300 °K. The measurements made by the pulse echo method are then compared with the theory of Woodruff and Ehrenreich and that of Bommel and Dransfeld.
