Magneto-elastic effects in single-crystal USb0.8Te0.2

A high resolution X-ray diffraction study has been performed on a single crystal of USb_0.8Te_0.2. At room temperature this material has the cubic FCC (NaCl) structure. On cooling through the Néel point of 205 K USb_0.8Te_0.2 is known to enter a triple-q antiferromagnetic phase. Below T_C 175 K USb_0.8Te0.2 is a ferromagnet with net magnetisation along the 1 1 1 body diagonal. Four-circle X-ray scattering data from high-symmetry {h O h} and {h h h} reflections are presented as a function of temperature. We confirm that this magnetisation directly leads to a magneto-elastic distortion of the crystal lattice with a rhombohedral extension along the 1 1 1 body diagonals. At 130 K, this distortion is found to correspond to a rhombohedral bond angle of = 89.856° ± 0.004° consistent with previous results. In this single-crystal study we also present evidence for additional associated effects which we attribute to inter-grain stresses and sample mosaic structure.     

Enhanced magnetic properties of Nd–Fe–B thin films crystallized by heat treatment

Nd₂Fe₁₄B Φ phase crystallites were formed in Nd_16.7Fe_65.5B_17.8 thin films prepared by RF sputtering with subsequent heat treatment. The 2 μm-thick films were deposited onto 0.1 mm Mo sheets at an average substrate temperature (T_s) of 365°C. The enhanced magnetic properties of the magnetically anisotropic thin films were investigated using different heating rates (h_r) of 10°C, 20°C, 50°C and 100°C/min in an annealing experiment. Transformation from the amorphous phase to the crystalline phase is clearly manifested by the formation of fine crystallites embedded as a columnar matrix of Nd₂Fe₁₄B phase. High-resolution scanning electron microscope data of the cross-section of the annealed films show columnar stacking of Nd₂Fe₁₄B crystallites with sizes <500 nm. Transmission electron microscope observations revealed that the microstructure of these films having out-of-plane magnetization consists of uniformly distributed Φ phase with grain size around 400 nm together with small Nd rich particles. This grain size of Φ phase is comparable to the single domain particle diameter of Nd₂Fe₁₄B. Significant change in _iH_c, 4πM_r and 4πM_s with h_r was confirmed. Annealing conditions with a heating rate of 50°C/min to an annealing temperature (T_a) of 650°C for 30 min was consequently found to give optimum properties for the NdFeB thin films. The resulting magnetic properties, considered to be the effect of varying h_r were _iH_c= 1307–1357 kA/m, 4πM_r=0.78–1.06 T and 4πM_s=0.81–1.07 T.     

Energy levels of Si

Proton energies and strengths of (p, γ), (p, p₁), (p, p₂) and (p, ₁γ) resonances of the ²⁷Al + p reaction were determined for E_p < 2.5 MeV. The γ-decay of 78 resonances and 46 bound levels was established and four new bound levels were found at E_x = 9793.8±2.0, 10513.5±1.0, 10884.1±2.0 and 11778.8±1.5 keV. The level at E_x = 11.43 MeV was found to be a doublet. Lifetimes of 25 bound levels were measured by means of the Doppler-shift attenuation technique. Angular distributions were measured at 15 resonances.     

The energy levels of Al

The energies and strengths of ²⁵Mg(p, γ), ²⁵Mg(p, p₁) and ²⁵Mg(p, p₂) resonances were determined in the energy range E_p < 2 MeV. Four new (p, γ) resonances were observed at E_p = 736.2±0.7, 818.1±0.7, 834.6±0.7 and 1514.7±0.7 keV. The γ-decay of 44 resonances and 49 bound levels was established. The energies of 41 and mean lifetimes of 25 bound levels were determined. Two new levels at E_x = 4952.4±1.4 keV and 5141.7±2.0 keV were observed. Angular distributions yielded unique spins for five resonance levels. Weisskopf estimates allowed unique spin assignments for five resonances. The Q-value of the ²⁵Mg (p, γ) ²⁶Al reaction was found to be 6305.0±1.2 keV.     

Stokes parameters measurement of light over a wide wavelength range by judicious choice of azimuthal settings of quarter-wave plate and linear polarizer

The Stokes parameters (S_p, S₁, S₂, S₃) of light are measured at an arbitrary wavelength over a wide wavelength range. The Stokes parameters S₁ and S₂ are easily obtained. But the Stokes parameter S₃ at the arbitrary wavelength is affected by the phase difference error Δ₁ of the quarter-wave plate mismatch and the Stokes parameters S₁ or S₂. Therefore, in this paper, S₃ is obtained from both the intensity measurements by the circular polarizer rotating the quarter-wave plate by 0° and 90°. Then, S₃ is obtained by considering only Δ₁, but is not affected by S₁ and S₂. Also, though Δ₁ is not accounted for, S₃ is measured more accurately.     

Pressure effect on the lattice spacings of an Fe0.7Ni0.3 invar alloy

The bulk moduli of both paramagnetic (B_p) and ferromagnetic (B_f) phases of a 30 Ni-Fe Invar alloy were measured from -40 to 150°C up to about 60 kbar. B_p is about 600 kbar larger than B_f. Both B_f and B_p are found to increase wit decreasing temperature.     

Temperature dependence of the specific heat of anisaldazine close to phase transitions

The experimental data for the specific heat C_p is analyzed at various temperatures close to the nematic–isotropic liquid (T_NI = 180.5 °C) and the solid–nematic (T_SN = 168.9 °C) transitions in anisaldazine.

Values of the critical exponent for the specific heat, which describe a λ-transition between nematic and the isotropic liquid, and a jump discontinuity between solid and nematic phases, are deduced for anisaldazine. They are compared with the model predictions.     

Hydrogen adsorption on Gd(0001)

The electronic structure of hydrogen adsorbate-induced states on Gd(0001) was investigated by means of photoelectron spectroscopy with linearly polarized radiation. The E vector of the incoming photon beam is rotatable. Clean and well-ordered rare-earth (0001) surfaces exhibit a highly localized surface state near the Fermi edge. After the adsorption of hydrogen, the surface state disappears and an additional sharp feature at about 4 eV binding energy is observed. For this latter state, the ratio of the radial matrix elements as well as the relative phase shifts were determined to be R=R_p/R_f=2.4±0.3 and δ_f−δ_p=310±10°, respectively. The removal of the Gd surface state by hydrogen adsorption was investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). The removal of the surface state exhibits domain-like behavior, with surface steps acting as domain boundaries. The tunneling spectra reveal that hydrogen adsorption causes a dramatic reduction in the differential conductivity near the Fermi level.     

Magnetic moment of the Δ (1232) resonance

The reaction γp → π^°γ′p has been measured with the TAPS BaF₂ calorimeter at the Mainz Microtron accelerator facility MAMI for energies between √2 = 1221–1331 MeV. Cross sections differential in angle and energy have been determined for the photon γ′ in three bins of the excitation energy. This reaction channel provides access to the magnetic dipole moment of the Δ⁺(1232) resonance and, for the first time, a value of μ_Δ+ = (2.7^+1.0_−1.3(stat) ± 1.5(syst) ± (theor)) π_N has been extracted.     

The influence of different fabrication processes on characteristics of excess Bi2O3-doped 0.95 (Na0.5Bi0.5)TiO3–0.05 BaTiO3 ceramics

In this study, we will develop the influences of the excess x wt% (x=0, 1, 2, and 3) Bi₂O₃-doped and the different fabricating process on the sintering and dielectric characteristics of 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃ ferroelectric ceramics with the aid of SEM and X-ray diffraction patterns, and dielectric–temperature curves. The 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ ceramics are fabricated by two different processes. The first process is that (Na_0.5Bi_0.5)TiO₃ composition is calcined at 850 °C and BaTiO₃ composition is calcined at 1100 °C, then the calcined (Na_0.5Bi_0.5)TiO₃ and BaTiO₃ powders are mixed in according to 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions. The second process is that the raw materials are mixed in accordance to the 0.95 (Na_0.5Bi_0.5)TiO₃–0.05 BaTiO₃+x wt% Bi₂O₃ compositions and then calcining at 900 °C. The sintering process is carried out in air for 2 h from 1120 to 1240 °C. After sintering, the effects of process parameters on the dielectric characteristics will be developed by the dielectric–temperature curves. Dielectric–temperature properties are also investigated at the temperatures of 30–350 °C and at the frequencies of 10 kHz–1 MHz.     

Influence of the ferroelastic twin domain structure on the {100} surface morphology of LaAlO3 HTSC substrates

LaAlO₃ crystals have been investigated with differential scanning calorimetry (DSC), high-precision X-ray powder diffraction (XRD) and scanning force microscopy (SFM). The DSC measurements show the second-order phase transition of LaAlO₃ at 544°C, where LaAlO₃ changes its symmetry from the cubic Pm3m high-temperature phase to the pseudocubic rhombohedral low-temperature phase. This paraelastic to improper ferroelastic phase transition causes twinning in the {100} and {110} planes of the pseudocubic lattice. The twin angles between the surface {100}_pseudocubic planes of twin domains were measured by SFM on the surface of a macroscopic (100)_cubic cut crystal plate. The misorientation angle ω₁₀₀ between {100} twins is 0.195(8)°, while {110} twinning gives an angle of ω₁₁₀=0.276(7)°. The two twin kink angles correspond to a rhombohedral angle of the pseudocubic cell of the phase as ₁=90.0973(40)° and ₂=90.0975(30)°, respectively. The XRD result for this rhombohedral angle is =90.096(1)°. The orientation of the misfit steps formed during annealing after mechanical surface polishing depends on the domain orientation and pattern during polishing. Any heating close to or above T_c changes the domain pattern. Footprints of previous domain patterns can thus be found on the surface in the form of surface corrugation and changes in the shape and orientation of misfit steps.     

Transient behavior of the thermal ion emission from KCl(0 0 1) upon polarity reversal of the electric extraction field

The technique of polarity reversal of the external electric extraction field (strength: 10² V/cm) was applied to study the relaxation of the thermal ion emission from the KCl(0 0 1) single crystal surface. Transient currents of the K⁺ and K₂Cl⁺ ions upon switching from the emission suppression to the ion extraction mode were recorded as a function of the evaporation time, the temperature, and the time of field reversal. The temperature dependence of the time constants of the K⁺ ions obtained from the exponential decreases of the emission currents to their steady-state emission resulted as logτ_h(s)=−(13.39±0.56)+(12.42±0.49)10³/T in a high temperature interval of 826–930 K after a prolonged heating period and as logτ_l(s)=−(20.65±1.04)+(16.77±0.81)10³/T in a low temperature interval of 750–801 K at the initial stage of evaporation, with corresponding activation energies of E_h(K⁺)=2.47±0.14 eV and E_l(K⁺)=3.32±0.16 eV, respectively. The transient currents can be interpreted by a partial adsorption of the suppressed ion currents at the kinks of the surface steps. The differences in the high- and low-temperature runs may be attributed to a strong coarsening of the surface at higher temperatures, which occurs as a bunching of monosteps to macrosteps and/or to an enrichment and segregation of divalent impurities at the surface. The transient behavior of the molecular K₂Cl⁺ ions seems to be strongly correlated with that of the K⁺ ions. This correlation is possibly caused by changes of the strength or the sign of the local electrical field connected with the excess charge at the kinks.     

Temperature dependence of the dielectric properties of SBN in the frequency range of 10kHz–10MHz

The dielectric constant of Sr_0.75 Ba_0.25 Nb₂O₆ and its dielectric loss have been measured from -20°C to +90°C at various frequencies in the range of 10 KHz– 10 MHz. The result show, that despite the low half-wave field distance product (E·l)λ/2, the use for light modulation purposes is limited because of the high value of tan σ.     

Use of the ac inductive method to assess the quality of bulk melt textured superconductors

Through a measurement of flux profiles, the inductive method of Campbell has not only been used for determining the bulk critical current density of large sized samples but has also been extended to study the coupling strength of the domains in melt textured NdBa₂Cu₃O_7−δ superconductors. An analysis of the profiles measured for the slow (1°C/h) and fast cooled (40°C/h) samples through peritectic temperature, T_p clearly showed that the domain boundaries in the fast cooled samples are decoupled in the presence of external dc fields. The observed features of flux profiles are correlated to the microstructural features of the samples. It is proposed that this technique can be used routinely to assess the quality of large size melt textured samples for high current applications like magnetic bearings or quasi permanent magnets.     

New experimental evidence of the structural modification in single crystal Bi2Sr2CaCu2Oy by X-ray diffraction observation

The variations of the high angle 00 peak-shape by means of X-ray l scans of the 00l fundamental reflections were investigated in detail for a highly oriented Bi₂Sr₂CaCu₂O_y (Bi2212) crystal with sufficiently small intrinsic mosaicity and the same crystal annealed in air at 250, 300, 400, 600, and 750°C for 20 h in consequence. For the first time, we observed a new additional reflection almost overlapped original 00l fundamental reflection at annealing temperature below 400°C by X-ray diffraction measurement, which shows that there coexisted two sets of lattice periodicity in the c-direction of the annealed crystal. The new additional reflection appeared at 250°C and disappeared at 400°C. Its intensity was increased at 300°C. The measurements of the AC susceptibility, c-axis parameter and full width at half maximum (FWHM) of the 00 peaks showed that the new additional reflection was associated with the oxygen diffusion in CuO₂ planes and the changes of strain field. The results provide the new experimental evidence that the structural distortion is more sensitive to the oxygen diffusion in CuO₂ planes than to that in Bi–O layers.     

Effective activation energy Ue(T,J,H) in textured Bi1.84Pb0.4Sr2Ca2Cu3Oy silver-clamped thick films

We have measured the resistivity of textured Bi_1.84Pb_0.4Sr₂Ca₂Cu₃O_y silver-clamped thick films as a function of temperature, current density ranging from 10 to 1×10³ A/cm² and magnetic field up to 0.3 T. We find that the effective activation energy U_e follows U_e(T,J,H)=U₀(1−T/T_p)^mln(J_c0/J)H⁻ with m=1.75 for Hab-plane and 2.5 for Hc-axis and =0.76 for Hab and 0.97 for Hc, for the current density regime above 100 A/cm², where T_p is a function of applied magnetic field and current density. This result suggests the effective activation energy U_e be correlated with the temperature, current density and magnetic field. The possible dissipative mechanisms responsible for the temperature, current density and magnetic field dependence of the effective activation energy are discussed.     

Magnetic properties in BaFe12O19 nanoparticles prepared under a magnetic field

It was observed that the nanocrystallites of BaFe₁₂O₁₉ formed at 140°C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T_c), a direct measuring of the strength of superexchange interaction of Fe³⁺–O²⁻–Fe³⁺, increased from 410°C for the nanoparticles prepared without an external field applied to 452°C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and Mössbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles.     

Inelastic electron scattering from He

The spectra of electrons scattered inelastically from ⁴He have been measured at incident energies from 150 MeV to 400 MeV for scattering angles from 38° to 90°. Through the use of a liquid ⁴He target, a high momentum resolution (≈0.25%) was obtained, and the 20 MeV 0⁺ state of the -particle was observed for the first time in electron scattering. The excitation energy and the total width of this state were determined and are in good agreement with the results from other experimental methods. It was found that the total disintegration cross section appears be smaller than 2(σ_p+σ_n) by more than an order of magnitude at the lowest q²(≈ 0.33 fm⁻²). In earlier work the assumption of a total cross section of 2(σ_p+σ_n) was found to be quite accurate for higher q². As a new result, the partial radiative width for the 0⁺ state is determined to be Γ_rad = 1.1±0.3 meV.     

The pion form factor at a timelike momentum transfer of approximately 1 fm from π electroproduction at threshold

From an extrapolation of threshold π⁺ electroproduction data combined with a kinematical constraint at an unphysical point, we derive the value of the pion form factor at a time-like value of the momentum transfer k²_π 1 fm⁻². The result is F_π(k²_π) = 1.19 ± 0.1. From this value, using a twice subtracted dispersion relation, we obtain r_π 0.98 ± 0.24 fm.     

Raman study of the phase separation in ZrO2-12 mol% CeO2 ceramic

We present Raman spectra of the ZrO₂-12 mol% CeO₂ ceramic heat- treated at 1000°C, 1200 °C and 1420 °C in the flowing mixture gas of H₂+Ar. Raman spectra reveal the tetragonal to monoclinic phase transition and three Raman bands are observed at 279 cm^-1, 410 cm^-1 and 456 cm^-1 at temperatures above 1200 °C; it turned out that these Raman bands are assigned to neither of the monoclinic phase nor the tetragonal phase. It is suggested that the starting ceramic undergoes phase separation in the heat-treatments in the mixture gas of H₂+Ar; the phase separation probably takes place as a result of the valence change of Ce during the heat-treatments, which leads to destabilization of the original tetragonal phase.