Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Studies on strontium substituted rare earth manganites

Sintering, electrical conductivity and thermal expansion behaviour of combustion synthesised strontium substituted rare earth manganites with the general formula Ln_1−xSr_xMnO₃ (Ln=Pr, Nd and Sm; x=0, 0.16 and 0.25) have been investigated as solid oxide fuel cell cathode materials. The combustion derived rare earth manganites have surface area in the range of 13–40 m²/g. Strontium substitution increases the electrical conductivity values in all the rare earth manganites. With the decreasing ionic radii of rare earth ions, the conductivity value decreases. Among the rare earth manganites studied, (Pr/Nd)_0.75Sr_0.25MnO₃ show high electrical conductivity (>100 S/cm). The thermal expansion coefficients of Pr_0.75Sr_0.25MnO₃ and Nd_0.75Sr_0.25MnO₃ were found to be 10.2×10⁻⁶ and 10.7×10⁻⁶ K⁻¹ respectively, which is very close to that of the electrolyte (YSZ) used in solid oxide fuel cells.     

Direct evidence for charge ordering and electronic phase separation in BixSr1−xMnO3 at room temperature

We present a study of charge ordering and electronic phase separation (EPS) phenomenon in Bi_xSr_1−xMnO₃, for an exhaustive range of x (0.25x0.75), by STM/STS at room temperature (RT) and specific heat measurements at high temperatures (350–650 K). Atomically resolved STM images of the samples, in real space, show the presence of stripe-like charge-ordered (CO) phase coexisting with charge-disordered (CD) phase. The STM images further reveal that the fraction of CO phase increases with an increase in x. The conductance spectra of these phases measured at nano level by STS are discussed. The transition to CO phase above RT is corroborated by specific heat measurements in all samples, giving a T_CO(x) phase diagram for this system.     

Magnetic pair making and breaking effect of Ru in La0.7Sr0.3Mn0.9Ru0.1O3 and La0.5Sr0.5Co0.9Ru0.1O3

The substitutional effect of Ru on the magnetic and transport properties of double exchange ferromagnets, La_0.7Sr_0.3MnO₃ and La_0.5Sr_0.5CoO₃ has been investigated. It is found that substitution of 10% Ru at the Mn site of La_0.7Sr_0.3MnO₃ decreases the Curie temperature by 20 K than that of the parent compound. However, a large decrease in the Curie temperature, ΔT_c80 K and the system undergoes a transition from metallic state to insulating state is observed when 10% Ru is doped in La_0.5Sr_0.5CoO₃. The marginal effect of Ru in the Mn–O–Mn sublattice in comparison to the Co–O–Co sublattice could be due to the magnetic exchange interaction between Mn and Ru by virtue of the fact that Ru exhibits variable valence states, Ru⁺⁴/Ru⁺⁵. The e_g and t_2g parentage of Ru⁺⁵ is similar to Mn⁺⁴ and therefore, Ru⁺⁵ ion appears to participate in the double exchange mediated ferromagnetic (FM) interaction. On the other hand, Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co⁺³: t_2g⁵e_g¹), forcing a double exchange FM state to anti-FM state.     

Tuning the multiferroic properties of Pb(Fe1/2Nb1/2)O3 by cationic substitution

A series of substituted lead iron niobate compounds with the general formula Pb²⁺_(1−x)A^Z_x(Fe_{{(1−(2−Z)x)/2}}Nb_{{(1+(2−Z)x)/2}})O₃ (0<x<0.6 and A=La³⁺, K⁺ or Sr²⁺) were prepared by a modified solid-state synthesis. The relative concentrations of Fe³⁺ and Nb⁵⁺ were adjusted to compensate the charge imbalance due to the aliovalent substitution. The dielectric constant and magnetic susceptibilities were studied as a function of temperature. The temperature of the dielectric maximum, T_M, of the substituted compounds decreased linearly with increasing concentration of the substituent ions. The magnetic measurements showed an antiferromagnetic transition at temperatures T_N1 due to the superexchange interactions mediated by Fe–O–Fe and an additional antiferromagnetic-type transition at T_N2. T_N1 linearly increased with the increasing concentration of Fe³⁺ ion at the B-site of ABO₃-type substituted compounds. T_M is shown to be directly dependent on the concentration of the ferroactive Nb⁵⁺ ions at the B-site and Pb²⁺ ions at the A-site.     

Metal/insulator manganite multilayers

We have studied the transport and magnetic properties of strongly textured metal/insulator La_1−xSr_xMnO₃ (x=0.4,0.1) bilayers and trilayers, grown by DC magnetron sputtering over MgO and SrTiO₃ substrates. The multilayers present transport properties similar to those of the La_0.6Sr_0.4MnO₃ films, being very sensitive to deposition conditions. Magnetic multilayers show a metal–insulator transition around T_c (250 K) and colossal magnetoresistance which is maximum around T_c. No extrinsic magnetoresistance associated with the multilayered structure was observed, probably due to the presence of ferromagnetic coupling between the metallic layers, as suggested by magnetization measurements.     

Double peak behavior of resistivity curves in Cd doped LaMnO3 perovskite systems

The effect of Cd doping on transport, magnetotransport, and magnetic properties has been investigated in the perovskite La_1−xCd_xMnO₃ (0x0.5) systems. The ρ(T) curves exhibit a sharp metal insulator transition (T_p1), which is close to paramagnetic to ferromagnetic transition (T_c) obtained from M−T curves for all samples. In addition, ρ(T) curves for Cd doped samples exhibit another broad transition (T_P2) below T_p1. This transition becomes more prominent and the transition temperature (T_p2) shifts to lower temperature with increasing Cd content. Such double peak behavior in the ρ(T) curve is attributed to the phase separation between the ferromagnetic metallic phases and the ferromagnetic insulating phases induced by the electronic inhomogeneity in the samples.     

High hydrostatic pressure effect on magnetic state of anion-deficient La0.70Sr0.30MnOx perovskite manganites

The magnetic state of La_0.70Sr_0.30MnO_x (x=2.85, 2.80) anion-deficient manganites is studied experimentally under hydrostatic pressure up to 5.2 GPa. These materials possessing the properties of a spin-glass state. The La_0.70Sr_0.30MnO_2.85 exhibits a coexistence of the rhombohedral (R3¯c) and tetragonal (I4/mcm) crystal structures and below T_f 50 K the spin-glass state is formed. The La_0.70Sr_0.30MnO_2.80 exhibits the tetragonal (I4/mcm) crystal structure. For this compound a phase-separated magnetic state below T_f is formed, involving coexistence of C-type antiferromagnetic (AFM) clusters within spin-glass matrix. In both compounds the crystal structure and magnetic states remain stable upon compression. Under the hydrostatic pressure up to 1 GPa, the La_0.70Sr_0.30MnO_2.85 is a spin glass with a smeared phase transition to the paramagnetic state. The freezing temperature T_f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T_MO increases at a rate of 12.90 K/GPa. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit cell parameters.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

Magnetic transition behavior of perovskite manganites Nd_(0.5)Sr_(0.3)Ca_(0.2)MnO_3 polycrystalline

A polycrystalline sample Nd_(0.5)Sr_(0.3)Ca_(0.2)MnO_3 is prepared by the conventional solid state reaction method. The structure and magnetic properties are investigated with x-ray diffraction(XRD) patterns, a superconducting quantum interference device(SQUID), and electron spin resonance(ESR). The sample is in single phase with the space group Pbnm symmetry. With the decrease of temperature, Nd_(0.5)Sr_(0.3)Ca_(0.2)MnO_3 undergoes three magnetic transitions: ferromagnetic transition at TC≈ 210 K, charge-ordering at TCO≈ 175 K, and antiferromagnetic transition at TN= 155 K. In addition, the activation energy Ea ≈ 52.78 me V can be extracted by curve fitting.     

High temperature susceptibility of semimagnetic-semiconductor Zn1-xMnxS

This paper gives an analysis of the high temperature susceptibility of diluted semimagnetic-semiconductor Zn_{1 − x}Mn_xS. The high-temperature susceptibility of Zn_{1 − x}Mn_xS was found to behave in accordance with the Curie-Weiss law. From _χ(T) measurements the exchange integral of Mn²⁺ -Mn²⁺ interaction 2J₁/k_B = (-34.6±0.5) K (effective exchange integral) was obtained. A spin S = 2.6±0.1, close to its atomic value S = , was also found. The role of the superexchange in this alloy is shortly discussed at the end of the paper.     

Trapping mechanism of superconductivity suppression induced by Pr substitution in the Ho1−xPrxBa2Cu3O7−δ system

Pr concentration dependence of the superconducting transition temperature T_c in the Ho_1−xPr_xBa₂Cu₃O_7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y_1−xPr_xBa₂Cu₃O_7−δ reference system. Both systems have the same value of the critical concentration x_c=0.58, in accordance with nearly equal ionic radii of Ho³⁺ and Y³⁺ ions. It has been shown that the T_c(x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr^IV-ions, if one takes also into account that the number of these ions changes with x.     

钙钛矿锰氧化物(La1-xGdx)4/3Sr5/3Mn2O7 (x=0, 0.025) 磁性和输运性质研究

采用传统的高温固相烧结法制备了双层钙钛矿锰氧化物(La_1-xGd_x)_4/3Sr_5/3Mn₂ O₇ (x=0, 0.025)多晶样品. 通过X射线衍射仪研究发现样品为Sr₃Ti₂O₇型四方结构, 空间群为I4/mmm; 磁性测量表明, Gd³⁺掺杂后的样品(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂O₇的三维磁有序转变温度(T_C1^3D)、磁化强度(M)均降低, 这是由于Gd³⁺的掺杂引起晶格的畸变, 从而使得晶格常数发生改变, 减弱了铁磁耦合而导致的; 通过电子自旋共振谱测量发现, 在T_C^3D<T<300 K温度范围内, 两样品在顺磁的基体上均有短程的铁磁团簇存在, 出现了相分离现象. 电性测量表明: 两样品分别在T_C1^3D (La_4/3Sr_5/3Mn₂O₇ 样品的三维磁有序转变温度, T_C0^3D)<T<300 K温度范围内均以三维变程跳跃的方式导电, 分析得出Gd³⁺的掺杂使得载流子局域长度的减小. 这表明载流子需要吸收更多的能量才能克服晶格的束缚进行跳跃, 因此(La_0.975Gd_0.025)_4/3Sr_5/3Mn₂ O₇ 样品的电阻较高.     

Effect of Co doping on the magnetic properties of La0.85Ag0.15(Mn1−yCoy)O3

The mixed valent manganites (La_0.85Ag_0.15)MnO₃ with perovskite structure has been prepared by doping up to 50% of Co at the Mn site. Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all the prepared materials. However, the long-range magnetic ordering observed in (La_0.85Ag_0.15)MnO₃ is systemically reduced to cluster glass-type (short-range) of FM ordering due to the introduction of Co. The FM transition temperature was found to decrease with increase in Co doping up to 20% and for further increase in Co doping, the T_c was found to increase. They are explained on the basis of competition between FM double exchange interactions in Mn–O–Mn and Co–O–Co networks. In addition to PM–FM transition, evidences of FM to antiferromagnetic (AFM), and AFM to reentrant spin-glass transitions have been observed. The shift in spin-glass freezing temperature, T_f has been observed from the frequency variation of ac susceptibility measurements. The observed magnetic transitions are explained on the basis of magnetic interactions in different Mn–O–Mn, Mn–O–Co and Co–O–Co networks and such transitions are also observed from the measurement of third harmonic susceptibility. Metal–insulator transition and colossal magneto-resistivity have been observed up to 10% of Co doping.     

Ion-size effects on the structure and superconductivity of La-214 compounds at 1/8 doping

A structural transition to a low-temperature tetragonal (TLT) phase occurs below T_d2≈60 K in La_2−xBa_xCuO₄ only around x=0.125, accompanied by sharp suppression of superconductivity. In this study we investigated the effects of partial substitution of smaller divalent cations for Ba²⁺ ions (Ba²⁺ > Sr²⁺ > Ca²⁺) on these anomalous low-temperature properties. The suppression of superconductivity is strongly affected by the appearance of the TLT phase for this carrier concentration. We also show that the variations of T_d2 and T_c are quantitatively scaled by the averaged size of cations at the La site. This dependence can be interpreted as chemical effects acting effectively as applied pressure. We discuss how our observation is related to recent reports which claim that the TLT structure itself does not necessarily destroy the superconductivity.     

Charge ordering in the electron doped Ca1−xYxMnO3 manganites

Structural studies on the electron doped Ca_1−xY_xMnO₃ are presented. At 300 K, orthorhombic O-phase was observed in all cases, associated to low electric resistivity and high Curie–Weiss temperature. For samples with x>0.07, structural phase transitions to more distorted orthorhombic and monoclinic phases were found at T<170K. In these phases only weak ferromagnetic interactions were observed.     

Interplay of structure and magnetism in ruthenocuprates: a Raman scattering and dilatometry study

We present a Raman scattering and dilatometry study of polycrystalline samples of the magnetic superconducting ruthenocuprates RuSr₂Gd_2−xCe_xCu₂O_10+δ (RuGd₁₂₂₂) and RuSr₂GdCu₂O₈ (RuGd₁₂₁₂). In the Raman spectra a high-temperature diffusive-like laser-tail develops below the magnetic ordering temperature (T_M) into an underdamped peak which shifts up to 130 cm⁻¹. A line assigned to O(Ru) phonons hardens, narrows and strengthens strongly below T_M. Finally, a phonon peak appears below T_M at 590 cm⁻¹. These three magnetic-order-dependent features are observed for RuGd₁₂₁₂ and for RuGd₁₂₂₂ with x=1.0, but do not appear for x=0.5. Dilatometry measurements, on the other hand, evidence a change of the expansion coefficient at T_M. These results point to a structural effect accompanying the magnetic order, and suggest a complex interplay of spin and lattice degrees of freedom in these ruthenocuprates.     

Properties of a large La1.92Sr0.08CuO4+δ single crystal grown by the travelling-solvent floating-zone method

A centimeter size single crystal of La_2−xSr_xCuO_4+δ (volume=1.32 cm³) with x_Sr=0.08 has been grown by the travelling-solvent floating-zone (TSFZ) method using a double ellipsoidal-type optical furnace as the heat source. The crystallised phase was checked solvent free by X-ray powder diffraction experiments, the crystal dimensions and quality being investigated by X-ray and neutron Laüe techniques. Several rocking curves of the Bragg peaks were performed by neutron diffraction giving a full width at half maximum (FWHM) of 0.200° for (006) reflection and clearly showing the presence of twin domains as expected for such an orthorhombic structure. The superconducting critical temperature of the as-grown crystal under 2 oxygen bar was determined by SQUID measurements with T_c=18–20 K. Thermal treatments at different oxygen pressures were carried out showing no significant improvement of the transition sharpness and the T_c value. Normal state susceptibility was also measured from 6 to 800 K for two different field orientations and can be interpreted as an antiferromagnetic insulating state behaviour. The resistivity measurements display an insulating behaviour perpendicular to the CuO₂ planes and a metallic behaviour in the planes, with a high anisotropy ratio R_c/R_b350 at room temperature and a zero resistivity achieved at 27 K in both directions. The specific heat measurements have revealed no anomalies in the temperature range 15–300 K.     

Dy,Co共掺杂对BiFeO3陶瓷磁特性和磁相变温度Tc的影响

采用快速液相烧结法制备BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0, 0.05, 0.1, 0.15)陶瓷样品. 实验结果表明: 所有样品的主衍射峰与纯相BiFeO₃相符合且具有良好的晶体结构, 随着Co³⁺掺杂量的增大, Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品的主 衍射峰由双峰(104)与(110)逐渐重叠为单峰(110), 当掺杂量x>0.05时, 样品呈现正方晶系结构; SEM形貌分析可知: Dy³⁺, Co³⁺共掺杂使BiFeO₃晶粒尺度由原来的3—5 μ减小到约1 μ. 室温下, BiFeO₃样品表现出较弱的铁磁性, 随着Dy³⁺和Co³⁺掺杂, BiFeO₃样品的铁磁性显著提高. 在外加磁场为30 kOe的作用下, Bi_0.95Dy_0.05Fe_1-xCo_xO₃ (x=0.05, 0.1, 0.15)的M_r分别为0.43, 0.489, 0.973 emu/g; M_S分别为0.77, 1.65, 3.08 emu/g. BiFeO₃和Bi_0.95Dy_0.05Fe_1-xCo_xO₃样品磁矩M随着温度T的升高而逐渐减小, Dy掺杂使BiFeO₃样品的T_N由644 K升高到648 K, 而T_C基本没有变化. Dy和Co共掺杂导致BiFeO₃样品磁相变温度T_C由870 K降低到780 K, 其T_C变化主要取决于Fe-O-Fe反铁磁超交换作用的强弱和磁结构的相对稳定性. 关键词： 铁磁电材料 磁滞回线 磁相变温度     

The effect of gallium substitution on the superconductivity of the BiSrCaCuO system

Magnetic susceptibility, X-ray diffraction and resistivity measurements of the system Bi_1.4Pb_0.6Sr₂Ca_2−xGa_xCu₃O_y are reported for 0 x 2. The high-T_c 2223 phase with a T_c of 107 K for x = 0 exists for x 0.3. The low-T_c 2212 phase with a T_c of 75 K for x = 0 exists for the full range of x. The highest values of the critical temperature and the largest volume fraction of the low-T_c phase in compounds with Ga occur for x = 0.5 ± 0.1. The identification of CaO by X-ray diffraction for x 0.6 indicates that Ga replaces Ca in the compound.