Low-temperature properties of pseudoternary R(Rh1−x Ir x )4B4 (R=Ho and Dy) systems

Pseudoternary Ho(Rh_1–x Ir_x)₄B₄ and Dy(Rh_1–x Ir_x)₄B₄ systems were investigated with respect to the interplay between superconductivity (SC) and long-range magnetic ordering by means of specific heat (C) and upper critical magnetic field (H _c2) measurements. For Ho(Rh_1–x Ir_x)₄B₄ withx0.2 and Dy(Rh_1–x Ir_x)₄B₄ withx0.15, ferromagnetic (FM) ordering, reminiscent of spin-1/2 mean-field theory, was observed. In particular, FM ordering in Ho(Rh_1–x Ir_x)₄B₄ with 0.07x0.2 was found to destroy SC at a second critical temperatureT _c2 below the superconducting transition temperatureT _c, and was accompanied by a spike-shaped feature inC atT _c2. Coexistence of antiferromagnetic (AFM) ordering with SC was observed in both systems for 0.2x0.7. The compounds withx0.5 exhibited Néel temperaturesT _N that were smaller thanT _c and enhanced values ofH _c2 belowT _N. TheT _c andT _N undergo an abrupt decrease and increase, respectively, nearx0.5, and then cross atx0.6, above whichT _N>T _c. For Ho(Rh_1–x Ir_x)₄B₄ with 0.275x0.4, two different AFM transitions were detected in a double-peak feature inC.     

Superconductivity and magnetic interactions in Yb1.2−x Eu x Mo6S8 pseudoternary compounds

The superconducting and magnetic properties of the Yb_1.2–x Eu _x Mo₆S₈ series of compounds have been investigated by means of ac magnetic susceptibility, electrical resistivity, and upper critical magnetic fieldH _c2 measurements. The superconducting transition temperature at ambient pressure is depressed by Eu additions, until superconductivity disappears completely atx0.7. For Eu concentrationsx<0.45,H _c2 is enhanced at low temperatures and exceeds the values for pure Yb_1.2Mo₆S₈. The temperature dependence ofH _c2 for the various Yb_1.2–x Eu _x Mo₆S₈ compounds is analyzed using a multiple pair-breaking theory that includes the compensation of the applied magnetic field by the negative exchange field produced by antiferromagnetic interactions between the conduction electron spins and the Eu²⁺ spins (Jaccarino-Peter effect). Hydrostatic pressure was found to induce superconductivity in all Eu-rich compositions that were not superconducting at ambient pressure.     

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.     

Electronic structure and magnetic properties of the ThxY1−xCo4B solid solution

Detailed theoretical and experimental investigations of the electronic and magnetic properties of the Th_xY_1−xCo₄B compounds have been performed. The neutron powder diffraction shows that by Th for Y substitution, Th atoms occupy preferentially one (1a) of the two Y sites (1a and 1b). The preferential occupation on 1a and 1b crystal sites investigated by theoretical calculations is in good agreement with powder neutron diffraction results. The magnetic properties of the Th_xY_1−xCo₄B compounds are strongly influenced by the Th content x. The Curie temperature of the compound decreases with Th content from 380 K for YCo₄B to 303 K for ThCo₄B. The Th for Y substitution alter the magnetic coupling of the Co 2c atoms, which have the dominant contribution to the magnetization of the system. The magnetic moments behavior in the Th_xY_1−xCo₄B compounds is also related to the preferential occupation of crystallographic 1a and 1b sites by Th and Y atoms, respectively. The agreement between the calculated magnetic moments and the corresponding experimental results is discussed. All theoretical investigations of the electronic and magnetic properties of the system have been done using the Korringa-Kohn-Rostoker (KKR) band structure method in the ferromagnetic state. The substitutional disorder in the system has been accounted for by means of Coherent Potential Approximation (CPA).     

Transition from antiferromagnetism to ferromagnetism in the superconducting mixed ternary system (Sm1−x Er x )Rh4B4

The mixed ternary system (Sm_1–x Er _x )Rh₄B₄ has been investigated by means of measurements of ac electrical resistance in various applied magnetic fields, ac magnetic susceptibility, and specific heat. The low-temperature superconducting and magnetic transition temperature versus Er concentrationx phase diagram reveals the occurrence of a single transition to long-range magnetic order for all concentrations, changing from antiferromagnetism for SmRh₄B₄ to ferromagnetism for ErRh₄B₄ atx 0.2. The upper critical magnetic field versus temperature curves for various Er concentrations exhibit combined features characteristic of both SmRh₄B₄ and ErRh₄B₄, rather than a gradual evolution from one type to the other. A model for such a mixed magnetic system is discussed, and comparisons with other mixed ternary systems are made.Research supported by the U.S. Department of Energy under Contract No. DE-AT03-76ER70227.     

Structural, transport, magnetic, and dielectric properties of La1−x Te x MnO3 (x = 0.10 and 0.15)

In this study, we have investigated the structure, temperature-dependent resistivity, magnetization, and dielectric properties of La_1?x Te _x MnO_3±δ (x = 0.10 and 0.15). X-ray diffraction analysis confirms the rhombohedral crystal symmetry with space group R $ \overline{3} $ c. For both the samples, the temperature dependence of magnetization plots show paramagnetic-to-ferromagnetic phase transition. The Curie temperature (T _c) and magnitude of magnetization increase with the Te concentration. Field-dependent magnetization produces the asymmetric hysteresis loop that has been attributed to the magneto crystalline anisotropy induced by lattice distortion and the rare earth spin coupling at room temperature. Temperature-dependent resistivity plots exhibit metal–insulator transition (MIT) and charge-ordering state. These plots have been fitted using variable range hopping model, and the density of states [N(E_F)] has been estimated. Magnetoresistance is measured as a function of temperature in the field of 1T, 5T, and 8T. The dielectric constant shows an anomaly near MIT. The dielectric constant exhibits a peaking behavior with the applied frequency and the temperature dependence of dielectric constant attains colossal values at high temperatures.     

Formation and properties of amorphous (Fe1−x Nb x ) l B100−l

Amorphous (Fe_1–x Nb _{x l} B_100–l alloys with 0 x 0.15 and 74 T _g, crystallization temperatureT _x, and microhardnessH _v, but to decrease the magnetization and Curie temperatureT _c. The effects of niobium onT _x,H _v, andT _c in iron-based amorphous alloys are similar to those of chromium, manganese, molybdenum, tungsten and vanadium.     

First-principles investigations of structural, elastic, electronic and magnetic properties of Ga1−x Mn x P and In1−x Mn x P

We employ the full-potential linearized augmented plane wave plus local orbital (FP L/APW + lo) method based on the density functional theory (DFT) in order to investigate the structural, elastic, electronic, and magnetic properties of ordered dilute ferromagnetic semiconductors Ga_1?x Mn _x P and In_1?x Mn _x P at (x = 0.25) in the zinc blende phase, using generalized gradient approximation, GGA (PBE). To our knowledge the elastic constants of these compounds have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Results of calculated electronic structures and magnetic properties reveal that both Ga_0.75Mn_0.25P and In_0.75Mn_0.25P have stable ferromagnetic ground state, and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. Also we show the nature of the bonding from the charge spin-densities calculations. The calculated total magnetic moments are 4.0 μ_B per unit cell for both Ga_0.75Mn_0.25P and In_0.75Mn_0.25P, which agree with the Slater–Pauling rule quite well, and we observe that p–d hybridization reduces the local magnetic moment of Mn from its free space charge value and produces smaller local magnetic moments on the nonmagnetic Ga, In and P sites. The values of N ₀α and N ₀β exchange constants confirm the magnetic nature of these compounds. From the robust half-metallicity of Ga_0.75Mn_0.25P and In_0.75Mn_0.25P as a function of lattice constant is also investigated.     

Dielectric properties of the Sr1−x La x Sn1−x Co x O3 system

The possibility of the formation of a solid solution in the Sr_1–x La _x Sn_1–x Co _x O₃ system has been explored. Single-phase solid solution forms in the compositions for x0.10. All single-phase solid solution compositions have a cubic structure similar to SrSnO₃. The dielectric behaviour of these solid solution compositions has been studied as a function of temperature and frequency. The frequency dependence of dielectric constant and dielectric loss in these materials indicates that space charge polarization contributes significantly to their observed dielectric parameters. Microstructural studies show the presence of well-faceted grains. The average grain size in these samples is small.     

Preparation and thermoelectric properties of β-Fe1−x Ru x Si2

-Fe_1–x Ru _x Si₂ solid solution was synthesized by solid state reaction at 1100 °C for 48 h and subsequent annealing at 850 °C for 168 h in an evacuated silica tube. Single phase solid solution was obtained in the composition range 0 x 0.1. The thermal stability range of the -phase is extended to higher temperature region by partial substitution of Ru atom. The thermoelectric properties of Cr or Co-doped -Fe_1–x Ru _x Si₂ strongly depend on the sintering conditions. The samples with optimum thermoelectric properties are obtained by high-pressure sintering at 3 GPa and 800 °C for 1 h. The optimum compositions are found to be Fe_0.92Ru_0.05Cr_0.03Si₂ and Fe_0.92Ru_0.05C_0.03Si₂ for p-type and n-type materials, respectively. The power factors (²) of these materials are higher than that of -FeSi₂ based materials.     

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.     

Hydrothermal synthesis and magnetic properties of CoxFe1−x/CoyLazFe3−y−zO4 composites

A series of iron–cobalt alloy and cobalt–ferrite composites doped with La³⁺ (Co_xFe_1−x/Co_yLa_zFe_3−y−zO₄) in which the Fe–Co alloy has either a bcc or a fcc structure and the oxide is a spinel phase, have been synthesized by using the disproportionation of Fe (OH)₂ and the reduction of Co (II) by Fe⁰ in a concentrated and hot KOH solution. when x ≤ 0.1, the structures of the Fe_xCo_1−x alloy and cobalt–ferrite are fcc structure; and when x ≥ 0.25, the structures of the Fe_xCo_1−x alloy is bcc structure. The fcc structure of alloy is favored for [KOH] close to 9 N, Co(II)/Fe(II) ratios between 0.5 and 0.9 and short reaction time of synthesis. And the bcc structure of the alloy is favored for [KOH] close to 1 N, Co(II)/Fe(II) ratios between 0.1 and 0.5 and long reaction time of synthesis. A low [KOH] favors nucleation leading to octahedral of 1 μm. And [KOH] of 9–12 N favors particle growth. The metal occurs in square particles of 100–150 nm included within the spinel. Powder X-ray diffraction (XRD), thermal gravity analysis (TGA) and different thermal analysis (DTA), scanning electron microscope (SEM), transmission electron micrograph (TEM) and vibrating sample magnetometer (VSM) were employed characterize the crystallite sizes, structure, morphology and magnetic properties of the composites. And the effect of the Co(II)/Fe (II) ratio (0 ≤ Co/Fe ≤ 1), concentration of KOH, reaction time and substitution Fe³⁺ ions by La³⁺ ions on structure, magnetic properties of the composites were investigated.     

Structural, optical, magnetic and electrical properties of Zn1−x Co x O thin films

Despite a considerable effort aiming at elucidating the nature of ferromagnetism in ZnO-based magnetic semiconductor, its origin still remains debatable. Although the observation of above room temperature ferromagnetism has been reported frequently in the literature by magnetometry measurement, so far there has been no report on correlated ferromagnetism in magnetic, optical and electrical measurements. In this paper, we investigate systematically the structural, optical, magnetic and electrical properties of Zn_1−x Co _x O:Al thin films prepared by sputtering with x ranging from 0 to 0.33. We show that correlated ferromagnetism is present only in samples with x > 0.25. In contrast, samples with x < 0.2 exhibit weak ferromagnetism only in magnetometry measurement which is absent in optical and electrical measurements. We demonstrate, by systematic electrical transport studies that carrier localization indeed occurs below 20–50 K for samples with x < 0.2; however, this does not lead to the formation of ferromagnetic phase in these samples with an electron concentration in the range of 6 × 10¹⁹ cm⁻³ ∼1 × 10²⁰ cm⁻³. Detailed structural and optical transmission spectroscopy analyses revealed that the anomalous Hall effect observed in samples with x > 0.25 is due to the formation of secondary phases and Co clusters.     

Dielectric properties of the system Ba1−xLaxTi1−xCo x O3

The dielectric behaviour of compositions withx=0.01, 0.05, 0.10 and 0.20 in the system Ba_1–x La _x Ti-_1-x Co _x O₃ was studied in the temperature range 300–473 K. The compositions withx = 0.01 and 0.05 show a diffuse ferroelectric-paraelectric phase transition, while other compositions do not show this transition in this temperature range. The frequency dependence of dielectric constant and dielectric loss in the samples withx0.05 indicates that spacecharge polarization contributes significantly to their observed dielectric parameters.     

X-ray,transport, magnetic and catalytic studies of the system Cd1–x Cu x FeCrO4

The compounds of the system Cd_1–x Cu_xFeCrO₄, where 0.0 x 1.0, synthesized by the co-precipitation technique have been studied with a view to investigating the cation distribution and the transport and magnetic properties, and to correlating the physical properties of oxidic spinels with their catalytic behaviour in the decomposition of benzyl alcohol. All the compounds of the system were crystallized with the cubic-spinel structure. The activatio-energy values of electronic conduction varied between 0.69 and 0.16 eV. Thermoelectric power measurements indicated a p-type semiconducting nature for all the compounds. Magnetic hysteresis studies indicated that the compounds with x 0.4 were ferrimagnetic. The Curie temperatures increased with increases in the Cu²⁺ -ion concentration. A good correlation could be established between the physical properties and the catalytic behaviour of the system.     

The effect of zinc doping on the structural and magnetic properties of Ni1−x Zn x Fe2O4

Nanoparticles of Ni_1?x Zn _x Fe₂O₄ (x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) were synthesized by the sol–gel auto-combustion method using ethylenediamine tetra acetic acid as a complexion agent. The detailed analysis of X-ray diffraction revealed that the crystalline structure was cubic spinel and by increasing x, it underwent a phase transition from normal to inverse spinel. The crystal lattice constant was increased gradually with increasing zinc substitution from 0.8339 nm (x = 0.0) to 0.8443 nm (x = 1.0). Also, the average crystallite size, which is determined from Scherrer formula, was about 14–35 nm. The spinel phase formation was further monitored by the FTIR analysis. The vibration sample magnetometer data showed that by increasing Zn doping level up to x = 0.3, the magnetization was increased and it was decreased by further increase in x. This effect was discussed by metal cations distribution into the tetrahedral and octahedral sites. Also, the coercivity was decreased by increasing Zn content due to the decrease of magnetocrystalline anisotropy constant of the samples.     

Dielectric properties of the system Ca1−xYxTi1−xCoxO3(0.00 ⩽ x ⩽ 0.15)

The dielectric behaviour of the compositions withx0.15 in the system Ca_1–x Y _x Ti_1–x Co _x O₃ sintered and cooled in air has been studied. Space-charge polarization which arises due to the presence of chemical inhomogeneities at the micro-level contributes significantly to their dielectric constant. The composition with x=0.05 exhibits temperature and frequency-independent dielectric constant and very small dielectric loss.     

Structural,electrical and magnetic properties of polycrystalline La0.67(Ca1−x Sr x )0.33MnO3 manganites

Polycrystalline La_0.67(Ca_1?x Sr _x )_0.33MnO₃ with different substitution level of strontium element, were synthesized via solid state reaction. Structure of samples was characterized by X-ray diffraction (XRD). XRD patterns reveal that La_0.67Ca_0.33MnO₃ exhibits orthorhombic structure with space group Pnma. Phase transitions from orthorhombic to rhombohedral take place as Ca ions were gradually substituted by Sr ions. The XRD data were further analyzed by Rietveld refinement technique. The data show that Mn–O–Mn bond angle increases as x increases. Microstructures obtained from SEM show that substitution of Sr ions has demoted the grain growth and densification process during sintering. The substitution of Sr ions has greatly influenced the hopping integral of electron via double exchange interaction, thus affecting the electrical properties and magnetic properties as well. The resistivity decreases and the metal–insulator transition temperature (T _p ) shifts to higher temperature as x increases. The magnetoresistance (MR) effect gradually decreases and MR peak shifts to higher temperature as x increases. The magnetization measured at room temperature is found to be increasing as x increases.