Occurrence and stability of ferromagnetism in chemically synthesized cobalt doped TiO<Subscript>2</Subscript>

We report the development of ferromagnetism in ∼30 nm sized well-characterized Ti_1−x Co _x O₂ powders with x = 0.00015–0.006 and its absence for x > 0.006. In addition, these studies show the effect of Co doping on the structural stability and anatase to rutile phase transformation. X-ray diffraction data of samples synthesized by a wet chemical method and annealed at 450 °C indicate a limited solubility of ∼1.2% for Co in the anatase TiO₂ matrix, and with further increase, the CoTiO₃ phase is formed along with increased presence of rutile TiO₂. The bandgap (∼3.23 eV) of the anatase TiO₂ remained almost unchanged for x < 0.006, but decreased rapidly for x ≥ 0.006 approaching 2.8 eV for x = 0.03. The magnetic data from Ti_1−x Co _x O₂ samples with x = 0.006 showed a coercivity H _c ∼ 150 Oe and a weak magnetic moment of 0.2 μ_B/ion at 300 K. The ferromagnetism of Ti_0.994Co_0.006O₂ with open hysteresis loops continue up to a high superparamagnetic blocking temperature T _B ∼ 675 K, above which a superparamagnetic behavior was observed. Systematic changes in the structural, magnetic and optical properties suggest that Co doping is an excellent method to tailor the physical properties of TiO₂ nanoparticles.     

Investigations on green-emitting,Mn<Superscript>2+</Superscript>: BaAl<Subscript>12</Subscript>O<Subscript>19</Subscript> phosphors obtained by solution combustion process

Manganese-doped BaAl₁₂O₁₉ green phosphor was prepared using a self-propagating (combustion) synthesis. Powder X-ray diffraction and scanning electron microscopy were used to characterize the as-prepared combustion product. A room temperature photoluminescence study shows an emission line at 513 nm corresponding to a transition from the upper ⁴T₁ → ⁶A₁ ground state of Mn²⁺ ions. The electron paramagnetic resonance (EPR) spectrum exhibits six line hyperfine structure at g = 1.981. From the EPR spectrum, the spin-Hamiltonian parameters have been evaluated. The g value indicates that the site symmetry around Mn²⁺ ions is distorted tetrahedral. The number of spins (N) participating in the resonance for g = 1.981 is measured as a function of temperature. The paramagnetic susceptibility (χ) is calculated from the EPR data at various temperatures. From the plot 1/χ versus T, the Curie constant (C) and Curie paramagnetic temperature (θ_p) have been evaluated and discussed.     

Phase component and conductivities of Co-doped BaTiO<Subscript>3</Subscript> thermistors

BaTi_1−x Co _x O_3−δ (0.01 ≤ x ≤ 0.4) ceramics were prepared by a wet chemical process polymerized with polyvinyl alcohol. The phases and related electrical properties of the ceramics were investigated. The phase component of the ceramics changes from a tetragonal phase to a hexagonal one with the Co concentration increase. A pure hexagonal phase formed in the ceramic with x = 0.2. The measurement of the temperature dependence of resistances revealed that the ceramic resistivities increase with temperature rising at the temperatures (T) lower than half of the related Debye temperature (Θ_D), and the ceramics show a negative temperature coefficient (NTC) effect at T > Θ_D/2. The material constants B _50/120 of the BaTi_1−x Co _x O_3−δ NTC thermistors were calculated to be 3,187, 2,968 and 2,648 K for x = 0.2, 0.3 and 0.4, respectively. Narrow-band conduction and non-adiabatic hopping models are proposed for the conduction mechanisms at T < Θ_D/2 and T > Θ_D/2, respectively.     

Magnetic and Superconducting Properties of Doped and Undoped Double Perovskite Sr<Subscript>2</Subscript>YRuO<Subscript>6</Subscript>

We report here SQUID (magnetization) measurements, along with supporting specific heat, Raman, SEM (scanning electron microscope), EDX (energy dispersive X-ray) and XRD (X-ray diffraction) measurements, on Cu-doped and undoped double perovskite Sr₂²⁺Y³⁺Ru⁵⁺O₆^2-\mathrm{Sr}_{2}^{2+}\mathrm{Y}^{3+}\mathrm{Ru}^{5+}\mathrm{O}_{6}^{2-} (abbreviated as SrY2116) system grown as single crystal using high-temperature solution growth technique. These measurements show the undoped system to be a nonmetallic (insulating) spin glass (SG) and the ∼5–30% Cu-doped (i.e. Cu-concentration/(Cu + Ru-concentration) ∼5–30%) system to be a spin glass superconductor (SGSC) with T _c (critical temperature) ∼28–31 K and superconducting volume fraction, f _sc∼2.2–9%. To mention, similar measurements done on undoped and Cu-doped BaY2116 and BaPr2116 systems show for them the same (SG, SGSC) behaviors. However they show a decrease in T _c and f _sc when diamagnetic Y³⁺ ions are replaced by Pr³⁺ spins, presumably due to enhanced internal pair breaking, and also decreased Cu–O–Cu overlap, owing to Pr³⁺ presence; these phenomena are known to exist in the Pr123 compound, PrBa₂Cu₃O_7−δ (δ∼0), due to ∼10% of Pr³⁺ ions having tendency to occupy Ba²⁺ sites. Measurements done on undoped and Cu-doped SrHo2116 show similar SG and SGSC properties. Further, the undoped and Cu-doped SrY2116 crystals grown by hydrothermal growth technique (i.e., grown using lower temperature and high pressure) show same behaviors. From these investigations it can be said that the undoped Ru-double perovskites (A₂BB′O₆, B′=Ru) are SG systems and that Cu-doped Ru-double perovskites (A₂BB′O _δ , δ∼6, B′=Ru_1−x Cu _x , 0<x≲0.3) are SG superconductors (SGSCs). Results are discussed.     

Superconducting YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−δ</Subscript> thick film (T<Subscript>c</Subscript>(0) = 92 K) on a newly developed perovskite ceramic substrate

A complex perovskite oxide, YbBa₂NbO₆, as a non-reacting substrate for YBa₂Cu₃O_7-° super-conducting film has been developed. The dielectric constant and loss factor values of the material are in the range suitable for its use as substrate for microwave applications. A YBa₂Cu₃O_7−δ superconducting thick film dip coated on YbBa₂NbO₆ substrate gave a T_c (0) of 92 K and current density of ∼ 1.3 × 10⁴ A cm⁻².     

Synthesis and physical properties of new oxide AgMnO<Subscript>2</Subscript>

A novel oxide AgMnO₂ was prepared from LiMnO₂ via Ag⁺ → Li⁺ exchange in the eutectic melt AgNO₃-KNO₃. It crystallizes in a monoclinically distorted unit cell (SG C2/m) caused by the Jahn-Teller (J-T) ion Mn³⁺ (3d ⁴). The structure was refined by isotypy with the crednerite CuMnO₂. There are two long axial Mn–O of 264.2(0) pm and four equatorial bonds of 192.7(3) pm and Mn–O–Mn adjoining (83.07°) are bent below the ideal angle. The thermal variation of the magnetic susceptibility (χ/T ⁻¹) obeys a Curie-Weiss law with manganese in a trivalent, high spin (HS) state accommodated in elongated MnO₆ octahedra (14.8%). Direct coupling between Mn³⁺ involves negative exchange interactions through long-range antiparallel moments with a temperature θ _p = −436 K and a magnetic moment of 5.26 μ_B/Mn³⁺ slightly larger than the spin only moment. The title oxide is stable in air up to ∼680 °C before it decomposes into metal silver. It displays a semi-conducting behavior with an activation energy of ∼0.45 eV, characteristic of a conduction by low mobility polarons between Ag^+/2+ where nearly all polarons are bonded. The photoelectrochemical properties of AgMnO₂ have been investigated by photocurrent technique in 1 M KOH. The cathodic photocurrent J _ph provides unambiguous evidence of p-type character attributed to oxygen insertion (0.025 oxygen by formula unit) as required by the charge compensating mechanism. The valence band is made up of Ag−4d wave functions positioned at ∼5.14 eV below vacuum. A comparison with CuMnO₂ was also reported.     

Electric resistance and magnetoresistance of 30-nm-Thick La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> films elastically compressed in the (110) plane

We have studied the structure, electric resistance, and magnetoresistance of 30-nm-thick (110)La_0.67Ca_0.33MnO₃ (LCMO) films grown by laser deposition on (001)-oriented LaAlO₃ substrates. The unit cell parameters a and b (along the [100] and [010]LCMO axes, respectively) of these manganite films are significantly (by ∼1.2%) increased as compared to the corresponding values in the pseudocubic unit cell of bulk stoichiometric LCMO crystals. At T < 150 K, the temperature dependence of the resistivity of LCMO films is well described by the relation ρ = ρ₁ + ρ₂ (H) T ^4.5. The value of ρ 2 decreases with increasing magnetic field and is close to the analogous coefficient for manganite films grown on substrates with small lattice misfit.     

Martensitic phase transformation in single crystal Co<Subscript>5</Subscript>Ni<Subscript>2</Subscript>Ga<Subscript>3</Subscript>

The magnetic, thermal, and transport properties of martensitic phase transformation in single crystal Co₅Ni₂Ga₃ have been investigated. The single crystal Co₅Ni₂Ga₃ shows martensitic transformation at 251 K on cooling and 254 K on warming. Large jumps in the temperature-dependent resistance curve, temperature-dependent magnetization curve, and temperature-dependent thermal conductivity curve are observed at martensitic transformation temperature (T _M). Negative magnetoresistance due to spin disorder scattering was observed in Co₅Ni₂Ga₃ single crystal at all temperature range. The temperature-dependent negative magnetoresistance shows a peak at T _M, which indicates that the spin disorder increases in the process of phase transition. Co₅Ni₂Ga₃ sample exhibits a temperature dependence of thermal conductivity κ(T) (dκ/dT > 0) due to electrons being above temperature 100 K.     

Electron Spin Resonance Study of Polycrystalline La<Subscript>0.4</Subscript>Bi<Subscript>0.1</Subscript>Ca<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript>

We have inspected the magnetic properties of polycrystalline La_0.4Bi_0.1Ca_0.5MnO₃ using electron spin resonance (ESR) in the temperature range 150–280 K. The temperature dependence of magnetization indicates that the Curie temperature is T _C= 225 K. ESR spectra revealed that the sample is not completely paramagnetic above its Curie temperature through the presence of ferromagnetic interactions in the temperature range 225–270 K which can be attributed to the presence of Griffiths phase in this temperature range. The sample becomes completely paramagnetic above 270 K. The presence of Griffiths phase can be attributed to the disorder induced by the 6 s ² lone pair electrons of Bi³⁺ ions.     

Ac and dc magnetotransport properties of the phase-separated La<Subscript>0.6</Subscript>Y<Subscript>0.1</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The physical properties of the La_0.6Y_0.1Ca_0.3MnO₃ compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol–gel method revealed that specimens are single phase and have average grain size of ∼0.5 μm. Magnetization and 4-probe dc electrical resistivity ρ(T,H) experiments showed that a ferromagnetic transition at T _C ∼ 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature T _MI . The magnetoresistance effect was found to be more pronounced at low applied fields (H ≤ 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to T _C and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity ε′(T,H) is consistent with changes in the concentration of e _g mobile holes, a feature much more pronounced close to T _C .     

Enhanced Superconducting Properties of Air-Processed GdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7-δ</Subscript> Single Domains with BaCO<Subscript>3</Subscript>/BaCuO<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript> Addition

Single domain GdBa₂Cu_7-δ (Gd123) bulk superconductors were fabricated in air by top-seeding melt-texture growth. Performance of the air-processed Gd123 was successfully enhanced by addition of both BaCO₃ and BaCuO_2−x , which suppress the formation of Gd_1+x Ba_2−x Cu₃O_7-δ solid solutions. The optimum doping amount ranges from 0.05 to 0.15, M BaCO₃ and 0.05 to 0.1, M BaCuO_2−x per molar Gd123. The distribution of the second phase particles was observed by scanning electron microscopy. A narrow band formed by Gd₂BaCuO₅ particle concentration appeared around the seeding zone in both a–b plane and c-growth sector in Gd123 single grain. Trapped magnetic field density reached 0.67, T for sample with 24 mm in diameter and 8, mm in thickness and a high critical current density J _c up to 91,200, A/cm² was achieved at 77, K under self-field.     

Contributions from Inter-grain Boundaries to the Magneto-resistive Effect in Polycrystalline High-<Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> Superconductors. The Underlying Reason of Different Behavior for YBCO and BSCCO Systems

In order to clarify the mechanisms in charge of broadening of resistive transition R(T) in magnetic fields of bismuth-based polycrystalline high-T _C superconductor (HTSC), a comparative study of Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x (BSCCO) and YBa₂Cu₃O_7−δ (YBCO) have been performed. Magnetoresistive effects and irreversibility line obtained from magnetic measurements have been studied. It was established that (1) for YBCO, the smooth part of R(T) dependence unambiguously corresponds to dissipation in the intergrain boundaries for arbitrary magnetic fields; (2) for polycrystalline BSCCO, the smooth part of R(T) dependences correspond to dissipation within intergrain boundary subsystem in the field range H<10² Oe only, while standard measurements of R(T) dependences in magnetic field range H>10² Oe reflect the dissipation processes occurring both in intergrain boundary and HTSC grain subsystems; (3) for the high-field range, the contribution from intergrain boundaries of BSCCO can be distinguished from magnetoresistance R(H) dependences obtained at high enough current density on textured samples. It is proposed that various magneto-resistive properties of these classical HTSC systems are due comparatively weak pinning in BSCCO.     

Crystal Electric Field Excitations in the Non-Fermy Liquid Compound YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript>

We have calculated the crystalline electric field (CEF) splitting of the energy levels of Yb³⁺ (4f¹³) in the clean Yb-based heavy fermion compound YbRh₂Si₂. The data of inelastic neutron scattering and electron spin resonance measurements in YbRh₂Si₂, together with relevant structural, thermodynamic, and magnetic properties, were used as input in the calculations of the possible CEF level scheme in this non-Fermi-liquid compound. Two possible sets of the CEF parameters with the Γ₆ or Γ₇ ground-state symmetry are discussed. PACS: 71.27.+a; 75.20.Hr; 76.30.−v.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Interplay of Superconductivity and Ferromagnetism in UGe<Subscript>2</Subscript>

The emergence of pressure induced superconductivity (SC) under the background of ferromagnetic state in 5f-electron based itinerant ferromagnetic superconductor UGe₂ is studied in the single band model by using a mean-field approximation. The solutions to the coupled equations of superconducting gap (Δ) and magnetization (m) are obtained using Green’s function technique and equation of motion method. It is shown that there generally exists a coexistent (Δ≠0, m≠0) solution to the coupled equations of the order parameters in the temperature range 0<T<min (T _C,T _FM), where T _C and T _FM are respectively the superconducting and ferromagnetic transition temperatures. The study of electronic specific heat (C/T), density of states, free energy, etc. are also presented. The specific heat capacity at low temperature shows linear temperature dependence as opposed to the activated behavior. Density of states increases as opposed to the case of a standard ferromagnetic metal. Free energy study reveals that the superconducting ferromagnetic state has lower energy than the normal ferromagnetic state and, therefore, realized at low enough temperature. The agreement between theory and experimental results for UGe₂ is quite satisfactory.      

Low-loss,high-purity (TeO<Subscript>2</Subscript>)<Subscript>0.75</Subscript>(WO<Subscript>3</Subscript>)<Subscript>0.25</Subscript> glass

By melting a mixture of high-purity oxides in a platinum crucible under flowing purified oxygen, we have prepared (TeO₂)_0.75(WO₃)_0.25 glass with a total content of 3d transition metals (Fe, Ni, Co, Cu, Mn, Cr, and V) within 0.4 ppm by weight, a concentration of scattering centers larger than 300 nm in size below 10² cm⁻³, and an absorption coefficient for OH groups (λ ∼ 3 μm) of 0.008 cm⁻¹. The absorption loss in the glass has been determined to be 115 dB/km at λ = 1.06 μm, 86 dB/km at λ = 1.56 μm, and 100 dB/km at λ = 1.97 μm. From reported specific absorptions of impurities in fluorozirconate glasses and the impurity composition of the glass studied here, the absorption loss at λ ∼ 2 μm has been estimated at ≤100 dB/km. The glass has been drawn into a glass-polymer fiber, and the optical loss spectrum of the fiber has been measured.     

Ordered oxygen deficient ‘112’ perovskites,LnBaCo<Subscript>2</Subscript>O<Subscript>5·50+<Emphasis Type="Italic">δ</Emphasis></Subscript>: complex magnetism and transport properties

The ordered oxygen deficient ‘112’ perovskites, LnBaCo₂O_5·50+δ (Ln = lanthanide or Y), exhibit a very flexible structure which can either uptake (δ > 0) or release oxygen (δ < 0) depending on the experimental conditions of synthesis and on the size of the lanthanide. These compounds exhibit remarkably complex magnetic transitions, metal-insulator transition and exceptionally high magnetoresistance. We show herein that their physics is mainly dominated by three different ferromagnetic states, depending on the cobalt valency: FM1 for δ = 0 (Co³⁺), FM2 for δ < 0 (Co²⁺/Co³⁺) and FM3 for δ > 0 (Co³⁺/Co⁴⁺). The competition between ferromagnetism and antiferromagnetism in these phases and the various transitions are discussed taking into consideration the spin state of cobalt, the issue of phase separation and the effect of cobalt coordination and disproportionation. In honour of Prof. C N R Rao on his 75th birthday     

Ion-selective properties of metastable (VO)<Subscript>0.09</Subscript>V<Subscript>0.18</Subscript>Mo<Subscript>0.82</Subscript>O<Subscript>3</Subscript> · 0.54H<Subscript>2</Subscript>O

This paper describes a thin-film solid electrode with an ion-sensitive membrane based on the mixed oxide (VO)_0.09V_0.18Mo_0.82O₃ · 0.54H₂O. The electrode is selective for tetravalent vanadium in the concentration range 3 ≤ pC _V ⁴⁺ ≤ 5 and acidity range 4.5 ≤ pH ≤ 6, with a slope close to the theoretical value. In the range 1 ≤ pH < 5, the electrode responds to changes in hydrogen ion concentration, with a slope of 50 ± 2 mV/pH. Its alkali-metal-ion response shows up in the range 1 ≤ pC _M ⁺ ≤ 4 for pH ≥ 6. We examine the effect of the Li⁺, K⁺, Na⁺, Cs⁺, Rb⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Co²⁺, Ni²⁺, Mn²⁺, Al³⁺, Cr³⁺, and VO₂⁺ ions on the potential of the electrode and determine its selectivity coefficients for these cations.     

Solution-sol-gel processing of superconductors

Bulk materials and thin films of pure and homogeneous YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x compounds were prepared by a nanocomposite solution-sol-gel (SSG) method. The superconducting oxides of YBa₂Cu₃O_7−x and Bi₂Sr₂CaCu₂O_8+x were prepared at very low temperatures i.e. 750°C and 850°C, respectively by SSG method. Pellets sintered from these nanophasic sol powders showed sharp resistivity drops atT _c ∼ 90°K for YBa₂Cu₃O_7−x andT _c∼67°K for Bi₂Sr₂CaCu₂O_8+x . Thin films were prepared using triphasic sol of Y, Ba, Cu and tetraphasic sol of Bi, Sr, Ca and Cu on MgO and SrTiO₃ substrates. The triphasic sol coated on SrTiO₃ substrates and calcined at 800°C for 12h showed the formation of superconducting phase, YBa₂Cu₃O_7−x with preferred orientation along theC-axis. X-ray diffraction patterns of the Bi₂Sr₂CaCu₂O_8+x films on MgO substrate showed the formation of the superconducting phase with preferential orientation along the C-axis and the microwave absorption data as a function of temperature of this film revealed the onset temperature to be 90°K.     

Thermodynamic and Magnetic Properties of Superconductors with Anisotropic Energy Spectrum,MgB<Subscript>2</Subscript>

The behavior of the superconducting transition temperature T _c and that of the jump of electron heat capacity (C _S −C _N )/C _N of the compound MgB₂ at T=T _c with substitution of boron and magnesium atoms by other atoms from the periodic table of the elements, corresponding to introduction of additional electrons or holes in this compound are researched. The microscopic superconductivity theory in MgB₂ systems in the magnetic field parallel to the crystallographic axis (H ‖ c) is built. The magnitude of the upper critical field H _c2 is determined and its temperature dependence in two-band systems with different and identical topologies of Fermi surface cavities of the corresponding bands is studied. The obtained results and their comparisons with the experimental data demonstrate that all kinds of anomalies of the physical properties of the compound MgB₂ are effectively described by the two-band model.