Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.     

Structural,electrical and magnetic properties of copper-cadmium ferrites prepared from metal oxalates

Seven samples of the system Cu_1–xCd_xFe₂O₄ were prepared by thermal decomposition of mixed metal oxalates with x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0. The formation of the ferrispinel was studied by XRD, FT-IR, Mössbauer spectroscopy, electrical conductivity and magnetic susceptibility measurements. XRD showed single cubic spinel phase for all the samples except for CuFe₂O₄ sample which shows a tetragonal structure. An increase in the lattice parameter of the ferrispinel was observed with increasing the cadmium content. The temperature variation of ac conductivity showed four definite regions with three transitions which attribute to the change in the conduction mechanism with increasing temperature. Magnetic properties of the sample with x 0.5 showed a decrease in the effective magnetic moment with the increase in the cadmium content which means that the ferromagnetics are widely separated and enclosed by non-magnetic cadmium ions. A well defined hyperfine Zeeman spectrum is observed for samples with x 0.3 at room temperature and resolved into two sextets corresponding to the octahedral and tetrahedral sites. The propable ionic configuration of the system proposed is (Cd_xCu_yFe_1–x–y)[Cu_1–x–yFe_1+x+y]O₄.     

Effects of Non-Magnetic Impurities on Cu-Spin Dynamics and Superconductivity in La2−x Sr x Cu1−y Zn y O4 Around x = 0.115

Muon-spin-relaxation (SR) and magnetic susceptibility measurements have been carried out in La_2–x Sr _x Cu_1–y Zn _y O₄ with x=0.13 changing y finely up to 0.10, with the aim at clarifying effects of the non-magnetic impurity Zn on the Cu-spin dynamics and superconductivity. It has been found that the y dependence of the volume fraction of the superconducting region estimated from susceptibility measurements highly correlates with that of the magnetically ordered region estimated from the SR results. The rapid decrease in the superconducting region and the rapid increase in the magnetically ordered region by the doping of a small amount of Zn can be interpreted as follows; Zn pins the dynamical spin correlation or the dynamical stripe correlations and hence the superconductivity is destroyed around itself.     

Electrical properties of BaSnO3 in substitution of antimony for tin and lanthanum for barium

Polycrystalline materials of BaSn_1–x Sb _x O_3– and Ba_1–y La _y SnO_3– were prepared. Substitutional solubilities of antimony for tin and lanthanum for barium, respectively, in BaSnO₃ were obtained to be x=0.18 for BaSn_1–x Sb _x O_3– and y<0.052 for Ba_1-y La _y SnO_3–. The X-ray photoemission spectroscopy measurements showed the valence of antimony and tin is mixed in our samples of BaSn_1–x Sb _x O_3–. At lower temperature, magnetic susceptibilities of BaSn_1–x Sb _x O_3– and Ba_1–y La _y SnO_3– satisfy the Curie law, indicating the existence of non-interacting localized electrons at the Sn⁴⁺ site, and forming a Sn⁴⁺+e^– state in these systems. By substitution of antimony and lanthanum in BaSnO₃, the conductive properties are semiconductor-like. To explain this conductive behaviour, three types of mechanism were taken into consideration.     

Alloying (In,Mn)As and (Ga,Mn)As: Ferromagnetic (In,Ga,Mn)As Lattice-Matched to InP

An effect of alloying two ferromagnetic semiconductors (In,Mn)As and (Ga,Mn)As on the ferromagnetic properties of resultant (In,Ga,Mn)As alloys is reported. For conditions close to lattice-matching to InP substrates, y = 0.53 in (In _y Ga_1–y )_1–x Mn _x As, ferromagnetism up to Curie temperatures T _C = 100–110 K could be achieved for a Mn composition x = 0.13. Trends in the Curie temperature in (In,Ga,Mn)As are compared with (Ga,Mn)As and (In,Mn)As as a function of Mn content. Hole concentrations determined from magnetotransport, taking into account the anomalous Hall contribution to Hall resistance, gives p/Mn = 0.03 ratio to Mn composition in metallic case for x = 0.13. We mention the possible role of chemical ordering (short range) of Mn impurity atoms on hole concentration and, consequently, for the ferromagnetic properties.     

Phase Relations in the Systems Ln1 – x Ba x CoO3 – δ(0 < x ≤ 0.66; Ln = Nd, Sm)

Data are presented on the structural and magnetic properties of Ln_{1 – x} Ba _x CoO_{3 –} (Ln = Nd, Sm; 0 < x 0.66) samples slowly cooled in air after synthesis. The composition stability limits of the intermediate phases Ln_{1 – y} Ba_{1 + y} Co₂O_{5 +} and solid solutions are determined. The magnetic phase diagram of the Nd_{1 – x} Ba _x CoO_{3 –} system is constructed.     

Synthesis and thermoelectric properties of filled skutterudite compounds CeyFexCo4?xSb12 by solid state reaction

Filled skutterudite compounds, Ce _y Fe _x Co_4–x Sb₁₂(x= 0–1.0, y= 0–0.15), were synthesized by solid state reaction using Co, Fe, Sb and CeCl₃· nH₂O powders as source materials. The lattice constants of Ce _y Fe _x Co_4–x Sb₁₂increased with increasing substitution of Fe at Co sites and with increasing Ce filling fraction in the Sb-icosahedron voids. All specimens showed p-type conduction. Carrier concentration (p) and electrical conductivity () increased with increasing substitution of Fe at Co sites and decreased with increasing Ce filling fraction. The Seebeck coefficient () decreased with increasing Fe content and increased with increasing Ce filling fraction. Thermal conductivity () decreased with increasing substitution of Fe at Co sites and with increasing Ce filling fraction. A maximum ZTvalue of 0.8 was obtained for Ce_0.12Fe_0.71Co_3.29Sb₁₂at 750 K.     

The nonstoichiometry and physical properties of the Nd1−x Ca1+x FeO4−y system

The solid solutions of the Nd_1–x Ca_1+x FeO_4–y system for compositions ofx=0.000,0.125, 0.250,0.375, and 0.500 are prepared by drip pyrolysis. XRD analysis shows all the solid solutions are tetragonal I_4/mmm. The Fe⁴⁺ ratio to the total Fe ions or value has a maximum for the compositionx=0.375. From the X-ray powder diffraction analysis and the Mössbauer spectroscopy, the distortion and symmetry change of oxygen octahedra of Fe ions are observed. The structural change of oxygen octahedra of Fe ions strongly affects the physical properties. The solid solution whenx=0.000 shows a weak ferromagnetic behaviour due to the spin canting of the distorted octahedra. The other solid solutions withx=0.125, 0.250, 0.375, and 0.500 show a paramagnetic behaviour over room temperature. The decrease of the magnetic transition temperature is due to the distortion of oxygen octahedra of Fe ions and the existence of the Fe⁴⁺ ion. The formation site of oxygen vacancies plays an important role in the conductivity of the Nd_1–x Ca_1+x FeO_4–y system. Although the oxygen vacancies in [Nd, Ca]-O layer have little effect on conductivity, the oxygen vacancies in the FeO₂ plane of the perovskite layer act as electron trapping sites and thus increase the activation energy.     

Synthesis of (Sr1 – xCax)1 – yCuO2(0 < x< 0.86, 0 < y < 0.1) at High Pressures and Temperatures

(Sr_{1 – x} Ca _x )_{1 – y} CuO₂(0 <x< 0.86, 0 < y< 0.1) infinite-layer solid solutions were prepared at high pressures and temperatures using low-pressure, orthorhombic Sr_{1 – x} Ca _x CuO₂as a precursor, and their low-temperature magnetic susceptibility was measured.     

μSR Study of Magnetic Order in LTO and LTT Phase of La2−x M xCu1−y Zn y O4 (M=Sr, Ba)

In order to investigate the origin of so-called 1/8 anomaly in La(Sr,Ba)-214, we have performed SR experiments for the low-temperature orthorhombic (LTO) and low-temperature tetragonal (LTT) structure phases in La_2–x Sr _x Cu_1–y Zn _y O₄ (LSCO) and La_2–x Ba _x Cu_1–y Zn _y O₄ (LBCO), and decided the magnetic order temperature T _m where -spin coherent rotation starts. 1% Zn-substitution depresses magnetic order in the LTT phase of LBCO while it induces or enhances magnetic order in the LTO phase of LSCO. With doping Tm rises first and keeps constant around x=1/8 and decreases in both Zn-substituted LSCO and LBCO. This implies that doped holes are never uniformly distributed on the CuO₂ plane, suggesting the segregation of spin and hole around x=1/8.     

Magnetic susceptibility and specific heat of Pr2?xCexCuO4?y

We report here the measurements of magnetic susceptibility of Pr_2–xCe_xCuO_4–y (x=0, 0.05 and 0.15) from 2K to 300K and the specific heat of Pr_1.85Ce_0.15CuO₄ between 6K and 60K. Fitting of the magnetic susceptibility data to a Curie-Weiss law gave effective magnetic moment (p_eff) values close to the free ion value of 3.58_B for Pr³⁺. p_eff was found to decrease with increase in cerium concentration. The site symmetry of Pr³⁺ in Pr_2–xCe_xCuO_4–y (PCCO) host is D_4h and a crystal field (CF) Hamiltonian appropriate to tetragonal symmetry was used to analyze the magnetic susceptibility data. J-mixing of all the thirteen multiplets of Pr³⁺ and intermediate coupled wave functions have been included. A best set of CF parameters obtained not only explains the magnetic susceptibility but also the inelastic neutron scattering spectra (INS) reasonably well. The Schottky component together with Debye and Einstein contributions successfully accounts for the measured specific heat.     

Magnetic scattering of conduction carriers in 3d transition-metal intercalates of M x TiS2 (M=Mn,Fe, Co,and Ni)

Galvanomagnetic measurements on layer-structured 3d transition-metal intercalates of M_xTiS₂ (M = Mn, Fe, Co, and Ni;x0.33) have been made over the temperature range 0.34–20 K. Resistivity minima and negative magneto-resistances are observed for paramagnetic Mn, Fe, and Ni intercalates with low guest concentrations (x0.1), as found in well-known dilute alloys. However, in the case of the weak-ferromagnetic phase of Co_xTiS₂ (0.10x0.33) with the Curie temperatureT _c =120–140 K, such anomalous behaviors are also observed, which is not commonly found in the magnetically ordered phases of various magnetic materials. With these data we have given qualitative discussions on the magnetic scattering process of conduction carriers. Further, some of the experimental results are discussed in connection with the band calculations for M_1/3TiS₂.     

Ab Initio Study of Magnetism in III-V- and II-VI-Based Diluted Magnetic Semiconductors

Electronic structure and magnetic properties of Ga_1–x Mn _x As, Ga_1–x Mn _x N, Zn_1–x M _x O, and Zn_1–x M _x Te (M=V, Cr, Mn, Fe, and Co) diluted magnetic semiconductors (DMS) are calculated by the tight-binding LMTO method in the 64-atom supercell. Calculations are made at several x with varied spatial distribution of dopant atoms and codoping of DMSs. The results show that stability of the ferro- and antiferromagnetic (FM and AFM) states in DMSs strongly correlates with the occupation and energy position of 3d-dopant bands. Adequacy of the double exchange and superexchange mechanisms for explanation of the FM vs. AFM competition is discussed.     

p–T–x Phase Equilibria in the Cd–Zn–Te System

The p–T–x–y phase equilibria in the Cd–Zn–Te system are analyzed. The p–T and T–x–yprojections of the p–T–x–y phase diagram and a T–x–y isobar (for pressures at which Cd_1–x Zn _x Te_{1 ±} solid solutions sublime congruently in terms of Te) are mapped out. The key features of the sublimation behavior of the solid solution are examined. The p–T projection is studied by static vapor pressure measurements at temperatures from 700 to 1300 K and pressures of up to 101.3 kPa. The p–T sections of the phase diagram are constructed for x = 0.05, 0.10, 0.15, 0.25, 0.50, 0.75, 0.90, and 1. The solid solution containing 35 mol % ZnTe is found to phase-separate at 473 K.     

Magnetic field properties of 1 at.% transition metal substituted Nb3Ge

Measurements of the critical magnetic field slope near the superconducting transition temperature T _care reported for samples of Nb_1–x Ge _x , (Nb_0.99Ti_0.01)_1–x Ge _x , and (Nb_0.99Zr_0.01)_1–x Ge _x for 0.15x0.25. The samples investigated were produced by trisputtering onto heated polycrystalline alumina substrates. The critical magnetic field slope near T _cfor substituted samples is comparable to that of Nb₃Ge alone. The highest T _cvalues, the highest resistance ratios, and the narrowest transitions, however, occur for samples with excess transition metal component.Supported by PSC-BHE Research Award Program.     

X-ray,electrical conductivity and infrared studies of the system Zn1−x CO x Mn1−x Fe x CrO4

The system Zn_1–x Co _x Mn_1–x Fe _x CrO₄ is tetragonal in the range 0.0x0.4 and cubic in the range 0.5x1. Electrical resistivity temperature behaviour obeys Wilson's law for all the compounds and thermoelectric coefficient values vary between 135 to 226V K^–1. All compounds exhibit P-type semiconductivity and possess low mobility values (10^–9 cm² V^–1 sec^–1). The infrared spectra show the presence of two strong absorption bands around 500 and 600 cm^–1. The probable ionic configuration for the system is suggested to be Zn _1–x ²⁺ Fe _x ^–y3+ -Co _x ²⁺ [Mn _1–x ³⁺ Fe _y ³⁺ CO _x ^–y2+ Cr³⁺]O₄.     

Synthesis and Physicochemical Properties of La–Ce–Ni–O and Sr–Ce–Ni–O Oxides

La_{2 – x} Ce _x NiO _y and Sr_{2 – x} Ce _x NiO _y materials were prepared, and their properties were studied. Nearly single-phase Sr_{2 – x} Ce _x NiO _y samples (tetragonal K₂NiF₄ structure) could be obtained at x= 0.25 and 0.3. The lattice parameters, weight change, relative length change, and electrical resistivity of Sr_1.7Ce_0.3NiO _y were measured from 20 to 1000°C. The oxygen content of this material, determined by hydrogen reduction and iodometric titration, was found to vary widely, depending on heat-treatment conditions. The room-temperature resistivity of Sr_1.7Ce_0.3NiO _y is (2–5) × 10^–2 cm. In the range 20–450°C, this material exhibits n-type conductivity. Its thermoelectric power varies from –12 V/K at 20°C to –34 V/K at 450°C. The temperature variation of resistivity for Sr_1.7Ce_0.3NiO₄ in the first heating–cooling cycle below 450°C is shown to depend on the thermal history of the sample. The resistivity reaches a maximum between 500 and 800°C. The structural and transport properties of the mixed oxide are shown to be correlated with its oxygen content.     

Phase Equilibria and Structure of Solid Solutions in the La–Co–Fe–O System at 1100°C

Phase equilibria in the La–Co–Fe–O system are studied at 1100°C in air using samples prepared by the citrate, nitrate, and conventional ceramic routes. The stability regions and structures of solid solutions in the La–Co–Fe–O system are determined by x-ray powder diffraction: LaCo_{1 – y} Fe _y O_{3 –} (0 < y 0.25, sp. gr. R c; 0.775 y< 1, sp. gr. Pbnm), Co_{1 – y} Fe _y O (0 < y 0.13, NaCl-type structure, sp. gr. Fm3m), and Fe_{3 – x} Co _x O₄ (0.84 x 1.38, sp. gr. Fd3m). The structural parameters of phase-pure solid solutions are determined by the Rietveld method. The composition dependences of lattice parameters are presented for LaCo_{1 – y} Fe _y O_{3 –} (0 < y 0.25) and Fe_{3 – x} Co _x O₄ (0.84 x 1.38). The 1100°C isotherm of the pseudoternary system La₂O₃–CoO–Fe₂O₃ in air is constructed.     

The Influence of Lithium Halides on the Superconducting Properties of YB2Cu3O7?x

The effects of addition to YBa₂Cu₃O_7–x of lithium halides (YBa₂Cu₃O_7–x (LiF) _y , and YBa₂Cu₃O_7–x (LiCl) _y ) on the structural, electric, magnetic, and transport properties are analyzed. Both structural and superconducting properties depend weakly on the lithium content up to y= 0.10. The critical temperature keeps on a value well above 91 K for a small amount of lithium halide (reaching 93.48K. for y= 0.02 in YBa₂Cu₃O_7–x (LiF) _y and 91.30 K in YBa₂Cu₃O_7–x (LiCl) _y ), but for higher concentration of Li it rapidly decreases (81.68K for y= 0.20). The same behavior is exhibited in the lower intragranular critical field. The intragranular critical current density depends on the magnetic field as a power law:j _cB ^–, with a lithium-concentration-dependent . The voltage–current characteristics follow a law typical for granular superconductors, V(I–I _c(B,T)) ^n(B,T). The dependence of the intergranular critical current, I _c, and of the exponent, n, on temperature, magnetic field, and concentration is analyzed.     

Effect of Ca Substitution on the Superconductivity of La2.5Y0.5CaBa3(Cu0.88Fe0.12)7Oz

The structural and superconducting properties of single-phase La_2.5–y Y_0.5Ca_1+y Ba₃ (Cu_0.88Fe_0.12)₇O _z (LYCaBCuFe) (y= 0.0–1.0) compounds with triple perovskite structure are investigated using X-ray diffraction, resistivity, a.c. susceptibility, and oxygen content measurements. Increasing Ca substitution for La resulted in a decrease in unit cell axes and volume. T _c ^R=0 shows a marginal increase from 31 K to 37 K for y = 0.0–0.21 and thereafter it decreases with increasing y leading to zero T _c ^R=0 at y 0.84. This shows that the suppression of T _c from 80 K to 31 K by Fe doping at x = 0.12 La_2.5Y_0.5CaBa₃(Cu_1–x Fe _x )₇O _z cannot be compensated by appropriate hole doping with Ca in LaYCaBCuFe.