Influence of Ni doping on the properties of perovskite molybdates SrMo1−xNixO3 (0.02≤x≤0.08)

Systematic studies of the structural, transport, magnetic and specific heat behavior have been performed on the perovskite molybdates SrMo_1−xNi_xO₃ (0.02≤x≤0.08). Ni doping at the Mo site does not change the structure of all samples, but increases the lattice parameter a monotonically. All of the doped samples keep their metallic behavior. The magnetic properties keep a Pauli paramagnetism in the high-temperature region, but have a ferromagnetic (FM) transition at about 50 K. The resistivity, ρ, and magnetic susceptibility, χ, increase, while the electronic specific heat coefficient, γ_e, decreases monotonically with the increase of Ni doping content, x. The electronic transport of all samples shows a T² dependence in the low-temperature region and a T dependence in the high-temperature region, respectively. The temperature dependence of the specific heat can be well described by the formula C_p(T)/T=γ_e+β_pT² in the low-temperature range. These behaviors can be explained by the competition between the decrease in the density of states (DOS) at the Fermi level and the electron localization due to the disorder effect induced by the random distribution of Ni at the Mo site in the samples.     

Influence of Pr-doping on structural, electronic transport, magnetic properties of perovskite molybdates Sr1−xPrxMoO3 (0≤x≤0.15)

The effect of Pr-doping on structural, electronic transport, magnetic properties in perovskite molybdates Sr_1−xPr_xMoO₃ (0≤x≤0.15) has been investigated. The Pr-doping at Sr-site does not change the space group of the samples, but decreases the lattice parameter a. The magnitude of resistivity ρ increases initially (x≤0.08) and then decreases with further increasing Pr-doping level x and ρ(T) behaves as T² and T dependence in the low-temperature range blow T* and high-temperature range of 150 K<T<350 K, related to the electron-electron (e-e) and electron-phonon (e-ph) scattering, respectively. The magnetic susceptibility χ value of the sample increases with increasing x and the χ(T) curve for all samples can be well described by the model of exchange-enhanced paramagnetism. The specific heat magnitude in the low-temperature region increases with increasing Pr-doping level. The specific heat value agrees with the classical Dulong-Petit phonon specific heat, C_cl=3k_BrN_A=124.7 J/mol K in the high-temperature region and the temperature dependence of the specific heat can be well described by the formula C_p(T)/T=γ_e+β_pT² in the low-temperature range. These behaviors can be explained by the competition between the increase in the density of state (DOS) at Fermi energy level and the disorder effect due to Pr-doping.     

The correlation between structure and magnetic properties in the manganites La0.7Ca0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Ca_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show an orthorhombic structure (O′-Pbnm) at room temperature. It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase in the Te content. All samples exhibit an insulator-metal (I-M) transition and the resistivity increases with the increase in the Te-doping level. Additionally, the Curie temperature T_c decreases and the transition becomes broader with increasing Te-doping level, in contrast, the magnetization of Te-doping samples at low temperatures decrease with increasing x as x≤0.10 and then increase with further increasing x to 0.15. The results are discussed in terms of Jahn-Teller (JT) vibrational anisotropy Q₃/Q₂ and the opening of the new DE channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ ions for Ca²⁺ ions.     

Magnetoresistance in La1−xCaxMnO3 (0≤x<0.4)

The magnetic and transport properties of La_1−xCa_xMnO₃ (0≤x<0.4) have been systematically studied. The magnetoresistance (MR) maximum appears at x=0.2-0.25 and the temperature dependence of MR for x>0.25 shows a much broader profile than that of samples for x=0.2-0.25. Based on a scenario in which there is a short-range charge ordering (CO) state coexisting in the ferromagnetic state matrix for x>0.25, and the least or even no short-range CO state exists in samples for x=0.2-0.25, the above observations can be understood.     

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.     

The effect of Fe doping on structural, magnetic and electrical transport properties of CaMn1−xFexO3 (x=0-0.35)

A comprehensive study of the effect of Fe doping on CaMnO₃ is carried out by means of experiments on the structural, transport conduction, and magnetic properties of CaMn_1−xFe_xO₃ (0≤x≤0.35). With a sol-gel process for sample preparation, Fe is substituted for Mn up to x=0.35. This substitution substantially brings out the lattice expansion and gradually suppresses the antiferromagnetism. For x=0.08 and 0.10 in particular, the magnetization curves with a field-cooled mode under the field of 1 kOe behave as those of a ferrimagnetic-like system and present low-temperature negative magnetization. For x≥0.15, the negative magnetization phenomenon disappears, and a ferromagnetic component coexists with an antiferromagnetic one, but the antiferromagnetic interaction still dominates in these compounds. Electrical transport measurements show insulating behavior for all compositions. Fe doping, even at a level as low as x=0.02, can cause a marked resistivity increase in the temperature range studied. Further increasing the Fe content causes the resistivity to gradually decrease due to the increasing carrier presence.     

Structure and superconductivity of Mg1−xPbxB2

A series of polycrystalline samples of Mg_1−xPb_xB₂ (0≤x≤0.10) were prepared by a solid state reaction method and their structure, superconducting transition temperature and transport properties were investigated by means of X-ray diffraction (XRD) and resistivity measurements. Mg_1−xPb_xB₂ compounds were shown to adopt an isostructural AlB2-type hexagonal structure in a relatively small range of lead concentration, x≤0.01. The crystalline lattice constants were evaluated and were found to exhibit slight length compression as x increases. The superconducting transition temperature (T_c) steadily decreases with Pb doping. It is suggested that the mechanism of superconductivity reduction by lead doping can be attributed to the chemical pressure effect.     

Structure, magnetic, and transport properties of the Co-doped manganites La0.9Te0.1Mn1−xCoxO3 (0≤x≤0.25)

The effect of Co doping at Mn-site on the structural, magnetic and electrical transport properties in electron-doped manganties La_0.9Te_0.1Mn_1−xCoxO₃ (0≤x≤0.25) has been investigated. The room temperature structural transition from rhombohedra to orthorhombic (Pbnm) symmetry is found in these samples with x≥0.20 by the Rietveld refinement of X-ray powder diffraction patterns. All samples undergo the paramagnetic-ferromagnetic (PM-FM) phase transition. The Curie temperature T_C of these samples decreases and the transition becomes broader with increasing Co-doping level. The magnetization magnitude of Co-doping samples increases at low temperatures with increasing Co-doping level for x≤0.15 and decreases with increasing Co-doping content further. The metal-insulator (M-I) transitions observed in the sample with x=0 are completely suppressed with Co doping, and the resistivity displays semiconducting behavior within the measured temperature region for these samples with x>0. All results are discussed according to the changes of the structure parameters and magnetic exchange interaction caused by Co-doping. In addition, the different effects between the Co doping and Cu doping in the Mn site for the electron-doped manganites are also discussed.     

Bulk Sn1−xMnxO2 magnetic semiconductors without room-temperature ferromagnetism

Polycrystalline Sn_1−xMn_xO₂ (0≤x≤0.05) diluted magnetic semiconductors were prepared by solid-state reaction method and their structural and magnetic properties had been investigated systematically. The three Mn-doped samples (x=0.01, 0.03, 0.05) undergo paramagnetic to ferromagnetic phase transitions upon cooling, but their Curie temperatures are far lower than room temperature. The magnetization cannot be attributed to any identified impurity phase. It is also found that the magnetization increases with increasing Mn doping, while the ratio of the Mn ions contributing to ferromagnetic ordering to the total Mn ions decreases.     

Structure, transport and magnetic properties in La2xSr2−2xCo2xRu2−2xO6

The perovskite solid solutions of the type La_2xSr_2−2xCo_2xRu_2−2xO₆ with 0.25≤x≤0.75 have been investigated for their structural, magnetic and transport properties. All the compounds crystallize in double perovskite structure. The magnetization measurements indicate a complex magnetic ground state with strong competition between ferromagnetic and antiferromagnetic interactions. Resistivity of the compounds is in confirmation with hopping conduction behaviour though differences are noted especially for x=0.4 and 0.6. Most importantly, low field (50 Oe) magnetization measurements display negative magnetization during the zero field cooled cycle. X-ray photoelectron spectroscopy measurements indicate the presence of Co²⁺/Co³⁺ and Ru⁴⁺/Ru⁵⁺ redox couples in all compositions except x=0.5. Presence of magnetic ions like Ru⁴⁺ and Co³⁺ gives rise to additional ferromagnetic (Ru-rich) and antiferromagnetic sublattices and also explains the observed negative magnetization.     

Structural, magnetic and transport properties in the manganites La0.7Sr0.3−xTexMnO3 (0≤x≤0.15)

The effect of Te-doping at La-site on structural, magnetic and transport properties in the manganites La_0.7Sr_0.3−xTe_xMnO₃ (0≤x≤0.15) has been investigated. All samples show a rhombohedral structure with the space group . It shows that the Mn-O-Mn bond angle decreases and the Mn-O bond length increases with the increase of Te content. The Curie temperature T_C decreases with increasing Te-doping level, in contrast, the magnetization magnitude of Te-doping samples at low temperatures increase with increasing x as x≤0.05 and then decrease with further increasing x to 0.15. The results are discussed in terms of the combined effects of the opening of the new double exchange (DE) channel between Mn²⁺-O-Mn³⁺ due to the introduction of Mn²⁺ ions because of the substitution of Te⁴⁺ for Sr²⁺ and the reduction of the transfer integral b due to the decrease of the Mn-O-Mn bond angle.     

Magnetic/structural phase diagram and enhanced giant magnetoresistance in Zn-doped antiperovskite compound Ga1−xZnxCMn3

The structural, magnetic and electrical transport properties of Zn-doped antiperovskite compounds Ga_1−xZn_xCMn₃ (0≤x≤0.30) have been investigated. After partial substitution of Zn for Ga, the Curie temperature increases monotonously and the ground antiferromagnetic (AFM)-ferromagnetic intermediate (FI) phase transition is gradually suppressed. With increasing the doping level x, the saturated magnetizations decreases gradually firstly for x≤0.20, then increases with increasing x. The electrical transport properties of Ga_1−xZn_xCMn₃ are studied at different magnetic fields. Enhanced giant magnetoresistance (GMR) was observed around the AFM-FI transition. With increasing x, the maximal values and peak widths of GMR increase. Particularly, for x=0.20, GMR reaches a maximum value of 75%, spanning a temperature range of 80 K at 50 kOe and displays the behavior of strongly depending on the magnetization history. The possible origins are discussed.     

Magnetic/structural diagram, chemical composition-dependent magnetocaloric effect in self-doped antipervoskite compounds Sn1−xCMn3+x (0≤x≤0.40)

The effects of Mn substitutions on the crystal structure, magnetic properties, and magnetocaloric effect (MCE) of antiperovskite Sn_1−xCMn_3+x (0≤x≤0.40) have been investigated detailedly. Both the Curie temperature (T_C) and the magnetizations at 40 kOe decrease with increasing x firstly for x≤0.10, and then increase with increasing x further. The type of magnetic transition changes from first-order to second-order around x=0.10 with increasing x. Chemical composition-dependent MCE is also studied around T_C. With increasing x, the maximal magnetic entropy changes decrease and the magnetic phase transitions broaden. Accordingly, the relative cooling power (RCP) increases with increasing x, reaching the largest values of ∼0.56 J/cm³ (∼75 J/kg) and ∼1.66 J/cm³ (∼221 J/kg) with the magnetic changes of 20 kOe and 48 kOe, respectively. Considering the large RCP, inexpensive, and innoxious raw materials, these serial samples Sn_1−xCMn_3+x are suggested to be potential room-temperature magnetic refrigerant materials.     

The effect of growth ambient on the structural and optical properties of MgxZn1−xO thin films

Mg_xZn_1−xO (x = 0.0-0.20) thin films have been deposited by sol-gel technique on glass substrates and the effect of growth ambient (air and oxygen) on the structural, and optical properties have been investigated. The films synthesized in both ambient have hexagonal wurtzite structure. The c-axis lattice constant decreases linearly with the Mg content (x) up to x = 0.05, and 0.10 respectively for air- and oxygen-treated films, above which up to x = 0.20, the values vary irregularly with x. The change in the optical band gap values and the ultraviolet (UV) peak positions of Mg_xZn_1−xO films show the similar change with x. These results suggest that the formation of solid solution and thus the structural and optical properties of Mg_xZn_1−xO thin films are affected by the growth ambient.     

Influence of Ce doping on electrical and thermal properties of La0.7−xCexCa0.3MnO3 (0.0≤x≤0.7) manganites

The effect of Ce-doping on structural, magnetic, electrical and thermal transport properties in hole-doped manganites La_0.7−xCe_xCa_0.3MnO₃ (0.0≤x≤0.7) is investigated. The structure of the compounds was found to be crystallized into orthorhombically distorted perovskite structure. dc Susceptibility versus temperature curves reveal various magnetic transitions. For x≤0.3, ferromagnetic regions (FM) were identified and the magnetic transition temperature (T_C) was found to be decreasing systematically with increasing Ce concentration. The electrical resistivity ρ(T) separates the well-define metal-semiconducting transition (T_MS) for low Ce doping concentrations (0.0≤x≤0.3) consistent with magnetic transitions. For the samples with 0.4≤x≤0.7, ρ(T) curves display a semiconducting behavior in both the high temperature paramagnetic (PM) phase and low temperature FM or antiferromagnetic phase. The electron–phonon and electron–electron scattering processes govern the low temperature metallic behavior, whereas small polaron hopping model is found to be operative in PM phases for all samples. These results were broadly corroborated by thermal transport measurements for metallic samples (x≤0.3) in entire temperature range we investigated. The complicated temperature dependence of Seebeck coefficient (S) is an indication of electron–magnon scattering in the low temperature magnetically ordered regime. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The observation of a significant difference between ρ(T) and S(T) activation energies and a positive slope in thermal conductivity κ(T) implying that the conduction of charge carriers were dominated by small polaron in PM state of these manganites.     

Optical properties of bulk Zn1−xCoxO magnetic semiconductors

Polycrystalline Zn_1−xCo_xO (x=0, 0.02, 0.05, 0.10 and 0.15) oxides have been synthesized by solid state reaction via sintering ZnO and Co powders in open air. X-ray diffraction analyses using Rietveld refinement indicate that a stoichiometric single phase with a wurtzite-like structure was found in Zn_1−xCo_xO samples with x up to 0.10. The elemental mapping using energy dispersive X-ray spectroscopic analyses presents a uniform distribution of Co. Optical transmittance measurements show that several extra absorption bands appear in the Co-doped ZnO, which is due to the transitions between the crystal-field-split 3d levels of tetrahedral Co²⁺ substituting Zn²⁺ ions. Raman measurements show that limited host lattice defects are induced by Co doping. Magnetization measurements reveal that the Co-doped ZnO samples are paramagnetic due to the absence of free carriers and in low temperature the dominant magnetic interaction is nearest-neighbor antiferromagnetic.     

Magnetic and related properties of the solid solution CeCuxGa4−x

Physical properties of polycrystalline samples of CeCu_xGa_4−x (x = 0.2–1.4), crystallizing in the tetragonal BaAl₄-type structure (space group I 4/mmm), were studied by means of X-ray powder diffraction, magnetization, specific heat, electrical resistivity and magnetoresistivity measurements in wide temperature and magnetic fields ranges. The unit-cell volume of the system was found to decrease with increasing x (in total by about 4%) but the magnetic moments of Ce³⁺ ions remain localized in the whole x-range studied. The alloys exhibit ferromagnetic order at low temperatures, which manifests itself as distinct and relatively sharp anomalies in all the temperature characteristics measured. The ordering temperature decreases with increasing the Cu content from 5.5(1) K for x = 0.2 down to 1.35(5) K for x = 1.4, and the electrical transport properties of the system show some features characteristic of Kondo lattices.     

Magnetic and magnetocaloric properties of Gd1−xScxNi2 solid solutions

Magnetic and heat capacity measurements have been carried out on the polycrystalline Gd_1−xSc_xNi₂ solid solutions (0≤x≤1), which crystallize in the cubic C15 Laves phases superstructure (space group F4?3m). These solid solutions are ferromagnetic with a Curie temperature below 76 K. Their Curie temperature decreases from 75.4 K for GdNi₂ to 13.6 K for Gd_0.2Sc_0.8Ni₂. At high temperatures, all solid solutions, except ScNi₂, are Curie-Weiss paramagnets. The Debye temperature as well as phonon, conduction electron and magnetic contributions to the heat capacity have been determined from heat capacity measurements. The magnetocaloric effect has been estimated both in terms of isothermal magnetic entropy change and adiabatic temperature change for selected solid solutions in magnetic fields up to 3 T.     

Low-field AC susceptibility in Pr1−xHoxMn2Ge2

We report the structure and magnetic properties of Pr_1−xHo_xMn₂Ge₂ (0.0≤x≤1.0) germanides by means of X-ray diffraction (XRD), differential scanning calorimetry (DSC) techniques and AC magnetic susceptibility measurements. All compounds crystallize in the ThCr₂Si₂-type structure with the space group I4/mmm. Substitution of Ho for Pr leads to a linear decrease in the lattice constants and the unit cell volume. The samples with x=0 and x=0.8 have spin reorientation temperature. The results are collected in a phase diagram.