57Fe mössbauer study of high-T c Y−Ba−Cu oxide superconductors

⁵⁷Fe Mössbauer measurements have been performed for Fe-doped YBa₂ (Cu_1-x Fe_x)₃O_7-v (x=0.005–0.10) superconducting oxides. Fe atoms mainly substitute at Cul chain sites and the fraction of Fe at Cu2 plane sites is discussed thermodynamically. The spin direction of Fe at Cu2 was determined to be normal to thec-axis using an oxygen deficient specimen having a strong texture along thec-axis. A magnetically broadened spectrum at 4.2 K for Fe at Cul is compared with a simulated one which takes the distribution of hyperfine fields into account.     

Non-equilibrium population and slow relaxation of Fe2+ in57Co:LiNbO3 and57Co:LiTaO3 single crystals

The Fe²⁺ fraction observed in the Mössbauer emission spectra of⁵⁷Co:LiNbO₃ and⁵⁷Co:LiTaO₃ exhibits both slow electronic relaxation and nonthermal populations of them _s sublevels of the⁵A_lg orbital singlet ground state at low temperatures (T<15 K) in high magnetic fields. The relaxation rates depend on temperature and on the angle ? between the magnetic field and the crystallographicc-axis.     

Temperature dependence of the anomalous emission line intensities in57Co:LiTaO3-initial population and relaxation

Mössbauer spectra of⁵⁷Co: LiTaO₃ single crystals were recorded in an external longitudinal magnetic field of 6 T at different temperatures between 4.2 K and 150 K. The spectra were taken at two different orientations of the crystallographicc-axis relative to the magnetic field. The line intensities of the Fe³⁺-subspectra show a temperature dependent anomalous population of the three Kramers doublets of the Fe³⁺ spinS=5/2 ground state. A relaxation broadening is observed at higher temperatures, which cannot successfully be reproduced within a relaxation model taking into account only the six lowest lying electronic states.     

Antiferromagnetism in57Co-doped La2CuO4−y studied by Mössbauer spectroscopy

Mössbauer effect studies of⁵⁷Co-doped La₂CuO_4?y were performed at temperatures between 4.2 K and room temperature. These confirm the antiferromagnetic ordering of these compounds below room temperature. Temperature dependence of the quadrupole splitting shows that the hyperfine field is at an angle with thec-axis.     

Iron inc-axis oriented thin films of YBa2Cu3O7: A combined cems,SEM, EDX and XRD study

Fullyc-axis oriented thin films of YBa₂(Cu_1?x ⁵⁷Fe _x )₃O₇ on 〈100〉 SrTiO₃ substrate (x=0.15%, 1%, 3%, and 6%) were investigated by means of x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX-analysis), inductiveT _c measurement, and conversion electron Mößbauer spectroscopy (CEMS). For all iron concentrations the films are fullyc-axis oriented and iron is homogeneously distributed in the film. Forx=0.15%, 1%, and 3% thec-axis length is the same as for undoped films, forx=6% it is enlarged. With increasingx the surface of the films, which shows an undulated structure, gets smoother. TheT _c values forx=0.15% and 1% are the same as for the undoped films, forx=3% and 6%T _c is reduced. CEMS spectra for allx values can satisfactorily be fitted with only three quadrupole doubletsA, B, andC, but with different relative intensities for differentx values. The quadrupole splitting of doubletB decreases with increasingx values. The Debye Waller factor does not change significantly withx.     

Anisotropy of the lattice vibration of Eu and Cu in highTc superconductor Ba2EuCu3O7 analyzed by151Eu and57Fe Mössbauer effects

The anisotropy of superconductivity in highTc superconductors indicates that it is important to clarify the anisotropies of microscopic properties in these materials. In this work, we observed the anisotropy of the lattice vibration ofEu andCu inBa ₂ EuCu ₃ O ₇ by¹⁵¹ Eu and⁵⁷ Fe Mössbauer effect. Thec-axis aligned samples were prepared by healing the mixture of epoxy andBa ₂ EuCu ₃ O ₇ powder in a magnetic field. For⁵⁷ Fe Mössbauer measurement the sample doped with⁵⁷ Fe,Ba ₂ Eu(Cu _0.99 Fe _0.01)₃ O ₇, was used. The temperature dependence of Mössbauer spectrum was measured in two cases,c-axis ‖ γ-ray andc-axis ⊥ γ-ray. The Debye temperature ofEu alongc-axis (230 K) was lower than that inab-plane (265 K). The Debye temperature of⁵⁷ Fe substituted forCu(2) was lower (378 K) alongc-axis than inab-plane (417 K). The Debye temperature of⁵⁷ Fe substituted forCu(1) having no oxygen atom ina-axis was 435 K alongc-axis and 416 K inab-plane. These results are interpreted by the oxygen configuration around the Mössbauer atoms.     

Measurement of the energy flux in acousto-electric domains by birefringence

The change of the birefringence by a travelling acousto-electric domain of anS ₄ wave is used to study the generation, amplification, and dissipation of acoustic flux in CdS. Different geometries for the direction of light incidence and light polarization are discussed theoretically. Experimental results are compared with the theory for the cases where thec-axis is perpendicular to the light direction and polarization either parallel or under 45 ° to thec-axis as well as for cases, where thec-axis is parallel to the light incidence direction. From the measurements a lower limit for the absolute value of the acoustic flux was found to be 5 kW/cm2.     

Interpretation of neutron scattering and μSR data on paramagnetic Gd in terms of magnetic clusters and coherent dissipative structures

Fullyc-axis oriented thin films of YBa₂(Cu_1−x ⁵⁷Fe _x )₃O_7−y were studied by varying the Fe- and O-content and the substrate. For the two substrates MgO and SrTiO₃ no difference in the CEMS spectra was observed. Measurements for differentx andy values and their comparison with results from powder samples proved that the solubility of iron is the same in the films and powders. The substitution of Cu(2) by Fe in the films is the highest ever reported. The CEMS spectrum of a reduced film shows that the direction of the magnetic hyperfine field for doubletC is perpendicular to thec-axis.     

Heavy ion irradiation effects inc-axis oriented thin films of YBa2(Cu0.97 57Fe0.03)3O7 studied by CEMS

Fullyc-axis oriented thin films of YBa₂ (Cu_0.97 ⁵⁷Fe_0.03)₃O₇ on <100> SrTiO₃ substrate were irradiated with different fluences ? of 500 MeV¹²⁷I parallel to thec-axis. The samples were investigated by means of x-ray diffraction, resistiveT _c measurements and conversion electron Mößbauer spectroscopy (CEMS). Irradiation results in the appearance of a fourth quadrupole doubletE in the CEMS-spectra (E _Q=1.1 mm/s, IS=0.26 mm/s), whose intensity increases with increasing ?. The Debye Waller factor of the new Fe-speciesE is only about 60% of that of the speciesA, B, andC proving that irradiation with high energy heavy ions results in latent tracks of lower density. from the hyperfine parameters of speciesE we conclude that the latent tracks are mainly formed by the “green phase” Y₂BaCuO₅. We derived the radius of the latent tracks to be about 26 Å.     

Determination of the orientation of the electric field gradient at the Cu(1) site from the Mössbauer spectra of textured pellets made of YBa2Cu3O7−δ superconductor

A textured pellet of YBa₂Cu₃O_7?δ was prepared and doped with⁵⁷Co. After subsequent heat treatments the cobalt was found at the two copper sites. The degree of alignment of individual crystallites (about 40%) was determined from the Mössbauer spectrum by using a method developed by the authors [1]. The same method showed that the most characteristic doublet component of such spectra (δ=0.08 mm/s; |ΔE _Q|=1.96 mm/s) is associated with a site, most probably a Cu(1) site, where the EFG is parallel to thec direction of the unit cell. The sign ofV _zz turned out to be negative, giving a negative quadrupole splitting (ΔE _Q=?1.96 mm/s) for that component.     

Mössbauer study on a two-dimensional triangular antiferromagnet YFeMnO4

Magnetic ordering of YFeMnO₄ has been studied with Mössbauer spectroscopy in the temperature range between 4.2 K and 300 K on the conditions of gamma rays parallel to and perpendicular to thec-axis. It is revealed that the spin system orders like a Heisenberg spin system in a triangular antiferromagnet with an anisotropic axis parallel to thec-axis.     

Surface acoustic wave device properties of (B, Al)N films on 128° Y-X LiNbO3 substrate

A c-axis orientated aluminium nitride (AlN) film on a 128° Y-X lithium niobate (LiNbO₃) surface acoustic wave (SAW) device which exhibit a large electromechanical coupling coefficient (k²) and a high SAW velocity property, is needed for future communication applications. In this study, a c-axis orientated (B, Al)N film (with 2.6 at.% boron) was deposited on a 128° Y-X LiNbO₃ substrate by a co-sputtering system to further boost SAW device properties. The XRD and TEM results show that the (B, Al)N films show highly aligned columns with the c-axis perpendicular to the substrate. The hardness and Young's modulus of (B, Al)N film on 128° Y-X LiNbO₃ substrates are at least 17% and 7% larger than AlN films, respectively. From the SAW device measurement, the operation frequency characteristic of (B, Al)N film on 128° Y-X LiNbO₃ is higher than pure AlN on it. The SAW velocity also increases as (B, Al)N film thickness increases (at fixed IDT wavelength). Furthermore, the k² of (B, Al)N on the IDT/128° Y-X LiNbO₃ SAW device shows a higher value than AlN on it.     

Mössbauer study of Lu2Fe3O7: A two-dimensional antiferromagnet on a triangular lattice

Mössbauer studies reveal that there are two kinds of Fe³⁺ spins with completely different characteristics in Lu₂Fe₃O₇: one is an Ising-like property and the other is a Heisenberg-like property in a two-dimensional antiferromagnet on a triangular lattice. The former spin orders ferrimagnetically along thec-axis at around 220 K. The latter spin is lying in thec-plane and a corresponding hyperfine magnetic field is observed at temperatures below 50 K. At very low temperatures, however, the latter spin has a component parallel to thec-axis and couples with the former spin. This finding is consistent with the modulated profile of the magnetic scattering in neutron diffraction and the result of a magnetization measurement.     

Channelingeffekte bei der Lichtausbeute von Anthrazen-, Naphthalin- undp-Terphenylkristallen bei Anregung mit α-Teilchen

The scintillation light yield of anthracene atT=300 °K andT=80 °K and of naphthalene andp-terphenyl atT=80 °K was investigated for impact directions in the (a, c)-plane. The light yield was measured for welldefined integration times. The relative depth of the channeling-minimum observed for impact directions parallel to thec-axis of anthracene is found to increase with increasing integration times. For the first time channeling-minima of the light yield were observed at impact directions parallel to thec- and the [102]-axis of naphthalene and thec- and [¯102]-axis ofp-terphenyl. The [¯102]-axis ofp-terphenyl shows a more pronounced effect than thec-axis.     

Strongly enhanced flux pining of GdBCO coated conductors with BaSnO3 nanorods by pulsed laser deposition

We report the effects of BSO addition on the crystallinity, texture, and the field dependency of critical current density (J_c) of GdBCO coated conductors (CCs) prepared by pulsed laser deposition (PLD). Undoped and BSO-doped GdBCO films showed only c-axis oriented growth, and the incorporated BSO nanorods exhibited epitaxial relationship with the GdBCO matrix. In comparison with undoped film, BSO-doped GdBCO film exhibited greatly enhanced J_c and higher pinning force densities in the entire field region of 0–5 T (H//c) at 77 and 65 K. The BSO-doped GdBCO film showed the maximum pinning force densities (F_p) of 6.5 GN/m³ (77 K, H//c) and 32.5 GN/m³ (65 K, H//c), ～2.8 times higher than those of the undoped sample. Cross-sectional TEM analyses exhibited nano-structured BSO nanorods roughly aligned along the c-axis of the GdBCO film, which are believed effective flux pinning centers responsible for strongly improved critical current densities in magnetic fields.     

Pressure dependence of c-axis elastic parameters of oriented graphite

The elastic c-axis moduli C₃₃ and C₄₄ of compression-annealed pyrolytic graphite and their derivatives with hydrostatic pressure have been measured at room temperature over a pressure range from 1 to 7000 bars (700 MNm^?2) using an ultrasonic pulse-echo technique. The experiments were also made with material irradiated with fast neutrons. The pressure dependence of the c-axis compressibility of these materials was calculated from the experimental data, enabling pressure derivatives of the c-axis moduli to be also expressed as c-axis strain derivatives.     

c-Axis orientation control of YBa2Cu3O7−x films grown on inclined-substrate-deposited MgO-buffered metallic substrates

Biaxially textured YBa₂Cu₃O_7−x (YBCO) films were grown on non-textured metal substrates with inclined-substrate-deposited (ISD) MgO as template. The biaxial texture feature of the films was examined by X-ray pole-figure analysis, φ-scan, and 2θ-scan. A tilt angle of 32° of the MgO[001] with respect to the substrate normal was observed. Epitaxial growth of YBCO films with c-axis tilt angle of 32° with respect to the substrate normal was obtained on these substrates with SrTiO₃(STO) as buffer layer. Whereas, by choosing yttria-stabilized ZrO₂ and CeO₂ instead of STO as buffer layer, a c-axis untilted YBCO film was obtained. Higher values of T_c=91 K and J_c=5.5×10⁵ A/cm² were obtained on the c-axis untilted YBCO films with 0.46 μm thickness at 77 K in zero field. Comparative studies revealed a unique role of CeO₂ in controlling the orientation of the YBCO films grown on ISD-MgO buffered metal substrates.     

A Mössbauer study of the bipyramidal lattice site (2b) in BaFe12O19

The hyperfine fieldsH _f(T), center shifts δ(T) and relative recoilless fractionsf _a(T) at different lattice sites of barium ferrite were determined by Mössbauer spectroscopy of⁵⁷Fe over a temperature range from 12 to 723 K. For ferric iron in the crystalline site (2b), the recoilless fractionf _a(T) decreases most with increasing temperature in comparison with that of other sites, corresponding to a characteristic temperature {ie219-1}. These results indicate that the ferric iron in the site (2b) oscillates between two equivalent positions (4e) on either side of the symmetry plane normal to thec-axis.     

Superconducting properties of Sm0.33Eu0.33Gd0.33Ba2Cu3Oy films by metal-organic deposition using metal 2-ethylhexanates

We have prepared Sm_0.33Eu_0.33Gd_0.33Ba₂Cu₃O_y (SEGBCO) films on LaAlO₃ single-crystal substrates by metal-organic deposition using 2-ethylhexanate solutions which contain no fluorine. The SEGBCO film fired at 850 °C under a low oxygen partial pressure of 2 × 10⁻⁶ atm exhibited strong c-axis oriented X-ray diffraction peaks and weak a/b-axis oriented peaks of the SEGBCO phase, and had a granular surface. High resolution cross-sectional transmission electron microscope images indicated the epitaxial growth of the SEGBCO film on the substrate with overgrowth of a/b-axis oriented grains. The critical temperature and critical current density values of the SEGBCO film was 86.5 K and 1.29 MA/cm², respectively, at 77.3 K at the self-field.     

A Mössbauer study of YBa2(Cu1−x)Fex3O7−σ

⁵⁷Fe Mössbauer spectra of a sample of YBa₂(Cu_0.985Fe_0.015)₃O_7−σ for which T_c≈59 K show that long-range magnetic order is established below ∼ K. A model in which the Fe spins are aligned with the crystallographic c-axis provides satisfactory agreement with the observed relaxation spectra. This result is discussed with reference to current theoretical models of high-T_c superconductivity which involve magnetic coupling mechanisms.