Impact of Pr on structural,superconducting and magnetic properties of Y1−xPrxBaSrCu3O7

We study the structural, superconducting and magnetic properties of the Y_1−xPr_xBaSrCu₃O₇ system with x=0.0, 0.20, 0.25, 0.30, 0.40, 0.50, 0.60, 0.75 and 1.0, from X-ray diffraction, AC and DC magnetic susceptibility measurements. X-ray diffraction results reveal that while Pr substitutes isostructurally until x=0.50 in Y_1−xPr_xBaSrCu₃O₇, for x=0.75 and 1.0 few small intensity un-reacted lines are observed in the spectrum. The orthorhombic distortion decreases with an increase in x. Both x=0.75 and 1.0 samples are tetragonal. The c lattice parameter of the substituted samples increases with x, indicating probable substitution of comparatively bigger ionic radii Pr^3+/4+ ion at Y³⁺ in Y_1−xPr_xBaSrCu₃O₇ system. The AC susceptibility results showed that the transition temperature T_c of the Y_1−xPr_xBaSrCu₃O₇ system decreases monotonically with an increase in x. Both x=0.75 and 1.0 samples do not show a superconducting transition down to 4.2 K. The critical concentration of Pr to completely suppress superconductivity in Y_1−xPr_xBaSrCu₃O₇ is higher (0.75) than that reported (0.55) for Y_1−xPr_xBa₂Cu₃O₇. DC magnetic susceptibility measurements done on PrBaSrCu₃O₇ and PrBa₂Cu₃O₇ samples at 0.5 T revealed that while the Pr ordering temperature seems to be 12 K for the former, the same is 17 K for the latter. These results indicate that the ordering temperature of Pr moments in RE_1−xPr_x : 123-type systems, which decides the Pr^4f hybridisation with the Cu–O conduction band has a direct connection to the suppression of superconductivity. The lower T_N of Pr explains the less destructive effect of the same on the superconductivity of the parent undoped system.     

High-temperature electronic transport properties of (YCa)BaCo4O7 compounds

Y_1−xCa_xBaCo₄O₇ (0.0≤x≤1.0) samples were prepared by the solid-state reaction method and their high-temperature electronic transport properties were investigated in nitrogen and oxygen respectively. Phase structure of Y_1−xCa_xBaCo₄O₇ transforms from hexagonal symmetry for x ≤0.6 samples to orthorhombic symmetry for x≥0.8 samples. In nitrogen, Y_1−xCa_xBaCo₄O₇ samples evolve three kinds of electronic transport behaviors with the increase of Ca content: thermal activation conduction, small polaron hopping conduction, and a possible mixed conduction. Ca doping increases the hole concentration and thus decreases Seebeck coefficients. In oxygen, the temperature dependence of electrical resistivity and Seebeck coefficients of Y_1−xCa_xBaCo₄O₇ samples displays similar change to their respective thermogravimetric curve, showing their electronic transport behavior under the control of their oxygen adsoption/desorption process.     

Magnetic properties of (GdxY1−x)Co2B2 compounds

Magnetic measurements were performed on the (Gd_xY_1−x)Co₂B₂ compounds, in the temperature range 2–800 K and fields up to 70 kOe. YCo₂B₂ is a paramagnet. The (Gd_xY_1−x)Co₂B₂ compounds with x≥0.2 shows a ferromagnetic type ordering. The saturation magnetization at 2 K coincides only with the contribution of gadolinium. The Curie temperatures are nearly linearly dependent on the composition. Above the Curie points, the thermal variations of the magnetic susceptibility can be described as a superposition of a temperature independent term ϰ₀ on a Curie-Weiss behavior. The Curie constants are determined by the contribution of Gd³⁺ ions only. The ϰ₀ values increase when the gadolinium content is greater. The observed properties are discussed in the wider framework of the magnetic behavior of cobalt in GdCo_xB_y compounds.     

Electron spin resonance on GdxMo6Se8 (x = 1.0 and 1.2)

Electron spin resonance of Gd³⁺ in Gd_xMo₆Se₈ (x = 1.0 and 1.2) has been measured in X-band and in K-band between 1.4 and 290 K. The results indicate that the conduction electron - Gd³⁺ spin exchange coupling constant is positive and equal to +0.011 eV, in agreement with previous determinations based on superconductivity and static magnetic susceptibility measurements.     

Synthesis and ionic conduction of C3A-type nasicon Na3+3x−yY1−xSi3−yPyO9

In the solid solutions of Na_3+3x−yY_1−xSi_3−yP_yO₉, new fast Na⁺ -conductors were obtained. The ionic conductivity, σ, and the activation energy, E, of the most conductive material, Na_4.425Y_0.375Si_2.55P_0.45O₉ were 5 × 10⁻³ S/cm at 300°C and 10 kcal/mol, respectively. Those values were strongly dependent upon the composition. With an increase of Na concentration, [Na], σ increased and E decreased. Accommodation of higher [Na] in the structure was madefeasible by decreasing [Y] and increasing [P]. Based on the crystal structure assumed for Na_3+3x−yY_1−xSi_3−yP_yO₉ and on the above results, a conduction model was presented. The structural consideration revealed that a conduction path is formed along the 〈1 1 1 〉 direction, where Y³⁺ ions locate in the way of Na⁺ ions.     

Local-moment formation and metal-nonmetal transition in Ca1−x Y x VO3 and Ca1−x Y x TiO3

Electron-doped metallic states of Ca_1−x Y _x VO₃ and Ca_1−x Y _x TiO₃ change into non-metallic states around x∼0.4 and 0.6, respectively. The residual resistivity in the metallic states increases with increasing effective magnetic moment or coefficient of T ² term of resistivity. The effective moment reaches ∼ 0.5 μ_B/molecule in Ca_1−x Y _x VO₃ and also in Ca_1−x Y _x TiO₃ near the metal-nonmetal phase boundary. In these metallic states. ∼ 10% of 3d atoms seem to have large localized magnetic moments. In electron-doped metallic sample of Ca_1−x Y _x VO₃, the temperature dependence of resistance shows no resistance-minimum. However, weak negative magneto-resistance is observed for the sample with x=0.2 up to 50 Tesla at 4.2 K.     

Phase transformation and magnetic properties of SmCo7−xBx alloys prepared by mechanical alloying

The phase transformation and magnetic properties of SmCo_7−xB_x (x=0, 0.2, 0.5 and 1) alloys prepared by mechanical alloying have been investigated systematically. The coercivities of the alloys without B increase with increasing annealing temperature, as a consequence of complete crystallization of TbCu₇-type phase. The substitution of B for Co is favorable to the formation of Th₂Zn₁₇- and CaCu₅-type phases when annealed at 650°C, accompanied by the enhancement of the coercivities. Increasing the annealing temperature causes the formation of soft magnetic phase Sm₂Co₁₄B in the B-substituted alloys. In the alloys with x=0.5 and 1 annealed at 850°C, the major phase is Sm₂Co₁₄B, which degrades the magnetic properties sharply. A remanence enhancement has been observed in the SmCo_7−xB_x alloys due to the exchange coupling of the nanoscale structure.     

Magnetism and superconductivity in (RE) Ni2B2C: The case of TmNi2B2C

The recently reported coexistence of an oscillatory magnetic order with wave vector Q = 0.241⁻¹ and superconductivity in TmNi₂B₂C is analyzed theoretically. It is shown that the oscillatory magnetic order and superconductivity interact predominantly via the exchange interaction between localized moments (LMs) and conduction electrons, while the electromagnetic interaction between them is negligible. In the coexistence phase of the clean TmNi₂B₂C the quasiparticle spectrum should have a line of zeros at the Fermi surface, giving rise to the power-law behavior of thermodynamic and transport properties. Two scenarios of the origin of the oscillatory magnetic order in TmNi₂B₂C are analyzed: (a) due to superconductivity and (b) independently of superconductivity. Experiments in a magnetic field are proposed in order to choose between them.     

Magnetic order in palladium-based heusler alloys: Part 2: Pd2MnIn1−ySby

Magnetization, susceptibility, X-ray and neutron-diffraction measurements have been on the Heusler alloy series Pd₂MnIn_1?ySb_y, for 0 < y < 1. All the alloys were chemically and magnetically ordered with a moment of ?4.2μ_B located at the Mn sites. Their magnetic structures change from antiferromagnetic fcc type 2 at the In rich end, to antiferromagnetic fcc type 3A and then to ferromagnetism as the Sb content is increased. The results closely parallel those already reported on the isostructural series Pd₂MnIn_1-xSn_x and Pd₂MnSn_1-xSb_x with the changes in magnetic structures o ccurring at the same conduction electron concentrations. The results strongly support the postulate that the exchange interactions in Heusler alloys are essentially dependent upon the conduction electron concentration. The correlation between the data and an indirect double resonance exchange coupling mechanism is discussed.     

Spinglass behaviour of Zn1−xMnxSe and Zn1−xMnxTe semimagnetic semiconductors

The magnetic susceptibility of Zn_1−xMn_xSe (0.02 ≤ × ≤ 0.53) and Zn_1−xMn_xTe (0.07 ≤ × ≤ 0.21) was investigated in the temperature range 10 mK < T < 40 K. A paramagnetic spinglass transition was observed in the whole concentration range. The concentration dependence of the freezing temperature T_f was found to be compatible with a radial dependence of the exchange interaction between manganese ions of the type J(R) ∝ R^−6.8. It appears from the available data that this radial dependence is rather universal for all II–VI wide gap semimagnetic semiconductors. A comparison is also made with other semimagnetic semiconductors and the physical exchange mechanism is discussed.     

Anomalous temperature dependence of the upper critical fields in magnetic-superconducting Y1−xGdxRh4B4

We report that the dominant mechanism governing the upper critical field H_c2 in Y_1?xGd_xRh₄B₄ with x ≥ 0.1 is the magnetic exchange field of Gd spins. The H_c2 vs. temperature curve determines the magnetic structure to be of spin glass for x = 0.1 and ferromagnetic for x ≥ 0.2.     

Austausch und Spinresonanz in den Gadolinium-Pniktiden GdP,GdAs und GdSb

Powder samples of mixed crystals Gd _x Y_1?x P, Gd _x Y_1?x As and Gd _x Y_1?x Sb have been studied by electron-spin-resonance for Gd-concentrations 0.01≦x≦1 between the ordering temperatures and room temperature. All measurements show a single exchange-narrowed and asymmetric absorption line of Lorentzian shape in the centre region. Theg-factors are 1.995±0.008 in agreement with the expected value for a⁸ S _7/2-state of the half-filled 4f-shell. The linewidths are explained by dipolar and exchange interactions. It is shown that the effect of exchange narrowing is proportional to the sum of the absolute values of the exchange parameters. Therefore a new method for studying the magnetic coupling far above the ordering temperature is proposed. The dependence of exchange on changes of the lattice parameters has been evaluated. The observed asymmetry of the resonance line gives a measure for the electric conductivity. In particular the beginning of critical scattering near the ordering temperature is observed.     

Magnetic ordering in Mg−Zn ferrites

The magnetic ordering of a series of magnesium-zinc ferrite, Zn_0.3Mg _x Fe_2.7?x O_4±δ (0.5≤x≤1.1; 0≤δ≤0.2) has been investigated using Mössbauer measurements in the temperature range 295–620 K. The samples were found to be magnetic at room temperature with a hyperfine field at each site which increases with iron content. The Curie temperature was also observed to increase in a similar manner. The slope of this increase forB _hf andT _c is steeper forx≤0.6 thanx≥0.7. It has also been observed that Mg²⁺ substitution by Zn²⁺ in MgFe₂O₄ affects the magnetic ordering and the internal hyperfine field. The Curie temperature decreases by ～200 K andB _hf by ～20%.     

Hyperfine interactions in Sc<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Y<Subscript>x</Subscript>Fe<Subscript>2</Subscript> cubic Laves alloys

Effects of hybridization of 3d bands of iron with 3d bands of scandium and 4d bands of yttrium in Sc_1?xY_xFe₂ cubic Laves alloys (0≤_x≤1) are studied by the nuclear magnetic resonance method. The concentration dependences of the lattice parameters a, saturation magnetization σ, and hyperfine fields at the ⁵⁷Fe, ⁴⁵Sc, and ⁸⁹Y nuclei—as well as the ²⁷Al impurity nuclei, whose atoms substitute iron atoms in the lattices of these alloys—are measured. The “local” and “induced” contributions to hyperfine fields at the ⁵⁷Fe nuclei are separated and the magnetic moments at iron atoms are estimated. It is found that the hybridization effect leads to the formation of magnetic moments at Sc and Y atoms (whose direction is opposite to the direction of the magnetic moment at iron atoms) and is responsible for the ferrimagnetic structure in Sc_1?xY_xFe₂ alloys.     

Theory for strong-coupling superconductivity in transition-metal alloys

Strong-coupling superconductivity in transition-metal alloys of the type A_xB_1?x is treated by applying the coherent-potential approximation to the Eliashberg theory of superconductivity. Expressions are given for the average electron-phonon coupling constant λ_eff, the average Coulomb-pseudopotential μ¹_eff, the superconducting transition temperature T_c, and the order parameter.     

Superconducting and magnetic properties of the oxide superconductors La1.85Sr0.15Cu1−xFexO4

Mössbauer and magnetic measurements have been made in the perovskite superconductors La_1.85Sr_0.15Cu_1−x ⁵⁷Fe_xO₄ for x=0.0025, 0.005, 0.01, 0.05 and 0.10. The superconducting transition temperature is rapidly depressed as x increases, reaching T_c=0 for x≈0.03. Mössbauer data for x=0.05 show behavior below ≈15 K which indicates magnetic ordering. Magnetic susceptibility data give an effective moment of 3–4 μ_B/Fe atom, and a hand-like susceptibility which increases with x.     

Observation of bulk Landau levels in transverse magnetotunneling in AlxGa1-xAs capacitors

Two phenomena are observed for transverse magnetotunneling (B⊥J) from an accumulation layer in n⁻ GaAs-undoped Al_xGa_1−xAs−n⁺ GaAs capacitors. One effect is a strong dependence of tunnel currents, J, at high applied voltage and high current densities, on the angle between J and the magnetic field, B. The second effect is the observation of structure in tunnel currents for applied voltages between 0.15 V and 0.6 V which is interpreted to result from tunneling into Landau levels formed in the n⁺ GaAs electrode.     

Antiferromagnetic spin correlations in palladium-based Pd-Fe, Pd-Fe-Ag, and Pd-Fe-Rh magnetic alloys

The magnetic structure of Pd_1?x Fe_x (x=0.03, 0.06, 0.10, 0.15, and 0.20) alloys is investigated using the method of ⁵⁷Fe-Mössbauer spectroscopy. The distribution functions P(B _hf) of hyperfine magnetic fields have a discrete structure defined by variations of the contribution to B _hf from the magnetic moment of the neighboring Fe atoms. The anomalies of intensities of components of the functions P(B _hf), which increase with the concentration of iron, are indicative of the instability of configurations with a large total spin and of the formation of local spin configurations with the antiferromagnetic orientation of magnetic moments. The probability of formation of such configurations is defined by the competition of the ferromagnetic Fe-Pd exchange interaction with the direct antiferromagnetic exchange between the nearest neighboring atoms of Fe. An Ag or Rh impurity effectively induces the process of spin flipping, which explains the anomalously strong effect of impurities on the magnetic ordering temperature. The results confirm the presence in Pd-Fe alloys of perturbations of long-range ferromagnetic order revealed by neutron diffraction.     

Crystal structure and magnetic properties of pseudoternary TbRh2−xMxSi2(M = Ru,Ir) compounds

The structural and magnetic characteristics of the pseudoternary TbRh_2−xM_xSi₂(M = Ru, Ir) compounds were studied. The compounds crystallize in the tetragonal ThCr₂Si₂-type structure. The magnetic data were collected in the temperature range 70–300 K. Their magnetic susceptibilities satisfy the Curie-Weiss law in the temperatures higher than 130 K. The magnetic moment of the rare earth atom is larger than of the free Tb³⁺ ion. A modified RKKY theory with included interaction between the conduction electrons was applied to explain the variation of properties of the compounds.     

Magnetic moments in heusler alloys

The Heusler alloys are a series of local moment ferromagnets of composition X₂MnY, with a magnetization of ～4mu_B per Mn atom. Magnetic measurements on the alloy series Ni₂Mn_xT_1?xSn, where T is Ti, V or Cr, indicate that the T site moment changes from ?mu_B (for Ti), through zero (for V), to ⁺mu_B (for CR). A simple physical interpretation is proposed for this sign change in the present work. This approach facilities the interpretation of several other features observed in Heusler alloys.