EuO Single Crystals Research Articles

Growth of EuO single crystals at reduced temperatures

Single crystals of $(\mathrm{E}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x})\mathrm{O}$ have been grown in a molten barium-magnesium metal flux at temperatures up to 1000 \ifmmode^\circ\else\textdegree\fi{}C, producing single crystals of $(\mathrm{E}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x})\mathrm{O}$ with barium doping levels ranging from $x=0.03$ to $x=0.25$. Magnetic measurements show that the ferromagnetic Curie temperature ${T}_{\mathrm{C}}$ correlates with the Ba doping levels, and a modified Heisenberg model was used to describe the stoichiometry dependence of ${T}_{\mathrm{C}}$. Extrapolation of the results indicates that a sample with Ba concentration of $x=0.72$ should have a ${T}_{\mathrm{C}}$ of 0 K, potentially producing a quantum phase transition in this material.

Neutrino-induced Doppler-broadening study on oriented EuO single crystals

The motion of excited recoiling atoms produced by \ensuremath{\beta} decay of ${}^{152}\mathrm{Eu}$ causes a Doppler broadening of subsequently emitted \ensuremath{\gamma} rays. The neutrino-induced Doppler-broadened \ensuremath{\gamma}-ray line shape is measured in oriented EuO single crystal targets using a high resolving power spectrometer. The motion and the slowing down of the emitting atoms depend on the forces exerted by the other atoms and on the direction of observation with respect to the crystal orientation. The comparison between measured and calculated \ensuremath{\gamma}-ray line shapes obtained by molecular dynamic simulation, permits to test different interatomic potentials and slowing down theories. The dependence of the observed line shape on the crystal orientation is also investigated.

Study of Atomic Motion in Oriented EuO Single Crystals Using Neutrino Induced Doppler Broadening

The motion of atoms caused by neutrino emission following electron capture is observed in EuO crystals by measuring the Doppler shift of subsequently emitted gamma rays. The slowing down of the recoiling atom, which depends on the forces exerted by other atoms in the crystal, is calculated by molecular dynamics simulations using different interatomic potentials. The comparison between measured and simulated gamma-ray line shapes opens the way to a precise determination of the interatomic potential and of the lifetime of the nuclear state. {copyright} {ital 1997} {ital The American Physical Society}

Reflectivity spectra of Eu rich EuO near the absorption edge

The reflectivity spectra at near normal incidence in the spectral range 1.5 μm to 4 μm of EuO single crystals slightly rich in Eu but almost stoichiometric have been measured in the temperature range 4.2 to 300 K, using the Voigt configuration. Two dips are observed at 1.85 and 2.20 μm in fairly good agreement with the location of the two absorption peaks observed in similar materials. These peaks are ascribed in the literature either to optical transitions of an electron bound to an oxygen vacancy or to transitions into F centres formed at the vacancies. Our measurements show a very small temperature dependence of the energy position of the dips in the whole range [4.2-300 K] and a small intensity of the dips, which is in favour of the first interpretation when the material is an insulator, and of an alternative one in the metallic configuration, i.e. an interband transition between up and down spin subbands. At 4.2 K, two extra dips appear at 1.74 and 2 μm. The magnetic field up to 30 kG does not affect the energy position of the different dips, but greatly affects their shape.

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

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.

Optical investigations of Eu-rich EuO near the absorption edge

Absorption spectra in the spectral range between 1.1 and 3 μ of Eu-rich EuO single crystals have been measured in the temperature range between 4.2 and 300 K. Below the band edge for the interband transitions two absorption lines are found. Room temperature spectra of these have previously also been reported in the literature [1]. They were ascribed to optical transitions of an electron bound to an oxygen vacancy. This electron is thought to form a bound magnetic polaron [2] and to be also responsible for the insulator-metal phase transition (IMT) of this material. The temperature dependence of these lines, however, suggests an alternative interpretation. As there are no strong changes of the spectra at the IMT we believe that they should be ascribed to transitions of 4f electrons into F + center like states at the oxygen vacancy.

The influence of the stoichiometry on the 153Eu nuclear spin-spin relaxation in EuO single crystals

The 153Eu spin-spin relaxation for two spherical EuO single crystals of different composition has been measured for two saturating field values of 2 and 6 T. The relaxation can be described by two time constants, a short one increasing with the magnetic field, arising from the Suhl-Nakamura coupling and a long one, due to the dipolar coupling, which is field independent. It is shown that the number of nuclei which are relaxed due to the dipolar coupling increases at increasing magnetic fields, in agreement with the Suhl-Nakamura theory. For the sample which is nearly stoichiometric the relative number of nuclei which is relaxed due to the SN coupling is much larger than for the sample which contains an excess of Eu atoms.

Effects of high nonstoichiometry on EuO properties

Electric, magnetic, and electron paramagnetic resonance (EPR) measurements have been performed on thin EuO films prepared either by reactive evaporation of Eu at 400°C or by oxidation of a Eu metal thin film at 200–250°C. The films obtained by reactive evaporation have electrical and magnetic properties very similar to those reported for EuO single crystals of various degrees of nonstoichiometry. The films prepared by oxidation of Eu films at 200–250°C are characterized by a much higher-valued Curie temperature (Tc≃148 K), while their resistivity is in the range 10−2−1 Ω cm. The EPR signal can be attributed to Eu spins, which shows that in these films there exist fluctuations of Eu doping due to nonstoichiometry. We analyzed the high concentration of oxygen vacancies in the bound magnetic polaron (BMP) model. A high level of defects and oxygen vacancies, inhomogeneously distributed in the film, can account for both their electrical and magnetic properties.

Exchange optics in Gd-doped EuO

This paper is concerned with magneto-optical properties of EuO single crystals, doped with up to 5-at.% Gd. The optical absorption and the Faraday rotation have been measured above and below the magnetic-ordering temperatures and in various magnetic fields. In addition the reflectivity between 13 eV and 250 \ensuremath{\mu}m has been observed and the Curie temperature has been determined. The measurements permit the evaluation of the magnetic short-and long-range order. The exchange interaction between free electrons and $4f$ spins can be determined and is found to decrease with increasing doping. The analysis of the optical data permits the derivation of the carrier concentration, their mobility, and temperature dependence. The carrier concentration shows an exchange-induced ionization near ${T}_{C}$ for nondegenerate samples and the mobility has a sharp minimum at ${T}_{C}$. The transport parameters also reflect the fact that the electron-$4f$-spin interaction decreases with increasing Gd concentration, probably owing to electrostatic-shielding effects.

Magnetic field-modulated magnetoreflectance of europium chalcogenides

The optical constants of EuO and EuS single crystals have been determined at 300 K by means of a Kramers-Kronig analysis of the reflectivity for photon energies up to 12 eV. For EuS the optical constants have also been determined above and below the Curie temperature in the energy region from 1.5 to 5.7 eV. A first tentative assignment of optical structure to interband transitions has been attempted on the basis of recent orthogonal plane wave (OPW) and earlier augmented plane wave (APW) band structure calculations. For photon energies from 1.2 to 3.8 eV a low magnetic field-modulated magnetoreflectance has been measured using circularly polarized light. By use of the Kramers-Kronig relation for the differential reflectance spectra in conjunction with our data of the optical constants, a detailed analysis of the magnetoreflectance spectra of EuS was carried out for the first time.

Infrared reflectivity and coupled modes in Gd-doped EuO

The reflectivity of EuO single crystals, doped with various amounts of Gd, has been measured between 250μ and 12 eV photon energy. A Kramers-Kronig analysis of the data reveals LO and TO phonon modes, plasmons and LO phonon-plasmon coupled modes.

Electrical switching and memory effects in doped ferromagnetic semiconductors

We have observed bidirectional threshold and memory type switching in 0.5 to 1.26 at.% Gd-doped EuO single crystals.

Growth of EuO, EuS, EuSe and EuTe single crystals

Single crystals of the europium monochalcogenides weighing up to 60 g have been grown from Eu-rich solutions and from stoichiometric melts by using a gradient cooling technique, Since all four compounds have high Eu partial pressures near their melting points, the crystals were grown in weld-sealed tungsten crucibles 1.25 or 2.5 cm in diameter. The oxide was synthesized and grown in the same run. Dried Eu 2O 3 was heated with an excess of Eu metal to about 2200 °C, and the solution obtained was directionally frozen from the bottom by cooling to room temperature at 5–50 °C/hr. The resulting boules were single crystals with total impurities ranging from 250 to 5000 ppm. These crystals were then remelted, either with or without additional Eu metal, and directionally frozen to obtain crystals containing total impurities of about 200 ppm primarily C and Ca. The crystals were also used subsequently for growing crystals doped with Gd or La. The other compounds were synthesized in sealed quartz ampoules by the reaction of Eu metal at about 600 °C with vapor of the other element supplied by a reservoir at a lower temperature. Single crystals similar in size and purity to those of the oxide were then grown by the gradient cooling technique. The effect of deviations from stoichiometry on optical and electrical properties are discussed. The melting point of EuO, measured with an optical pyrometer and a W-(W, 26% Re) thermocouple, was 1980 ± 20 °C. EuS, EuSe and EuTe all melt between 2250 and 2500 °C.

Pressure dependence of the absorption edge of the Europium-chalcogenides

The room temperature pressure dependence of the absorption edge of EuO, EuS, EuSe and EuTe single crystals was found to be −4.4; −7.9; −8.4 and −12 meV/kbar respectively. Using these data the deformation potential was calculated.

Infrared Transmission, Magnetic Birefringence, and Faraday Rotation in EuO

We have measured the infrared transmission, magnetic birefringence, and Faraday rotation of EuO single crystals in the wavelength range between 2 and 20 μ. These effects have been studied as a function of temperature between 5° and 200°K in magnetic fields up to 9 kG. The most transparent samples have an absorption coefficient less than 0.5 cm−1 in the range between 3 and 9 μ and less than 1.0 cm−1 at 10.6 μ. The transmission decreases gradually at longer wavelengths and is largely independent of temperature and magnetic field except for shifts in the short wavelength absorption edge and effects of domain scattering at low temperature. Below the Curie point at about 70°K these crystals show a strong magnetic birefringence and Faraday rotation. At 20°K and 9 kG the difference in the index of refraction for light polarized perpendicular and parallel to the magnetic field is n⊥-n∥=1.07×10−2 at 10.6 μ and increases gradually to 1.5×10−2 at 2.5 μ. Under these experimental conditions the Faraday rotation is approximately 660 degrees/cm at 10.6 μ corresponding to nr-nl≃3.9×10−3. This rotation increases to about 2.5×104 degrees/cm at 2.5 μ. This corresponds to a specific rotation per unit attenuation in excess of 150 degrees/dB at 10.6 μ and 104 degrees/dB at 2.5 μ.

Ultrasonic Propagation in EuO

The attenuation of 10 to 90-MHz longitudinal and shear ultrasonic waves propagating along the [100] and [110] directions of EuO single crystals was measured between 63° and 78°K. At zero applied magnetic field, the attenuation was constant above the Curie temperature, TC=70°K, but increased as the temperature was lowered through TC. The increase in attenuation was much greater for the shear modes than for the longitudinal modes. Preliminary experiments at 4.2°≤T≤78°K with 30-MHz longitudinal and 10-MHz shear waves propagating along the [100] direction showed that the excess attenuation at T<TC passes through a maximum near 60°K and is small at 4 2°K. An external magnetic field H≳5 kG removed the excess attenuation at T<TC. No attenuation peak was found at TC. The excess attenuation at T<TC is tentatively interpreted as being due to ferromagnetic domains. The elastic constants at 78°K are (in 1011 cgs units) c11 = 19.2±0.6, c44 = 5.42±0.13, c12 = 4.25±0.85. These lead to an adiabatic bulk modulus B = (9.2±0.6) ×1011 cgs units, and a Debye temperature θ = 350°K.