Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Electronic and magnetic properties of layered LnFePO (Ln=La and Ce)

Effects of the replacement of La with Ce on the electronic and magnetic properties of a layered superconductor LaFePO (T_c=∼5 K) were studied. Polycrystalline samples of CeFePO, prepared by a solid-state reaction, showed metallic conduction down to 2 K without exhibiting superconducting transition, although the resistivity decreased largely at temperatures below 30 K. Further, they showed an apparent positive magnetoresistance (MR) below ∼2 K, superposed on a negative MR. Temperature dependence of magnetic susceptibility is decomposed to a temperature-sensitive Curie-Weiss component presumably due to the Ce³⁺ ions with a magnetic moment of 1.98μ_B and a less temperature-sensitive component attributable to itinerant electrons. The magnetic interaction between Ce³⁺ ions and itinerant electrons in CeFePO likely suppresses the superconducting transition observed in LaFePO.     

Development of the method for low-temperature investigations of HTSC systems on an X-ray electron magnetic spectrometer

Peculiarities of the chemical structure of bulk polycrystalline samples of the high-temperature superconductors Bi₂Sr₂CaCu₂O₈, BiSrCaCu₂O_5.5, BiSrCaCu₃O₈, and YBa₂Cu₃O_{7 ? δ} have been investigated in detail at room and superconducting temperatures on an X-ray electron magnetic spectrometer equipped with an attachment for low-temperature studies. It is shown that covalent bonding is formed at a superconducting temperature between copper and oxygen due to Cu²⁺ ions. Due to the enhancement of the d(Cu)–p(O) hybridization of copper and oxygen electrons in the superconducting state, the d-electron density increases near E _F. The occurrence of additional peaks in the O1s and Sr3d (Ba3d) spectra after transition of the system to the superconducting state indicates changes in the nearest environment of O and Sr (Ba) atoms, in particular, the transition of Sr atoms to a higher oxidation state.     

Effects of divalent alkaline earth ions on the magnetic and transport features of La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) compounds

We have studied the magnetic and transport properties of Fe doped La_0.65A_0.35Mn_0.95Fe_0.05O₃ (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (T_C) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and T_C are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of T_C and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.     

Magnetism on Mg1−xZnxCyNi3

The magnetic phase diagram for Mg_1−xZn_xC_yNi₃ has been tentatively constructed based on magnetization and muon spin relaxation (μSR) measurements. The superconducting phase was observed to fade as x (y) increases (decreases). The low y samples show early stages of long-range ferromagnetism, or complete long-range ferromagnetism. In the phase diagram, the ferromagnetic phase exists in addition to the superconducting phase, suggesting that there is some correlation between superconductivity and ferromagnetism, even though the coexistence of ferromagnetism and superconductivity is not observed from the μSR measurements down to 20 mK for the superconducting sample (T_c=2.5 K, (x, y)=(0, 0.9)).     

Comparison of emission and absorption core profiles of Sr and Ba resonance lines in an air-acetylene flame

Under optically thin conditions, the core of emission-line profiles of the resonance lines (¹P₁-¹S₀ transition) of Sr and Ba have been measured with a pressure-scanned Fabry-Pe´rot interferometer in a laminar, pre-mixed, shielded air-acetylene flame at 1 atm (T = 2275 K). Under identical experimental conditions, where Doppler and collisional broadening are almost equally important, these profiles are compared with Zeeman-scanned absorption profiles of the same resonance lines. The values for the full width at one-half maximum (FWHM) and the shift of the emission and absorption core profiles for Sr and Ba proved to be identical within an experimental scatter of 5 and 20%, respectively. Obviously the radiation emitted by the Sr and Ba atoms in the flame is thermal, which means that the spectral shape of the line is consistent with Maxwell-Boltzmann equilibrium.     

Superconductivity in LaSr(Ba)CuO and related systems

Systematics in the superconducting behaviour of Ln _2−xSr _x(Ba _x)CuO ₄, La _2−xSr _x−yBa _y(Ca _y)CuO ₄, (La _1−xLn _x) _2−ySr _y(Ba _y)CuO ₄ with Ln = Pr, Nd or Y and related systems are presented. The role of oxygen stoichiometry and lattice parameters of these mixed valent copper oxides is indicated. These high T _c superconducting oxides exhibit high-temperature resistivities in the borderline of the metal-nonmetal transition.     

Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12−y high temperature superconductors

Measurements of the a.c.susceptibility (χ=χ′+iχ″) have been made on the Mg substituted high T_C superconducting system, CuBa₂(Mg_xCa_1−x)₃Cu₄O_12−y (Cu-1234) with x=0, 0.10 & 0.20, at different values of the a.c.field amplitude. Estimates of the intergranular critical current density(J_C) made from the field dependent χ″-T curves show an improvement in the Mg-substituted Cu-1234 system. Results have been analysed in the light of the crystal structure and the superconducting anisotropy factor (γ=ξ_ab/ξ_c) of the Cu-1234 system. Lower superconducting anisotropy emanating from Mg substitution has been found to be significant, resulting in better superconducting properties.     

Superconducting properties of SmFeAsO1−x prepared under high-pressure condition

Synthetic conditions such as stoichiometries, temperature and pressure are optimized to achieve a high quality oxygen deficient SmFeAsO_0.6 superconductor. Both electric and magnetic measurements show a sharp superconducting transition at about 55 K. Several important physical parameters are deduced. The apparent superconducting gap observed in heat capacity with 2Δ_o/k_BT_c of 4.57 larger than that of previous fluorine replaced samples indicate that this superconductivity will not strongly conflict with the phonon-mediated BCS mechanism. The mean free length ?=18.8 nm and the coherent length ξ=2.3-3.3 nm show that the superconductivity is in the clean limit.     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures T_cm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10⁻³ torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT² term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

Synthesis of NaxAy(H3O)zCoO2 ⋅ wH2O (A=Mg, Ca, Sr and Ba) superconductors via ion exchange

We have synthesized Na_xA_y(H₃O)_zCoO₂⋅wH₂O (A=Mg, Ca, Sr and Ba) superconductors via ion exchange at ambient conditions. The extent of incorporation of alkaline-earth elements was found to be related to the similarity in size between the guest ion and the host one. In the case of A=Ca, about half of the sodium was successfully replaced by calcium. All the synthesized samples were shown to be superconducting with the T_c from 3.1 to 4.5 K.     

Dielectric and leakage current properties of Li doped (Ba,Sr)TiO3 thick film interdigital capacitors on the alumina substrates

Li doped (Ba,Sr)TiO₃ thick films were fabricated by employing the screen printing method on the alumina (Al₂O₃) substrates. Interdigital capacitor patterns with seven fingers of 200 μm gap, 250 μm length were designed and screen printed on the alumina substrates. Ba_0.5Sr_0.5TiO₃ materials, paraelectric state at the room temperature, have been chosen for the microwave devices due to high dielectric permittivity and low loss tangent, however, the sintering temperature of (Ba,Sr)TiO₃ is over 1350 °C. In order to lower the sintering temperature, Li (3 wt%) was added to the (Ba,Sr)TiO₃ materials. Li doped (Ba,Sr)TiO₃ thick films screen printed on the alumina (Al₂O₃) substrates were sintered at 900 °C for 1.5 h. The structural feature was analyzed with X-ray diffraction method. Temperature dependent dielectric properties were characterized from 303 to 403 K at 1 MHz. Within the ±100 V of bias voltage, current-voltage characteristics of Li doped (Ba,Sr)TiO₃ films were investigated from 303 to 403 K. Through the current-voltage characteristics, the resistivity of Li doped (Ba,Sr)TiO₃ films were calculated.In this paper, the significant negative temperature coefficient of resistance (NTCR) of Li doped (Ba,Sr)TiO₃ films will be presented through the activation energy fitting. Measured activation energy is approximately 0.366 eV.     

Coexistence of superconductivity and magnetism in the La0.87−xLnxSr0.13FeAsO (Ln=Sm, Gd, Dy) system

The interplay between the superconducting phase and spin density wave order phase was studied. We report the magnetic and superconducting properties of the hole-doped FeAs-based superconducting compound La_0.87−xLn_xSr_0.13FeAsO (Ln=Sm, Gd, Dy; 0≤x≤0.06). Both resistivity and magnetic susceptibility measurements show that the superconducting transition temperature decreases with increase in composition of magnetic ions. The hysteresis loop of the La_0.87−xLn_xSr_0.13FeAsO sample shows a superconducting hysteresis in addition to a paramagnetic background. The experiment demonstrates that the magnetism and superconductivity coexist in hole-doped FeAs-based superconducting compounds. Among these three magnetic rare-earth elements, the influence of Dy³⁺ doping on superconductivity is more evident than that of Gd³⁺ doping, while the influence of Sm³⁺ doping is the weakest. The trend is consistent with the variation of the lattice parameter along c-axis.     

Spray drying: An alternative synthesis method for polycationic oxide compounds

Synthesis of polycationic compounds by the spray-drying technique is an interesting alternative in the domain of aqueous precursor synthesis methods. Spray drying yields high quality samples with good reproducibility. The possibility of scaling up for production of large quantities with fast processing time is well established by the commercial availability of powders of various compositions. In this paper, we have discussed the advantages and limitations of this method and demonstrated its interest by synthesizing a few polycationic compounds selected for their attractive properties of thermoelectricity [Bi_1.68Ca₂Co_1.69O₈, La_0.95A_0.05CoO₃ (A=Ca, Sr, Ba)] or magnetoresistance [La_0.70A_0.30MnO₃ (A=Sr, Ba)]. We have confirmed the quality of these samples by reporting their structure, magnetic and transport properties.     

Local magnetization study of the first-order superconducting transition in CeCoIn5

We report local magnetization measurements on the heavy fermion superconductor CeCoIn₅ using a micro-Hall probe. Below a critical point T₀ (), the local magnetization shows a clear jump at the superconducting transition temperatures for both H∥a and H∥c, indicating that the phase transition at the upper critical field H_c2 becomes a first-order phase transition. In addition, we observed an undershoot behavior of magnetization jump above (H∥c), which suggests a rapid change of textured superconducting structure with vortices and the nodal planes expected in the Fulde-Ferrell-Larkin-Ovchinnikov state.     

Tunable luminescent properties by adjusting the Sr/Ba ratio in Sr1.97−xBaxMgSi2O7:Eu0.01, Dy0.02 phosphors

The long afterglow phosphors Sr_1.97−xBa_xMgSi₂O₇:Eu²⁺_0.01, Dy³⁺_0.02 (x=0, 0.4, 0.8, 1.2, 1.6 and 1.97) were synthesized via high temperature solid-state reaction. The phase identification reveals that the crystal plane spacing becomes greater with the decrease in the Sr/Ba ratio. Phase transition occurs when x=1.97. A nonlinear relationship between the emission peak and the crystal plane spacing is obtained with the decrease of the Sr/Ba ratio. This ascribes to the splitting of the 5d level of the Eu²⁺ and the change of the crystal field strength. The duration of the afterglow becomes shorter with the decrease of the Sr/Ba ratio. It may ascribe to deeper trap depth, lower trap concentration and the embarrassment of the transfer of carriers.     

The unusual magnetic and electronic properties of Gd0.5Ba0.5CoO2.9

Magnetic and electrical properties of well-characterized Gd_0.5Ba_0.5CoO_2.9 have been studied carefully in order to compare them with those of other analogous cobaltates of the type Ln_0.5A_0.5CoO₃ (Ln=La, Nd and A=Sr, Ba) which are ferromagnetic. The results show that Gd_0.5Ba_0.5CoO_2.9, which has A-site cation ordering at room temperature, does not become a genuine ferromagnet at low temperatures, but the ferromagnetic interactions observed at 280 K give over to an antiferromagnetic (AFM) state on cooling to 230 K. The AFM state is rendered ferromagnetic on the application of high magnetic fields. The properties can be understood on the basis of phase separation induced by the large A-site cation-disorder, arising from the size mismatch.     

Atomic structure of Sr0.75Ba0.25Nb2O6 single crystal and composition-structure-property relationin (Sr,Ba)Nb2O6 solid solutions

The structure of the Sr_0.75Ba_0.25Nb₂O₆ single crystal has been investigated by x-ray diffraction. The occupancies of the Ba and Sr sites in two structural channels are determined. It is found that these sites are split in the large (pentagonal) channel. A qualitative correlation is revealed between the smearing of the phase transition and the displacement of the Sr atom from the m symmetry plane in the pentagonal channel at different [Sr]/[Ba] ratios. The degree of acentricity of the NbO₆ octahedra is analyzed as a function of the [Sr]/[Ba] ratio.     

Preparation of porous (Ba,Sr)TiO3 by adding corn-starch

A new method of preparing porous (Ba,Sr)TiO₃ ceramics has been introduced, using an ordinary ceramics processing technique. The effect of corn-starch on the positive temperature coefficient of resistivity characteristics and microstructure of the porous (Ba,Sr)TiO₃ ceramics has been investigated. When the corn-starch addition was 1-20 wt%, the PTCR jump was over 10⁶ and 1-2 orders higher than that of samples without corn-starch. Also, it was found that the (Ba,Sr)TiO₃ ceramics had porous microstructure by the addition of corn-starch. The porosity of the ceramics with 20 wt% corn-starch was 44%. The electrical properties of the (Ba,Sr)TiO₃ ceramics have been discussed, based on the microstructure, resistivity of grain boundaries, donor concentration of grains and the electrical potential barrier of grain boundaries.     

Magnetic-field-driven quantum critical behavior in graphite and bismuth

We study magnetotransport properties of graphite and rhombohedral bismuth samples and found that in both materials applied magnetic field induces the metal-insulator- (MIT) and reentrant insulator-metal-type (IMT) transformations. The corresponding transition boundaries plotted on the magnetic field-temperature (B − T) plane nearly coincide for these semimetals and can be best described by power laws T ∼ (B − B_c)^κ, where B_c is a critical field at T = 0 and κ = 0.45 ± 0.05. We show that insulator-metal-insulator (I-M-I) transformations take place in the Landau level quantization regime and illustrate how the IMT in quasi-3D graphite transforms into a cascade of I-M-I transitions, related to the quantum Hall effect in quasi-2D graphite samples. We discuss the possible coupling of superconducting and excitonic correlations with the observed phenomena, as well as signatures of quantum phase transitions associated with the M-I and I-M transformations.