First principles study of structural, electronic and magnetic properties of Mg1−xMnxTe alloys

Density functional FP-LAPW + lo calculations have been performed to study the structural, electronic and magnetic properties of Mg_1−xMn_xTe for compositional parameter x = 0.25, 0.50, 0.75 and 1. Our calculations reveal the occurrence of ferromagnetism in these compounds in which the transition-metal atom is ordered in a periodical way thereby interacting directly with the host atoms. Results extracted from electronic band structure and density of states (DOS) of these alloys show the existence of direct energy band gap for both majority- and minority-spin cases, while the total energy calculations confirm the stability of ferromagnetic state as compared to anti-ferromagnetic state. The total magnetic moment for Mg_1−xMn_xTe for each composition is found to be approximately 5 μ_B, which indicates that the addition of Mn content does not affect the hole carrier concentration of the perfect MgTe compound. Moreover, the s-d exchange constant (N₀α) and p-d exchange constant (N₀β) are also calculated which are in accordance with a typical magneto-optical experiment. The estimated spin-exchange splitting energies originated by Mn 3d states energies, i.e. Δ_X(s-d) and Δ_X(p-d), show that the effective potential for minority-spin is more attractive than that of the majority-spin. Also, the p-d hybridization is found to cause the reduction of local magnetic moment of Mn and produce small local magnetic moments on the nonmagnetic Mg and Te sites.     

Near room temperature magnetic entropy changes in as-cast Gd100−xMnx (x = 0, 5, 10, 15, and 20 at.%) alloys

A series of Gd_100−xMn_x (x = 0, 5, 10, 15, and 20 at.%) alloys were prepared by arc-melting. The Curie temperature (T_C) associated with the ferromagnetic-paramagnetic transitions, derived from M-T curves, show decrease in T_C for as-cast alloys (∼279 K) as compared to as-cast Gd (∼292 K). No appreciable decrease in the |ΔS_M|_max values ∼4.6 J/kg K (0-2 T) and ∼8.6 J/kg K (0-5 T) were observed upon alloying Gd with Mn up to x ≤ 15 at.%. Refrigerant capacity (q) showed negligible variation ∼195 J/kg (0-2 T) and ∼450 J/kg (0-5 T) with increasing Mn (up to x ≤ 15 at.%) content. Similar values of |ΔS_M|_max and q coupled with ∼13 K decrease in T_C for as-cast Gd_100−xMn_x (0 ≤ x ≤ 15) alloys as compared to Gd, suggests expansion of working temperature region of Gd upon alloying with Mn up to 15 at.%. Low cost, adjustable T_C, favorable magnetocaloric properties make Gd_100−xMn_x alloys potential candidates as second-order transition based magnetic refrigerants for near room temperature air-conditioning and magnetic refrigeration.     

Room temperature electroresistance in Sr2−xGdxMnTiO6 perovskites (0 ≤ x ≤ 1)

We report on the room temperature strong (∼80%) electroresistance (ER) in the double perovskite with mixed Mn valence: Sr_2−xGd_xMnTiO₆, 0 ≤ x ≤ 1. Both, continuous and pulsed current-voltage curves are almost identical which indicates that the observed electroresistance is not associated with heating. This is also supported by simultaneous temperature measurements. ER is negligible (absent) in the x = 0 compound and increases with the increase of Gd content ‘x’. The amplitude of ER has a maximum for x = 0.75, suggesting that ER is determined by both the double exchange and the Mn³⁺ concentration. At the same time, magnetic interactions change from the antiferromagnetic (x = 0) to ferromagnetic ones as x → 1, thus linking the ER with ferromagnetism.     

Growth and characterization of chemical bath deposited Cd1−xZnxS thin films

Cd_1−xZn_xS (0 ≤ x ≤ 1) thin films have been deposited by chemical bath deposition method on glass substrates from aqueous solution containing cadmium acetate, zinc acetate and thiourea at 80 ± 5 °C and after annealed at 350 °C. The structural, morphological, compositional and optical properties of the deposited Cd_1−xZn_xS thin films have been studied by X-ray diffractometer, scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), photoluminescence (PL) and UV-vis spectrophotometer, respectively. X-ray diffraction analysis shows that for x < 0.8, the crystal structure of Cd_1−xZn_xS thin films was hexagonal structure. For x > 0.6, however, the Cd_1−xZn_xS films were grown with cubic structure. Annealing the samples at 350 °C in air for 45 min resulted in increase in intensity as well as a shift towards lower scattering angles. The parameters such as crystallite size, strain, dislocation density and texture coefficient are calculated from X-ray diffraction studies. SEM studies reveal the formation of Cd_1−xZn_xS films with uniformly distributed grains over the entire surface of the substrate. The EDX analysis shows the content of atomic percentage. Optical method was used to determine the band gap of the films. The photoluminescence spectra of films have been studied and the results are discussed.     

Strong correlation between structural, magnetic and transport properties of non-stoichiometric Sr2FexMo2−xO6 (0.8 ≤ x ≤ 1.5) double perovskites

Sr₂Fe_xMo_2−xO₆ (x = 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5 wt.%) (SFMO) double perovskite oxides of different compositions have been prepared by sol-gel method. These materials were subjected to X-ray diffraction and found that crystal structure changes from tetragonal to cubic around x = 1.2 wt.%. Lattice parameters and unit cell volume have been calculated using X-ray diffraction data. Magnetization studies have been carried out using Vibrating Sample Magnetometer ranging from −15 kOe to +15 kOe and saturation magnetization (M_s) has been determined. Electrical resistivity and magnetoresistance studies have been carried out in the magnetic field range of −40 kOe to +40 kOe keeping the temperature constant at 5, 150 and 300 K using standard four-probe method. Resistivity studies have also been carried out in the temperature ranging from 5 to 300 K keeping the magnetic field constant at 0, 10, 20 and 40 kOe. Maximum degree of Fe/Mo ordering (η_max) of SFMO has been calculated and compared with magnetic and transport properties. It has been found that there is a strong correlation between 3 parameters η_max, M_s and MR (%), i.e. all of them show a maximum at x = 1.0 wt.% and decreases as x deviates from 1.0 in SFMO. It has been also found that there is a different resistivity behavior between x ≤ 1.2 wt.% and x > 1.2 wt.% samples of SFMO. Semiconductor metal transition temperature was found to be maximum at x = 1.0 wt.%.     

Infrared emissivities and microwave absorption properties of perovskite Sm0.5Sr0.5Co1−xFexO3 (0 ≤ x ≤ 0.5)

Sm_0.5Sr_0.5Co_1−xFe_xO₃ (0 ≤ x ≤ 0.5, SSCF) with perovskite-type structure has been successfully prepared by conventional solid-state reaction as a microwave and infrared multi-functional material. The effects of Fe incorporation on the structure, electrical conductivity, infrared emissivity and microwave-absorbing properties were investigated in detail. XRD results have shown that the perovskite structure of SSCF has an orthorhombic symmetry for 0 ≤ x ≤ 0.4 and a cubic symmetry for 0.5, respectively. The incorporation of Fe in SSCF could contribute to the decrease of electrical conductivity, while the infrared emissivities are increased. Moreover, microwave-absorbing properties in the frequency range of 2-18 GHz at room temperature are sensitive to Fe content. The complex permittivity, complex permeability and electromagnetic loss tangent have suddenly a step change at a certain frequency and the step-change frequency position moves slightly to lower frequencies with Fe increased. The optimal reflection loss calculated from the measured permittivity and permeability is 29.33 dB at 7.97 GHz with a thickness of 2.0 mm.     

Anisotropy compensation and high low-field magnetostriction of epoxy/Tb1−xHox(Fe0.8Co0.2)2 composites (0.60 ≤ x ≤ 1.0)

The anisotropy compensation and magnetostrictive properties of Tb_1−xHo_x(Fe_0.8Co_0.2)₂ (0.60 ≤ x ≤ 1.0) alloys have been investigated. The easy magnetization direction (EMD) at room temperature rotates from the 〈1 1 1〉 axis (x ≤ 0.75) to the 〈1 0 0〉 axis (x ≥ 0.90) through an intermediate state 〈1 1 0〉, subjected to the anisotropy compensation between Tb³⁺ and Ho³⁺ ions. Composition anisotropy compensation is realized near x = 0.75. The Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ alloy has a minimum anisotropy and a large spontaneous magnetostriction coefficient λ₁₁₁ (≈740 ppm) at room temperature. The strong 〈1 1 1〉-oriented 1-3 epoxy-bonded composite has been fabricated by curing under a moderate magnetic field. A high low-field magnetostriction of about 400 ppm at 3 kOe is obtained for the 1-3 epoxy/Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ composite with 40-vol% alloy particles, which can be attributed to the low magnetic anisotropy, EMD lying along 〈1 1 1〉 direction, the strong 〈1 1 1〉-textured orientation and the chain structure.     

Solvothermal synthesis of Hg1−xCdxTe nanostructures—Their structural and optical properties

This article describes a facile, low-cost, solution-phase approach to the large-scale preparation of Hg_1−xCd_xTe nanostructures of different shapes such as nanorods, quantum dots, hexagonal cubes of different sizes and different compositions at a growth temperature of 180 °C using an air stable Te source by solvothermal technique. The XRD spectrum shows that the crystals are cubic in their basic structure and reveals the variation in lattice constant as a function of composition. The size and morphology of the products were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The formation of irregular shaped particles and few nano-rods in the present synthesis is attributed to the cetyl trimethylammonium bromide (CTAB). The room temperature FTIR absorption and PL studies for a compositon of x = 0.8 gives a band gap of 1.1 eV and a broad emission in NIR region (0.5-0.9 eV) with all bands attributed to surface defects.     

XPS study of Pr1−xCaxMnO3 (x = 0.2, 0.33, 0.4 and 0.84)

We have studied the Mn 2p, Ca 2p, and Pr 4d core levels of Pr_1-xCa_xMnO₃ (x = 0.2, 0.33, 0.4 and 0.84) as a function of x using X-ray photoelectron spectroscopy both at room temperature as well as 77 K. Suppression of chemical potential shifts have been observed at 77 K compared to that of room temperature spectra. We have discussed this result by considering the concept of phase separation.     

Crystal structure and magnetic properties of DyCuxGa2−x (x = 0-2.0) antiferromagnetic compounds

DyCu_xGa_2−x (x = 0-2.0) compounds have been synthesized; meanwhile, their crystal structure and magnetic properties have been investigated by X-ray diffraction and magnetic measurements. The result shows that the continuous solid-solution series crystallize in three phases, with the structure types of AlB₂ (x = 0-0.2), DyCuGe (x = 0.3-0.6) and CeCu₂ (x = 0.7-2.0), respectively. The main reason to form the three structure types is considered to be the average atomic radius ratio of R to Cu/Ga. Magnetic-ordering transition of the compounds with x = 0.2-0.6 takes place at about 20 K and 113 K, while those of other compounds only takes place at about 20 K, which is attributed to the change of the near Dy-Dy distances and the ordered substitution of Ga by Cu.     

Dependence of magnetic properties on the film thickness and Pt atomic fraction of post-annealed Fe1−xPtx-C granular films on MgO(1 0 0) and SiO2/Si(1 0 0) substrates

The Fe_1−xPt_x-C granular films with different Pt atomic fractions (0.09 ≤ x ≤ 0.52) and film thicknesses (5 nm ≤ t ≤ 100 nm) were deposited on MgO(1 0 0) and SiO₂/Si(1 0 0) substrates by facing-target sputtering and post-annealing. With the increasing x, the ordered L1₀ FePt grains form. All of the films are ferromagnetic, and the easy axis is in the film plane. With the decrease of t, the films turn from hard ferromagnetic to soft ferromagnetic. The maximum coercivity of the 100-nm thick Fe_1−xPt_x-C granular films measured at a 10-kOe field is 3.7 kOe at x = 0.48. The coercivity of the Fe_0.56Pt_0.44-C granular films increases, and the magnetization measured at a 10-kOe field decreases with the increasing t. The reversal mechanism of the 100-nm thick Fe_1−xPt_x-C granular films turns from the domain wall motion to the Stoner-Wohlfarth rotation mode as x increases. However, the reversal mechanism of the Fe₅₆Pt₄₄-C granular films with different t approaches the Stoner-Wohlfarth rotation mode, and is film-thickness independent.     

Preparation and electrical properties of perovskite ceramics in the system BaBi1−xSbxO3 (0 ≤ x ≤ 0.5)

The effect of the composition on the electrical properties of BaBi_1−xSb_xO₃ (0 ≤ x ≤ 0.5) negative temperature coefficient (NTC) thermistors was studied. Major phases present in the sintered bodies of BaBi_1−xSb_xO₃ (0 < x < 0.5) ceramics were BaBi_0.5Sb_0.5O₃ compounds with a rhombohedral structure and BaBiO₃ compounds with a monoclinal structure. Most pores were located in the grains of BaBiO₃ and BaBi_0.5Sb_0.5O₃ ceramics. It was apparent that the ρ₂₅ and B_25/85 constant of the thermistors increased with increasing Sb content.     

MOCVD growth and characterizations of BxAl1−xAs and BxAl1−x−yInyAs alloys

Zinc-blende B_xAl_1−xAs and B_xAl_1−x−yIn_yAs alloys have been grown on exactly oriented (0 0 1)GaAs substrates by low pressure metalorganic chemical vapor deposition (LP-MOCVD). The influence of susceptor coating, growth temperature and gas-phase boron mole fraction on boron incorporation into AlAs has been comprehensively investigated. It has been found that boron incorporation into AlAs could be enhanced and the optimal growth temperature range of B_xAl_1−xAs alloys changed from 580 °C to 610 °C when SiC-coated graphite susceptors were replaced by the non-coated ones. In this study, the maximum boron composition x of 2.8% was achieved for the pseudomorphically strained B_xAl_1−xAs alloys. AFM measurements show that RMS roughness of B_xAl_1−xAs alloys increased sharply with the increase of gas-phase boron mole fraction. Raman spectra of B_xAl_1−xAs alloys show a linear increase of the BAs shift with boron composition x. Based on BAlAs deposition, bulk B_xAl_1−x−yIn_yAs (x = 1.9%) quaternary alloy was grown lattice-matched to GaAs successfully. Moreover, 10-period BAlAs/GaAs and BAlInAs/GaAs MQW heterostructures were also demonstrated.     

Charge ordering analysis by electrical and dielectric measurements in Ca2−xPrxMnO4 (x = 0-0.2) compounds

Ca_2−xPr_xMnO₄ (0 ≤ x ≤ 0.2) polycrystalline ceramic powders were synthesized by sol-gel method. The X-ray diffraction (XRD) profiles were indexed with a tetragonal and orthorhombic structure for Ca₂MnO₄ and Pr-doped compounds, respectively. Electrical properties were investigated by dc and ac electrical measurements. The dc measurements have revealed an insulating state for all compounds in 80-350 K temperature range. Both dc and ac measurements have highlighted a charge ordering (CO) transition at T_CO = 233 and 245 K for x = 0.175 and 0.2, respectively. The CO state was found to be accompanied by a jump of the hopping activation energy and a rapid rise of both dielectric permittivity and imaginary part of ac electrical impedance.     

Structural, magnetic and electrical properties of Bi1.4La0.6Sr2CaCu2−xMnxOy compounds

The Mn-doped compounds Bi_1.4La_0.6Sr₂CaCu₂O_y were prepared by sol-gel method. The structural variation was characterized systematically by X-ray diffraction (XRD), infrared (IR) spectra and Raman scattering spectra, respectively. The electrical and magnetic properties of the compounds were investigated by the temperature dependence of resistivity (R-T) and magnetic hysteresis loop (M (H)) measurements. Results indicate that the subtle change of lattice parameters has taken place in the compounds, which is attributed to CuO₂ planes canting and Mn valence alternation. In the condition of preserving Bi-2212 structure, Bi_1.4La_0.6Sr₂CaCu_2−xMn_xO_y compound has optimal resistivity and magnetism at x = 2%, which could provide a candidate as new barrier in Josephson junction in future.     

Effect of Ni substitution on the structural and transport properties of NixMn0.8−xMg0.2Fe2O4; 0.0 ≤ x ≤ 0.40 ferrite

Ni_xMn_0.8−xMg_0.2Fe₂O₄; 0.0≤ x ≤0.40 was prepared by standard ceramic technique, presintering was carried out at 900 °C and final sintering at 1200 °C with heating/cooling rate 4 °C/min. X-ray diffraction analyses assured the formation of the samples in a single phase spinel cubic structure. The calculated crystal size was obtained in the range of 75-130 nm. A slight increase in the theoretical density and decrease in the porosity was obtained with increasing the nickel content. This result was discussed based on the difference in the atomic masses between Ni (58.71) and Mn (54.938). IR spectral analyses show four bands of the spinel ferrite for all the samples. The conductivity and dielectric loss factor give nearly continuous decrease with increasing Ni-content. This was discussed as the result of the significant role of the multivalent cations, such as iron, nickel, manganese, in the conduction mechanism. Anomalous behavior was obtained for the sample with x = 0.20 as highest dielectric constant, highest dielectric loss and highest conductivity. This anomalous behavior was explained due to the existence of two divalent cations on B-sites with the same ratio, namely, Mg²⁺ and Ni²⁺.     

Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3

Superconductors Ba_1−xK_xBiO₃ and body-centered double perovskites Ba_1−xK_xBi_1−yNa_yO₃ have been selectively synthesized by a facile hydrothermal route. The appropriate ratio and adding sequence of initial reagents, alkalinity, reaction temperature and time are the critical factors that influence the crystal growth of the compounds. The purity and homogeneity of the crystals were detected by the ICP, SEM, EDX and TEM studies. Magnetic measurements show that the superconducting transition temperatures T_C of Ba_1−xK_xBiO₃ decrease from 22 K (for x = 0.35) to 8 K (for x = 0.55) with increasing the K doping level.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Effect of Ca-doping on the structural and electrical properties of CuY1−xCaxO2 (0 ≤x ≤0.10) ceramics

Delafossites CuY_1−xCa_xO₂ (0 ≤x ≤0.10) ceramics have been prepared by solid state reaction using Cu₂O, Y₂O₃ and CaCO₃. Liquid phase sintering, which obviously accelerates the reaction speed of Cu₂O-Y₂O₃-CaCO₃ system and promotes the formation of CuYO₂ phase is evidenced for the Ca-doped samples. During the sintering process, CuO can react with CaO to form two intermediate compounds, CaCu₂O₃ and Ca₂CuO₃, which decompose into CaO and liquid phase during 1273-1323 K. In the dopant range of 0 ≤x ≤0.10, both electrical conductivity and density of the samples are increased by Ca-doping. The room temperature conductivity of CuY_0.94Ca_0.06O₂ is more than four orders of magnitude higher than that of CuYO₂.     

DC magnetization investigations in Ti1−xMnxO2 nanocrystalline powder

In the present paper, DC magnetization investigation on the insulating nanocrystalline powder samples of Ti_1−xMn_xO₂ (x = 0, 0.05, 0.10, and 0.15) prepared by simple chemical route is reported. Structural measurements revealed phase pure anatase structure of TiO₂ when x ≤ 0.05 and a mixture of anatase and rutile TiO₂ along with the signature of Mn₃O₄ phase for x > 0.05. Magnetic measurements exhibited the presence of ferromagnetic ordering at room temperature in samples having either small fraction of Mn or no Mn at all. This ferromagnetic signature is accompanied with paramagnetic contribution which is found to dominate with increase in Mn concentration. The Ti_1−xMn_xO₂ sample having highest Mn concentration of x = 0.15 showed nearly paramagnetic behavior. However, at low temperatures, additional ferrimagnetic ordering arising due to Mn₃O₄ (T_C = 42 K) is evidenced in the doped samples. Consistent with the XRD investigations, the isofield DC-magnetization measurements under field cooled and zero field cooled (FC-ZFC) histories corroborated the presence of Mn₃O₄ phase. Also, distinct thermomagnetic irreversibility has been observed above 42 K. These results are suggestive of presence of weak ferromagnetic ordering possibly due to defects related with oxygen vacancies.