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The magnetotransport properties of [La.sub.0.7][Ca.sub.0.3-x][Ag.sub.x]Mn[O.sub.3] (0

The synthesis and magnetic properties of polycrystalline [Sr.sub.2]Cu([Re.sub.0.69-x]W.sub.x][Ca.sub.0.31])[O.sub.6] (0

The M-1212 compounds, with M=Fe,Co,Al and Ga are evaluated to solve the problem of M-cation intermixing with Cu and their structural peculiarities when compared to the parent Cu-1212 compound are presented and also the magnetization data along with neutron powder diffraction (NPD) structural refinement details is discussed. The oxygen content, based on oxygen occupancies, refined from the neutron diffraction data comes close to 7.0 for each.

Magnetic entropy and adiabatic temperature changes in and above the room-temperature region have been measured for La0.7Sr0.3Mn1-xMx′O3 (M′=Al,Ti) by means of magnetization and heat capacity measurements in magnetic fields up to 6 T. The magnetocaloric effect becomes largest at the ferromagnetic ordering temperature Tc that is tuned (from 364.5 K for x=0) to ∼300 K by the substitution of Al or Ti for Mn. While the substitution of Al for Mn drastically reduces the maximum magnetic entropy (-ΔSm) and adiabatic temperature (ΔTa) changes, it extends considerably the working temperature span and therefore improves the relative cooling power (RCP). Under a magnetic field change ΔH=2 T, -ΔSm (or ΔTa) of La0.7Sr0.3Mn1-xAlxO3 decreases from 2.66 J/kg K (or 1.65 K) for x=0 to 1.18 J/kg K (or 0.69 K) for x=0.1, while RCP increases from 80.3 to 108.8 J/kg, respectively. While Tc is largely suppressed, the magnetocaloric effect is only lightly affected by the Ti substitution. With ΔH=2 T, the La0.7Sr0.3Mn0.95Ti0.05O3 sample exhibits -ΔSm (or ΔTa) =2.44 J/kg K (or 1.38 K) with RCP=89.9 J/kg. The decrease of magnetic moment is found to be one possible reason behind the suppression of the MCE. The magnetocaloric effect in manganite materials seems to be inhibited by the existence of short-range ferromagnetic correlations above Tc. [ABSTRACT FROM AUTHOR]

We investigate the magnetotransport properties of La0.7Ca0.3-xAgxMnO3 (0<=x<=0.3) polycrystalline films prepared by spray pyrolysis. The virgin La0.7Ca0.3MnO3 (LCMO) (x=0.0) film shows paramagnetic-ferromagnetic [insulator-metal (IM)] transition at TC∼260 K (TIM∼245 K). Ag doping induces appreciable enhancement in TC/TIM and interestingly TIM of Ag doped films is higher than the corresponding TC value. In the 30% Ag doped film IM transition occurs at ∼325 K which is ∼10 K higher than its TC∼315 K. Strong irreversibility is observed in zero field cooled–field cooled magnetization of all the films and the irreversibility temperature Tirr>TC increases with Ag doping. This suggests the formation of magnetic clusters above TC. The pure LCMO film exhibits small magnetoresistance (MR) ∼20% and rather poor temperature coefficient of resistance (TCR) in the vicinity of the room temperature. In contrast, around room temperature the Ag doped films show large MR which is ∼60% for x=0.15 and decreases to ∼30% for the pure Ag doped film. Ag doped films also have good TCR ∼4%/K in the vicinity of room temperature. [ABSTRACT FROM AUTHOR]

The article explains the synthesis, properties and superconductivity exhibited by the Sm[Fe.sub.1-x][Co.sub.x]AsO (x = 0.0-0.30) compounds. The several different substitution effects of the system are also analyzed.

Numerous studies are conducted to explain the synthesis, as well as the variouss physical properties of the Fe[Se.sub.1/2][Te.sub.1/2] superconductor. An applied magnetic field is shown to lead to a decreese in the bulk superconductivity of the system.

The synthesis, structure, electrical transport and heat capacity of SmFeAsO are described. The studies have shown that the heat capacity [C.sub.P](T) peak at 4.5 K is because of the antiferromagnetic ordering of [Sm.sup.3+] spins and these results are in confirmation with ordering of Sm in [Sm.sub.2-x][Ce.sub.x]Cu[O.sub.4].

The perovskite [Sr.sub.1-x][Ho.sub.x]Co[O.sub.3-[delta]] has displayed room temperature ferromagnetism in a severely limited composition range 0.2

The measurement results for the magnetic entropy and adiabatic temperature changes in and above the room-temperature region for [La.sub.0.7][Sr.sub.0.3][Mn.sub.1-x][M'.sub.x][O.sub.] (M'= Al, Ti) is presented. The magnetocaloric effect in manganite materials is found to be inhibited by the existence of short-range ferromagnetic correlations above ordering temperature [T.sub.c].

The third harmonic nonlinear susceptibility of the [Ru.sub.0.9][Sr.sub.2]Y[Cu.sub.2.1][O.sub.7.9] magnetosuperconductor is examined. The results have shown that there are two well separated magnetic transitions, where the peaks grow as the amplitude of the ac field is increased and stay negative for the investigated temperature range.

High-pressure and high temperature method is used for synthesizing a 1212-type niobio-cuprate with an ideal formula of Nb[Sr.sub.2]Y[Cu.sub.2][O.sub.8]. The magnetic measurements have shown that the superconducting volume fraction is very small, but superconductivity and weak ferromagnetism have coexisted below 17 K in the [Nb.sub.0.9][Sr.sub.2]Y[Cu.sub.2.1][O.sub.7.9] sample.

Neutron diffraction measurements are used for analyzing the superconducting ruthenocuprates RE[Sr.sub.2][Cu.sub.2]Ru[O.sub.8] (RE = rare earth). Magnetic resonance and superconducting quantum interference device (SQUID) data have shown a complex picture, which is associated with Cu magnetic ordering.

Near single phase compounds of the composition RuSr2(LnCe2)Cu2O(sub 12.25) with Ln=Y and Dy are synthesized successfully through a HPHT solid-state reaction route. The results are compared with widely reported Gd/Ru-1222 and Ru-1212 (RuSr2GdCu2O8) compounds.

Roddlesden-Popper homologous series Sr(sub n+1)Co(sub n)O(sub 3n+1) (n=1, 2, 3, 4, and infinite) compounds are successfully synthesized by a high pressure and high temperature technique. Negative magnetoresistance is observed for Sr2CoO4 and found to be larger than that for SrCoO3.

The results of crystal growth, superconductivity and flux pinning in Gd and heavily Pb co-doped Bi-2212 crystals are discussed. Enhanced flux pinning was produced by Gd and heavily Pb doping.

We report synthesis, structural details and magnetization of SmFe1-xCoxAsO with x ranging from 0.0 to 0.30. It is found that Co substitutes fully at Fe site in SmFeAsO in an iso-structural lattice with slightly compressed cell. The parent compound exhibited known spin density wave (SDW) character below at around 140 K. Successive doping of Co at Fe site suppressed the SDW transition for x = 0.05 and later induced superconductivity for x = 0.10, 0.15 and 0.20 respectively at 14, 15.5 and 9K. The lower critical field as seen from magnetization measurements is below 200Oe. The appearance of bulk superconductivity is established by wide open isothermal magnetization M(H) loops. Superconductivity is not observed for higher content of Co i.e. x = 0.30. Clearly the Co substitution at Fe site in SmFe1-xCoxAsO diminishes the Fe SDW character, introduces bulk superconductivity for x between 0.10 and 0.20 and finally becomes non-superconducting for x above 0.20. The Fe2+ site Co3+ substitution injects mobile electrons to the system and superconductivity appears, however direct substitution introduces simultaneous disorder in superconducting FeAs layer and thus superconductivity disappears for higher content of Co. [ABSTRACT FROM AUTHOR]

One of the most important properties of very recently reported FeSe based superconductors is the robustness of their superconductivity under applied magnetic field. The synthesis and control of superconductivity in FeSe based compounds is rather a difficult task. Synthesis and physical property characterization for optimized superconductivity of FeSe1/2Te1/2 at 13 K is reported here. The compound crystallized in a tetragonal structure with lattice parameters a=3.8015(2) and c =6.0280(4) Å. Magnetization measurements indicated bulk superconductivity with lower critical field (Hc1) of around 180 Oe. By applying Ginzburg-Landau theory, the Hc2(0) value is estimated to be ∼1840 kOe for the 90% of resistive transition. A heat capacity measurement revealed bulk superconductivity by a hump at Tc near 13 K and an expected decrease in the same was observed under an applied magnetic field. [ABSTRACT FROM AUTHOR]

The perovskite Sr1-xHoxCoO3-δ shows room temperature ferromagnetism (Tc∼335 K) in a severely limited composition range 0.2<=x<=0.25. The formal Co valence is almost constant at +3 regardless of the Ho concentration (x>=0.2); thus, the high-spin state or the intermediate spin state of 3d6 Co should be mainly responsible for the magnetism. The ferromagnetism was found to accompany the formation of a superstructure 2ap×2ap×4ap (ap denotes a cubic perovskite cell). Besides, an additional superstructure 4(2ap)×4(2ap)×(4ap) was found at x=0.225, where the ferromagnetic volume in magnetic susceptibility was largest. The data suggest that the formation of the multiple superstructures probably plays a key role to establish the peculiar ferromagnetism. [ABSTRACT FROM AUTHOR]

We report the synthesis and magnetic properties of polycrystalline Sr2Cu(Re0.69-xWxCa0.31)O6 samples with x=0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6, which were prepared by high-pressure high-temperature synthesis technique. These phases had ordered cubic perovskite structures. The pristine compound (x=0) underwent a ferromagnetic (ferrimagnetic) transition with TC∼460 K. The ferromagnetism was suppressed by the W substitution and disappears almost completely for x=0.6. Mechanism of the suppression of the magnetism is discussed assuming a localized electron system. [ABSTRACT FROM AUTHOR]

The superconducting ruthenocuprates RESr2Cu2RuO8 (where RE is a rare earth or Y) have been interpreted as ferromagnetic superconductors, based on a strong response to applied field at the Ru Néel temperature (around 130 K). However, neutron diffraction measurements on the Gd compound by Lynn et al. [Phys. Rev. B 61, 1214964 (2000)] (using a separated isotope) show evidence only of antiferromagnetic order, while very limited data by Takagiwa et al. [J. Phys. Soc. Jpn. 70, 333 (2001)] on the Y compound show one antiferromagnetic Bragg peak and possible coexisting ferromagnetism. We have studied a sample of YSr2Cu2.1Ru0.9O7.9 prepared by a high pressure-high temperature route. Temperature dependent neutron powder diffraction shows antiferromagnetic order, manifested by the observation of the 12, 12, 12 and 12, 12, 32 reflections, which appear at 135 K [as suggested by the superconducting quantum interference device (SQUID) data]. Their intensities at the lowest temperature are consistent with a c-axis orientation of the Ru magnetic moments. Although the lowest order nuclear Bragg peak shows a small intensity increase as the temperature is lowered, this is attributed to magnetic ordering of the Cu planes and not to the presence of a ferromagnetic component, in agreement with the polarized neutron data of Lynn et al. [Phys. Rev. B. 61, 1214964 (2000)] Magnetic resonance and SQUID data on the same sample (to be reported elsewhere) show a complex picture, associated with Cu magnetic ordering. [ABSTRACT FROM AUTHOR]

We investigated the third harmonic nonlinear susceptibility of the Ru0.9Sr2YCu2.1O7.9 magnetosuperconductor and found the presence of two well separated negative peaks. The peak at TM=140 K is connected to the previously well known main magnetic ordering of the Ru sublattice, and the peak at around 180 K is probably associated with the formation of superparamagnetic particles above TM. The negative signals for both peaks will be discussed in terms of ensuing magnetic orderings of Ru spins at these temperatures. Further, these results will be compared with a recent study of nonlinear susceptibility on RuSr2GdCu2O8. [ABSTRACT FROM AUTHOR]

We have successfully synthesized a 1212-type niobio-cuprate with an ideal formula of NbSr2YCu2O8 by high-pressure and high temperature method. The phase-pure sample was obtained by Nb-poor Cu-rich starting composition, Nb0.9Sr2YCu2.1O7.9, at 6 GPa and 1450 °C for 2 h. The 1212-type niobio-cuprate has tetragonal lattices of a=3.853(1) Å and c=11.547(1) Å (space group P4/mmm), isostructural to Ru–1212Y phase. From magnetic measurements, it was found that superconductivity and weak ferromagnetism coexisted below ∼17 K in the Nb0.9Sr2YCu2.1O7.9 sample, although the superconducting volume fraction is still very small. [ABSTRACT FROM AUTHOR]

Roddlesden-Popper homologous series Srn+1ConO3n+1 (n =1,2,3,4, and ∞)compounds were successfully synthesized by a high pressure and high temperature technique. Structure refinement revealed that these compounds crystallize in tetragonal structures, while the compound n=∞ is cubic. These compounds are ferromagnetic with the Curie temperature decreasing from 255 K for n=1 to about 200 K for n=2-4 and down to 175 K for SrCoO3. Co4+ ions present as intermediate spin states for n=1-4, but in the low spin state in SrCoO3. Negative magnetoresistance was observed for Sr2CoO4 and found to be larger than that for SrCoO3. [ABSTRACT FROM AUTHOR]

RuSr2(LnCe2(Cu2O12.25 (Ru-1232) compounds with Ln=Y and Dy being synthesized by high pressure high temperature (6 GPa, 1200 °C) solid state synthesis route crystallize in space group P4/mmm in near single phase form with small quantities of SrRuO3 and RuSr2(RE1.5Ce0.5)Cu2O10 (Ru-1222). Both samples exhibit magnetic transitions (Tmag) at ~90 K with significant branching of zero-field-cooled (ZFC) and field-cooled (FC) magnetization and a sharp cusp in ZFC at ~70 K, followed by superconducting transitions at ~30 K. Both compounds show typical ferromagnetic hysteresis loops in magnetic moment (M) versus field (H) magnetization right up to Tmag. i.e., <90 K. To our knowledge these are the first successfully synthesized Ru-1232 compounds in near single phase with lanthanides including Y and Dy. The results are compared with widely reported Gd/Ru-1222 and Ru-1212 sRuSr2GdCu2O8d compounds. In particular, it seems that the Ru moments magnetic ordering temperature (Tmag) scales in the c-direction distance between magnetic RuO6 octahedras in Ru-1212/1222 or 1232 systems. [ABSTRACT FROM AUTHOR]

Bi2-xPbxSr2Ca1-yGdyCu2O8+δ (x=0.34 and y=0.18, 0.34) crystals were grown by the self-flux method. The crystals have a cleavage thickness of only half unit cell up to two unit cells with Tc only dropping 20 K as y is increased from 0.18 to 0.34 for as-grown crystals. However, Tc increased to almost the same value of about 80 K after annealing in air regardless of the Gd doping levels. The co-doping produced enhanced flux pinning compared to the sole Gd doping. A secondary peak effect presented in crystals with x=0.34 and y=0.34 was explained by phase segregations containing Gd-rich clusters. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Various MSr[SUB2]Ycu[SUB2]O[SUBz] (M-1212 with M = Fe, Co, Al, and Ga) compounds have been synthesized through a solid-state reaction route. All these compounds are derived from the basic CuBa[SUB2]Ycu[SUB2]O[SUB7-δ] structure. Rietveld structural refinement of the room-temperature neutron diffraction data for Fe-1212 reveals that nearly half of Fe remains at the M site, while the other half goes to the Cu site in the CuO[SUB2] plane. For the M = Co, Al, and Ga compounds, the basic unit cell gets doubled due to two distinct M sites in the M-O layer and the space group Ima2, without any cation intermixing is found to be more appropriate. For the M = Fe sample, magnetic ordering of Fe spins is seen from magnetization data below 70 K. In the case of the M = Co compound, a shallow down-turn in magnetic susceptibility is seen below 150 K, indicating low-dimensional magnetic characteristics of the Co-O layer. The compounds with M = Al and Ga show paramagnetic behavior in their magnetization data down to 5 K. The oxygen content, established for each compound based on oxygen occupancies, refined from the neutron diffraction data comes close to 7.0 for each. [ABSTRACT FROM AUTHOR]

We have successfully synthesized a 1212-type niobio-cuprate with an ideal formula of NbSr2YCu2O8 by high-pressure and high temperature method. The phase-pure sample was obtained by Nb-poor Cu-rich starting composition, Nb0.9Sr2YCu2.1O7.9, at 6 GPa and 1450 °C for 2 h. The 1212-type niobio-cuprate has tetragonal lattices of a=3.853(1) Å and c=11.547(1) Å (space group P4/mmm), isostructural to Ru–1212Y phase. From magnetic measurements, it was found that superconductivity and weak ferromagnetism coexisted below ∼17 K in the Nb0.9Sr2YCu2.1O7.9 sample, although the superconducting volume fraction is still very small. [ABSTRACT FROM AUTHOR]

The perovskite Sr1-xHoxCoO3-δ shows room temperature ferromagnetism (Tc∼335 K) in a severely limited composition range 0.2<=x<=0.25. The formal Co valence is almost constant at +3 regardless of the Ho concentration (x>=0.2); thus, the high-spin state or the intermediate spin state of 3d6 Co should be mainly responsible for the magnetism. The ferromagnetism was found to accompany the formation of a superstructure 2ap×2ap×4ap (ap denotes a cubic perovskite cell). Besides, an additional superstructure 4(2ap)×4(2ap)×(4ap) was found at x=0.225, where the ferromagnetic volume in magnetic susceptibility was largest. The data suggest that the formation of the multiple superstructures probably plays a key role to establish the peculiar ferromagnetism. [ABSTRACT FROM AUTHOR]

The superconducting ruthenocuprates RESr2Cu2RuO8 (where RE is a rare earth or Y) have been interpreted as ferromagnetic superconductors, based on a strong response to applied field at the Ru Néel temperature (around 130 K). However, neutron diffraction measurements on the Gd compound by Lynn et al. [Phys. Rev. B 61, 1214964 (2000)] (using a separated isotope) show evidence only of antiferromagnetic order, while very limited data by Takagiwa et al. [J. Phys. Soc. Jpn. 70, 333 (2001)] on the Y compound show one antiferromagnetic Bragg peak and possible coexisting ferromagnetism. We have studied a sample of YSr2Cu2.1Ru0.9O7.9 prepared by a high pressure-high temperature route. Temperature dependent neutron powder diffraction shows antiferromagnetic order, manifested by the observation of the 12, 12, 12 and 12, 12, 32 reflections, which appear at 135 K [as suggested by the superconducting quantum interference device (SQUID) data]. Their intensities at the lowest temperature are consistent with a c-axis orientation of the Ru magnetic moments. Although the lowest order nuclear Bragg peak shows a small intensity increase as the temperature is lowered, this is attributed to magnetic ordering of the Cu planes and not to the presence of a ferromagnetic component, in agreement with the polarized neutron data of Lynn et al. [Phys. Rev. B. 61, 1214964 (2000)] Magnetic resonance and SQUID data on the same sample (to be reported elsewhere) show a complex picture, associated with Cu magnetic ordering. [ABSTRACT FROM AUTHOR]