Magnetocaloric effect of GdCo2−xAlx compounds

The structure, magnetic property and magnetocaloric effect of GdCo_2−xAl_x (x=0, 0.06, 0.12, 0.18, 0.24, 0.4) compounds have been investigated by X-ray diffraction (XRD) and magnetic measurement techniques. The experimental results show that the GdCo_2−xAl_x (x≤0.4) compounds are single phase with a Laves-phase MgCu₂-type structure. The Curie temperature T_c initially increases, and then decreases with increasing Al content. The maximum value of T_c, 418 K, is reached for the compound with x=0.06. The magnetic entropy change, which is determined from the temperature and field dependence of the magnetization by the Maxwell relation, decreases almost linearly with increasing Al content.     

Magnetic entropy change in GdCo13−xSix intermetallic compounds

The magnetic entropy change in GdCo_13−xSi_x (x=3.8, 4, 4.1, and 4.2) intermetallic compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. It was found that the magnetic ordering temperatures decrease from 60 K at x=3.8 to 28 K for x=4.2. The magnetic entropy change is calculated from isothermal magnetization versus magnetic field at various temperatures using the Maxwell relation. As a result, the maximum magnetic entropy changes of the investigated compounds, at their Curie temperatures, decrease from 11.5 J/kg K for x=4.2 to 6.86 J/kg K for x=3.8 in a field change of 0-3 T, whereas it decreases from 5.13 J/kg K for x=4.2 to 2.60 J/kg K for x=3.8 in a field change of 0-1 T. Moreover, the maximum value of the magnetic entropy change obtained at a higher field for GdCo_13−xSi_x with x=4 (23.75 J/kg K at 5 T) is comparable to that of various types of compounds with a cubic NaZn₁₃-type structure. Finally, the maximum of the magnetic entropy change is found to decrease with increasing Si content.     

Behavior of the irreversibility line in the new superconductor La1.5+xBa1.5+x−yCayCu3Oz

The irreversibility properties of high-T_c superconductors are of major importance for technological applications. For example, a high irreversibility magnetic field is a more desirable quality for a superconductor [1]. The irreversibility line in the H–T plane is constituted by experimental points, which divides the irreversible and reversible behavior of the magnetization. The irreversibility lines for series of La_1.5+xBa_1.5+x−yCa_yCu₃O_z polycrystalline samples with different doping were investigated. The samples were synthesized using the usual solid estate reaction method. Rietveld-type refinement of x-ray diffraction patterns permitted to determine the crystallization of material in a tetragonal structure. Curves of magnetization ZFC–FC for the system La_1.5+xBa_1.5+x−yCa_yCu₃O_z, were measured in magnetic fields of the 10–20,000 Oe, and allowed to obtain the values for the irreversibility and critical temperatures. The data of irreversibility temperature allowed demarcating the irreversibility line, T_irr(H). Two main lines are used for the interpretation of the irreversibility line: one of those which suppose that the vortexes are activated thermally and the other proposes that associated to T_irr a phase transition occurs. The irreversibility line is described by a power law. The obtained results allow concluding that in the system La_1.5+xBa_1.5+x−yCa_yCu₃O_z a characteristic bend of the Almeida–Thouless (AT) tendency is dominant for low fields and a bend Gabay–Toulouse (GT) behavior for high magnetic fields. This feature of the irreversibility line has been reported as a characteristic of granular superconductors and it corroborates the topological effects of vortexes mentioned by several authors 1 and 2.     

The size and space arrangement roles on coercivity of electrodeposited Co1−xCux nanowires

Magnetization curves with various magnetic field orientations and nanowire diameters were measured at room temperature. The measured coercivity as a function of angle (θ) between the field and wire axis reveals that the coercivity decreases with increasing value of θ for various nanowires. Theoretically, based on Monte Carlo simulation we investigated the magnetization reversal modes of the Co_1−xCu_x nanowires and obtained also the θ dependence of the coercivity. Comparing the simulated with the experimental results, we find that the magnetocrystalline anisotropy plays an important role on the magnetic properties of Co_1−xCu_x nanowires, and the magnetization reversal process in the Co_1−xCu_x nanowires could not be understood by the classical uniform rotation mode in the chain-of-sphere model.     

X-ray photoemission and magnetometric studies of valence changes in Ce(Cu1−xNix)4Ga

The compounds Ce(Cu_1−xNi_x)₄Ga crystallize in the hexagonal CaCu₅-type structure for the whole doping range 0≤x≤1. The border compounds CeCu₄Ga and CeNi₄Ga represent a heavy fermion and fluctuating valence systems, respectively. We report on the studies of the valence evolution in Ce(Cu_1−xNi_x)₄Ga employing the X-ray photoemission spectroscopy (XPS) and magnetic susceptibility measurements. The photoemission of the Ce 3d peaks shows a gradual decrease of the occupation of the f states with Ni content. Simultaneously, the hybridization strength and the low temperature magnetic susceptibility are reduced. Within the valence band spectrum a transition from the dominance of the Cu 3d to the dominance of the Ni 3d states is well visible with the traces of the Ce 4f¹ states for up to x=0.5.     

Highly active Ce1−xCuxO2 nanocomposite catalysts for the low temperature oxidation of CO

A series of Ce_1−xCu_xO₂ nanocomposite catalysts with various copper contents were synthesized by a simple hydrothermal method at low temperature without any surfactants, using mixed solutions of Cu(II) and Ce(III) nitrates as metal sources. These bimetal oxide nanocomposites were characterized by means of XRD, TEM, HRTEM, EDS, N₂ adsorption, H₂-TPR and XPS. The influence of Cu loading (5-25 mol%) and calcination temperature on the surface area, particle size and catalytic behavior of the nanocomposites have been discussed. The catalytic activity of Ce_1−xCu_xO₂ nanocomposites was investigated using the test of CO oxidation reaction. The optimized performance was achieved for the Ce_0.80Cu_0.20O₂ nanocomposite catalyst, which exhibited superior reaction rate of 11.2 × 10⁻⁴ mmol g⁻¹ s⁻¹ and high turnover frequency of 7.53 × 10⁻² s⁻¹ (1% CO balanced with air at a rate of 40 mL min⁻¹, at 90 °C). No obvious deactivation was observed after six times of catalytic reactions for Ce_0.80Cu_0.20O₂ nanocomposite catalyst.     

Synthesis and magnetic properties of ferrites spinels MgxCu1−xFe2O4

Polycrystalline Mg_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and magnetic measurements.Using mean field theory and high-temperature series expansions (HTSE), extrapolated with the padé approximants method, the magnetic properties of Mg_1−xCu_xFe₂O₄ have been studied. The nearest neighbor super-exchange interactions for intra-site and inter-site of the Mg_1−xCu_xFe₂O₄ ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_c is calculated as a function of Mg concentration. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements.     

SIMS direct surface imaging of Cu1−xCrx formation

Cu-Cr alloys, irradiated with a low-energy, high-current electron beam, are analyzed by high-resolution secondary ion mass spectrometry. Mass spectra and images of Cu⁺ and Cr⁺ surface distributions finely reveal the regions enriched in Cu and Cr. For electron beam energies above a threshold value, the formation of a non-equilibrium Cu_1−xCr_x solid solution, extending over sub-micrometer areas is highlighted for the first time. A discussion of the process leading to Cu_1−xCr_x formation is given.     

Synthesis and characterization of Sr(ZnZr)x Fe12−2xO19-PANI composites

Strontium zinc zirconium hexaferrites/polyaniline (Sr(ZnZr)_xFe_12−2xO₁₉-PANI, x=0, 0.5, 1.0) composites were synthesized by oxidative chemical polymerization of aniline in the presence of ammonium peroxydisulfate (APS). The structure and morphology of the product was characterized by FTIR, TGA and SEM. The particle size of the core material was found to be about 250-500 nm. After coating with polyaniline, the particle size of Sr(ZnZr)_0.5Fe₁₁O₁₉-PANI composites grew upto 0.5-1.0 μm. XRD of the ferrites indicated that the structure of the core materials is hexagonal, with lattice constants around 5.886-5.885 Å. It was found that the saturation magnetization (M_S) and coercivity (H_C) for Sr(ZnZr)_xFe_12−2xO₁₉-PANI composites decreased after polyaniline coating. The composite under applied magnetic field, exhibited ferromagnetic hysteretic loops with high saturation magnetization (M_S=18.9-3.8 emu/g) and coercivity (H_C=3850.0-583.91 Oe).     

Superconductivity and magnetism of La2Cu1−xZnxO4+δ chemically oxidized by NaClO

The structural, superconducting and magnetic properties of La₂Cu_1−xZn_xO_4+δ (0≤x≤0.1) chemically oxidized by NaClO at room temperature were studied. All the samples before and after oxidation are single phase with orthorhombic structure, as indicated by their powder X-ray diffraction analysis. The iodometric titration results indicate that Zn-substituted La₂Cu_1−xZn_xO₄ is more favorable for the insertion of the excess oxygen, as compared to the Zn-free La₂CuO₄. The T_c suppression rate resulting from Zn substitution in La₂Cu_1−xZn_xO_4+δ is −12.4 K/%. The effective magnetic moment induced by the non-magnetic Zn ion is the order of one Bohr magneton, which decreases with increasing the Zn concentrations in the range examined. The latter two results are qualitatively well consistent with those obtained in La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping. This reveals that the non-magnetic Zn ions play the same role in both of the La₂Cu_1−xZn_xO_4+δ with the excess oxygen content of about 0.1 and the La_2−xSr_xCu_1−yZn_yO₄ with the Sr optimal doping.     

Non-Fermi-liquid behavior in UCu4+xAl8−x compounds

We report on experimental studies of the Kondo physics and the development of non-Fermi-liquid scaling in UCu_4+xAl_8−x family. We studied 7 different compounds with compositions between x=0 and 2. We measured electrical transport (down to 65 mK) and thermoelectric power (down to 1.8 K) as a function of temperature, hydrostatic pressure, and/or magnetic field.Compounds with Cu content below x=1.25 exhibit long-range antiferromagnetic order at low temperatures. Magnetic order is suppressed with increasing Cu content and our data indicate a possible quantum critical point at x_cr≈1.15. For compounds with higher Cu content, non-Fermi-liquid behavior is observed. Non-Fermi-liquid scaling is inferred from electrical resistivity results for the x=1.25 and 1.5 compounds. For compounds with even higher Cu content, a sharp kink occurs in the resistivity data at low temperatures, and this may be indicative of another quantum critical point that occurs at higher Cu compositions.For the magnetically ordered compounds, hydrostatic pressure is found to increase the Néel temperature, which can be understood in terms of the Kondo physics. For the non-magnetic compounds, application of a magnetic field promotes a tendency toward Fermi-liquid behavior. Thermoelectric power was analyzed using a two-band Lorentzian model, and the results indicate one fairly narrow band (10 meV and below) and a second broad band (around hundred meV). The results imply that there are two relevant energy scales that need to be considered for the physics in this family of compounds.     

Ab initio study of 5d-shells Ir substitution for Fe-based SmOFe1−xIrxAs

The lattice and electronic properties for 5d-shells Ir substituted Fe-based superconductor SmOFe_1−xIr_xAs (x=0,0.2,0.25,0.3) are investigated based on the density functional theory (DFT) with a spin generalized gradient approximation SGGA+U method. The electronic density of states (DOS) of SmOFe_1−xIr_xAs is studied and well compared with the results of experimental X-ray photoemission spectroscopy (XPS). The calculation indicated that iridium substitution at the Fe site induced a modification of the FeAs₄ tetrahedron and suppressed the magnetic ordering corresponding to the Fe-3d, which may be the main cause of inducing superconductivity in Ir-doped SmOFeAs system.     

High Bs nanocrystalline Fe84−x−yCuxNbySi4B12 alloys (x=0.0-1.4, y=0.0-2.5)

In this study, Cu and Nb content dependences of magnetic properties for annealed Fe_84−x−yCu_xNb_ySi₄B₁₂ alloy ribbons fabricated by melt spinning were investigated. In Fe_83−xCu_xNb₁Si₄B₁₂ alloy systems, the coercivity H_c markedly decreases with increasing x and exhibits a minimum at around x=1.0-1.2, while the saturation magnetic flux density B_s shows a slight variation. In Fe_83−yCu₁Nb_ySi₄B₁₂ alloy systems, H_c markedly decreases at around y=0.5, while B_s shows a monotonic decrease. Fe₈₂Cu₁Nb₁Si₄B₁₂ nanocrystalline alloy ribbons exhibit a high B_s of 1.78 T and a low H_c of 3.2 A/m. The core losses of the present alloys at 1.0 T at 400 Hz, P_10/400, and at 1.0 T at 1 kHz, P_10/1k, are 1.3 and 4.4 W/kg, respectively.     

Critical behaviour and magnetic properties of A-B spinel ZnxCu1−xFe2O4

Polycrystalline Zn_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using a solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and Mössbauer and magnetic measurements. These results have been compared to a more general theoretical study, on Zn_xCu_1−xFe₂O₄, based on mean field theory and high-temperature series expansions (HTSE), and extrapolated with the Padé approximant method. The nearest neighbour super-exchange interactions for the intra-site and the inter-site of Zn_xCu_1−xFe₂O₄ spinel ferrites, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_C is calculated as a function of Zn concentration. The theoretical results obtained are in good agreement with the experimental results obtained by magnetic measurements.     

Superconductivity coexisting with ferromagnetism in mixed-valence Mn doped La1.85−(4/3)xSr0.15+(4/3)xCu1−xMnxO4

The effect of Mn substitution for Cu in mixed-valence Mn doped La_{1.85−(4/3)x}Sr_0.15+(4/3)xCu_1−xMn_xO₄ (x=0.06) has been investigated by electric resistivity, magnetization and electron spin resonance experiments. Coexistence of superconductivity and ferromagnetism was observed.     

Magnetotransport properties and magnetocaloric effect in La0.67Ca0.33Mn1−xTMxO3 (TM=Cu,Zn) perovskite manganites

The magnetic and transport properties of the perovskites La_0.67Ca_0.33Mn_1-xTM_xO₃ were found to be sufficiently changed with the substitution of Mn-sites by other 3d transition-metal cations (TM=Cu,Zn; x=0.15). The values of T_C, T_M−I, and T_CMR were surveyed when Mn was replaced by Cu and Zn. The magnetic field induced resistivity and magnetic entropy change of these samples showed abrupt changes near T_C (194.2 and 201.5 K for Cu and Zn-doped case respectively) and attained the highest values among the doped cases (up to 20% Cu). The maximum values (obtained at H=4 kOe) of magnetoresistance ratio (CMR) were 27.8%, and 24.5% and of magnetic entropy change (−ΔS_M) were 3.9 and 3.2 J/kg K for Cu and Zn-doped, respectively.     

Electronic structure and optical properties of TbNi5−xCux

In this paper we present theoretical investigation of optical conductivity for intermetallic TbNi_5−xCu_x series. Within the framework of LSDA+U calculations, electronic structure for x=0, 1, 2 is calculated and additionally optical conductivity is obtained. Disorder effects of Cu for Ni substitution on a level of LSDA+U densities of states (DOS) are taken into account via averaging over all possible Cu ion positions in the unit cell for given doping level x. Gradual smoothing of optical conductivity structure at 2 eV together with simultaneous intensity growth at 4 eV corresponds to increase of Cu and decrease of Ni content.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Magnetocaloric effect in high Ni content Ni52Mn48−xInx alloys under low field change

Ni-rich Heusler alloys Ni₅₂Mn_48−xIn_x (x=15.5, 16 and 16.5) were prepared by the arc melting method. X-ray diffraction analysis revealed that the martensite has orthorhombic structure (S.G. Pmm2) at room temperature. The only alloy with x=15.5 has structural transmission from martensite to austenite without any magnetic transmission. The temperature dependence and the field dependence of the magnetization measurement indicated that the magnetization increased with the decreasing of the concerntration of Mn. The lesser the Mn atoms located in the In atom sites, the weaker the total AFM interaction in the system. Giant entropy changes ΔS_M(T, H) were found in Ni₅₂Mn_48−xIn_x alloys with the maximum ΔS_M value of 22.3 J kg K for the sample with x=16.5 at 270 K under the magnetic field change of 1.5 T.     

Room temperature ferromagnetism behavior of Sn1−xMnxO2 powders

In this paper, we have investigated Mn-doped SnO₂ powder samples prepared by solid-state reaction method. X-ray diffraction showed a single phase polycrystalline rutile structure. The atomic content of Mn ranged from ∼0.8 to 5 at%. Room temperature M-H loops showed a ferromagnetic behavior for all samples. The ferromagnetic Sn_0.987Mn_0.013O₂ showed a coercivity H_c=545 Oe, which is among the highest reported for dilute magnetic semiconductors. The magnetic moment per Mn atom was estimated to be about 2.54 μ_B of the Sn_0.9921Mn_0.0079O₂ sample. The average magnetic moment per Mn atom sharply decreases with increasing Mn content, while the effective fraction of the Mn ions contributing to the magnetization decreases. The magnetic properties of the Sn_1−xMn_xO₂ are discussed based on the competition between the antiferromagnetic superexchange coupling and the F-center exchange coupling mechanism, in which both oxygen vacancies and magnetic ions are involved.