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.     

Properties of nonstoichiometric Pr0.6−xSr0.4MnO3 as the cathodes of SOFCs

A series of samples Pr_0.6−xSr_0.4MnO₃ (x=0, 0.01, 0.05, 0.1, 0.15, 0.2) were synthesized by a solid state reaction method. Pr deficiency at the A site has a great effect on the properties of Pr_0.6−xSr_0.4MnO₃ as the cathode of SOFCs (solid oxide fuel cells). Compared to the commonly used La_0.6Sr_0.4MnO₃ and La_0.55Sr_0.4MnO₃ cathode, Pr_0.6−xSr_0.4MnO₃ is better in the properties of conductivity, overpotential and impedance. In all the samples, the one with x=0.05, Pr_0.55Sr_0.4MnO₃, revealed the best performance in the measured temperature range.     

Fitting of transport measurements in polycrystalline La2/3Ca1/3MnO3

Using a previous qualitative explanation to describe the transport properties of polycrystalline La_2/3Ca_1/3MnO₃ thick films, we achieved a good fit of the temperature dependence of the resistance R(T). Depending on the sample, we have observed different metal–insulator (MI) transitions while the magnetic behavior is always similar. Small regions of depleted T_c adjacent to the grain boundary could have an important resistance contribution without affecting the magnetic properties in an appreciable manner. In this work, we achieve a quantitative explanation for the different transport behaviors that we have observed experimentally.     

Superconductivity and crystallographic properties of La2 − xMxCuO4 − δ (M = Na, K)

Superconductivity and crystallographic properties of La_{2 − x}M_xCuO_{4 − δ} (M = Na, K) are studied. In the La_{2 − x}M_xCuO_{4 − δ} system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La_{2 − x}Sr_xCuO_{4 − δ} system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La_{2 − x}K_xCuO_{4 − δ} system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that T_c(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K₂NiF₄ structure at x ≈ 0.3 and only tetragonal samples show superconductivity.     

Spin-polarized tunneling in nanostructured La2/3Sr1/3MnO3

We present magnetic and transport properties of nanocrystalline La_2/3Sr_1/3MnO₃ powders prepared by a gel-combustion method using citric acid as the fuel. The coercive magnetic field H_c is significantly different to the field H_c^* for which the magnetoresistance (MR) is maximum. The MR at low fields (LFMR) exhibits a power-law dependence with magnetization, MR∝Mⁿ, with n=2.5–3.3 for temperatures ranging from 5 to 200 K. The results are discussed in terms of a distribution of particle size in our sample.     

The static and dynamic lattice effects in La1−xCaxMnO3

The structural and magnetic behavior of the perovskite insulator La_0.9Ca_0.1MnO₃ were studied as a function of temperature from 15 to 300 K by neutron powder diffraction. Although this compound shows an anomalous response of the lattice parameters around T_c (150 K), the behavior of the oxygen/manganese Debye-Waller factors is in clear contrast to its “colossal magnetoresistance” (CMR) counterpart La_0.65Ca_0.35MnO₃. We speculate that the difference is intimately associated with the metal-insulator transition in the latter compound.     

Effect of Sn doping on the magnetic and transport properties of LaMnO3+δ

We report the influence of the Sn doping on the magnetotransport properties of the LaMnO_3+δ perovskite. Two series of samples with nominal LaSn_xMn_1−xO_3+δ (I series) and La_{(1−x)/(1+x)}Sn_xMn_1−xO_3+δ (II series) compositions (x=0, 0.025, 0.05 and 0.10) were prepared at T_s=750°C. The M(T) data under 0.01 and 0.5 T for the I series reveal a depressed magnetization as the Sn content increases suggesting the presence of magnetic clusters with a superparamagnetic behavior. Resistivity measurements indicate an insulator material for all Sn content independently of the applied magnetic field. On the contrary, for the II series the M(T) and M(H) data reveal FM behavior and an improvement of the magnetization as Sn increases. These samples show magnetoresistance. The magnetotransport properties are discussed in terms of the presence of A-site cation vacancies.     

Influence of the Tl- and Hg-content on magnetic and transport properties of the Pb, Sr-doped Tl-1223 and Hg-1223 superconductors

The effects of the thallium and mercury content x on the as-sintered and post annealed samples of M_xPb_0.4Sr_1.6Ba_0.4Ca₂Cu₃O_8+δ {M: Tl (0.32≤x≤0.74) or Hg (0.18≤x≤0.68)} have been studied by magnetization and transport measurements. For Tl-1223 we have found the optimum Tl doping level to be x=0.53 regarding the grain properties, the content of superconducting phase, the first penetration field H_pl^wl, the transport (J_c^tr), magnetic intergrain (J_c^M) and intragrain (J_c^g) critical current densities. The critical temperature T_c of the as-sintered Tl-1223 sample decreased with increasing Tl content. Post-annealing in oxygen improved the T_c for Tl contents of x≥0.53 and had generally positive effects on the critical current densities. The intergrain properties of the Hg-1223 samples were much worse than those of the Tl-based superconductors.     

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.     

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.     

Chemical pressure effect on the superconductor MgCNi3

In order to investigate the positive and negative pressure effects on superconducting properties for MgCNi₃, chemical pressure was applied by means of Zn-doping to Mg site (Mg_1−xZn_xCNi₃) and by substituting Mg with Cd (CdCNi₃). The lattice constant decreases (increases) with increasing Zn (Cd) content. In the magnetic measurements, superconducting transition temperature (T_c) is decreasing with increasing Zn content and disappears at x > 0.3. While for CdCNi₃, T_c also decreases down to 3.4 K. The result seems not to be fully understandable in the case of CdCNi₃ since T_c seems to rise owing to the increase of density of states at Fermi energy caused by lattice expansion.     

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.     

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(La_(1-x)Eu_x)_(4/3)Sr_(5/3)Mn_2O_7(x=0,0.15)的磁性和电性研究

通过传统固相反应法制备了钙钛矿锰氧化物(La1-xEux)4/3Sr5/3Mn2O7(x=0,0.15)多晶样品,并且对其磁性和电性进行了研究.磁性测量表明:随着温度的降低,样品经历了一个复杂的转变过程,在温度为T*时经历二维短程铁磁有序转变,在温度为TC时进入三维长程铁磁态.随着Eu的掺杂,T*和TC减小,并且样品(La0.85Eu0.15)4/3Sr5/3Mn2O7在低温区表现出自旋玻璃行为.电性质测量表明:在母体La4/3Sr5/3Mn2O7中La位掺杂Eu后电阻率明显变大,金属绝缘转变温度TMI降低,磁电阻峰值增大.这些影响归因于较小的Eu3+离子替代La3+离子导致平均离子半径减小,晶格发生畸变.此外,较小的Eu3+离子优先占据层间岩盐层的R-site,使La3+,Sr3+,Eu3+离子在(La0.85Eu0.15)4/3Sr5/3Mn2O7中的分布更加有序,所以x=0.15的样品的ρ-T曲线只有一个峰.     

Hole distribution and madelung energy in the high-Tc oxide superconductors

Evidence is presented that in high-T_c superconductors the electrostatic interaction as expressed by the Madelung energy is the key factor in determining the charge distribution and in particular the distribution of the holes between the CuO₂-planes and other structural subunits. Specifically, this explains the phase diagram (T_c as a function of oxygen content and oxidizing power) of cation-substituted YBa₂Cu₃O_6+y and the oxygen ordering in YBa₂Cu₃O_6.5. Moreover, we argue that the dependence of T_c on strontium content in La_2−xSr_xCuO₄ is similarly due to the fact that holes go into the (La,Sr)O-sheets as well as into the CuO₂-planes.     

Magnetic anisotropy and superconductivity in a model for high-Tc copper oxides

The phase diagram for the CuO₂-based superconductors is found to be consistent with an extended Hubbard Hamiltonian with competing positive-and negative-U interactions on a 2D lattice where sites are plaquettes formed by clusters of Cu and O atoms. The negative-U effective interactions are implied by the XY anisotropy in the Cu-Cu spin couplings and local hole pairing corresponds to vortex-antivortex spin configurations. The phase progression observed with the variation of dopant fraction x can be obtained via gradual implementation of canonical transformation that maps the properties of the positive-U Hubbard model at half-filling into those of the negative-U model away from half-filling. In the strong-coupling limit this process is described in terms of percolation-driven dilute magnetism for both spins (U>0) and pseudospins (U−1_x=ξ⁻¹_o+η⁻¹_x for x→O as seen in La_2-xSr_xCuO₄. (ii) An x-dependent reduction of spin fluctuations at low temperatures that conforms with NMR studies of La_2-xSr_xCuO₄. And, (iii) a reduced superconducting transition locus T_c(x)/T_cmax in agreement with the universal shape and location revealed by analysis of experimental data.     

Disruption of antiferromagnetic order in Zn-doped La2CuO4

The magnetic phase diagram of La₂(Cu_1−xZn_x)O₄ has been determined from zero-field muon-spin-rotation (ZF-μSR) data taken at LAMPF for 0 ≤ x ≤ 0.10. Antiferromagnetic onset temperatures follow T_N(x) from susceptibility measurements on the same samples. However, the order becomes long range, as evidenced by a well-defined internal magnetic field, only at temperatures well below T_N. Extrapolation of our results yields T_N → 0 K at x = 0.11 for a single (Cu_1−xZn_x)O₂ plane, and comparison with YBa₂(Cu_1−xZn_x)₃O₆ implies identical disruption of magnetism by Zn doping in the single- and double-plane systems.     

Effect of Hg addition on synthesis of Bi-based superconductors

We have investigated the effect of Hg addition on the superconducting properties of BiSrCaCuO system. Polycrystalline samples with nominal composition Bi₂Sr_2−xHg_xCa₁Cu₂O_y and Bi₂Sr_2−xHg_xCa₂Cu₃O_y (x=0.3) were synthesized and used to investigate the phase evolution by XRD, superconducting behaviour by R–T measurement and the structural grain boundary effects by SEM. From these measurements, it has been noticed that the phases obtained with both types of compositions are the same as Bi2212 but the T_c values are different. With additional annealing, T_{c zero} values were raised from 60 to 72 K in Bi₂Sr_2−xHg_xCa₁Cu₂O_y and 64 to 92 K in Bi₂Sr_2−xHg_xCa₂Cu₃O_y. Also, an improved grain boundary linkage has been observed by SEM for the 92 K sample.     

Normal state magnetoresistance in stable magnetic fields up to 20 T in single crystalline La1.76Sr0.24CuO4

The in-plane normal state resistance R_n(T, H) in an overdoped La_2−xSr_xCuO₄ crystal (x = 0.24) has been measured in magnetic fields up to 20 T parallel to the c-axis. The R_n(T) curves in constant fields show a quadratic behavior in a wide range of temperature above T_c(H). Some characteristic features in R_n(H) are observed. In the low-field region R_n(H) increases with increasing H, reaches a maximum and then decreases with further increasing H. Possible origins for the observed unusual R_n(T, H) behavior are discussed.     

Reduced s-d model with antiferromagnetically coupled impurity spins

An antiferromagnetic equivalent-neighbour Heisenberg interaction H_i between impurity spins is added to the reduced s-d Hamiltonian H_r previously introduced by simplifying the Kondo s-d exchange Hamiltonian H_K. Asymptotic mean-field theory is developed for H_r + H_i, in the presence and absence of external magnetic field, and applied to (La_1−xCe_x)Al₂ alloys. Specific heat c_i(T) and zero-field susceptibility χ_i(0,T) curves for (La_1−xCe_x)Al₂ are depicted. The coupling constants of H_r + H_i and conduction bandwidth are adjusted so that T_c temperatures for x = 0.2, 0.1 are equal to the experimental values. c_i(T) exhibits a jump at T_c and is decreasing for T < T_c. χ_i(0,T) has a first order pole at T_c which corresponds to the maximum of experimental susceptibility and χ_i(0,0) > 0. These results improve those obtained earlier on the grounds of H_r theory.