Structural and magnetic heterogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.3 − x Bi x Mn1.1O3

The structural, resistive, magnetic, and magnetoresistive properties of the La_0.6Sr_{0.3 ? x} Bi _x Mn_1.1O₃ ceramics have been studied. The substitution of Bi ions for Sr ions increases the lattice parameter of the rhombohedral perovskite structure, decreases the metal-semiconductor and ferromagnet-paramagnet phase transition temperatures and the peak of the magnetoresistive effect, and increases the resistivity, approaching the system to the ferroelectric state. The ⁵⁵Mn NVR study indicates on the high-frequency Mn³⁺ ? Mn⁴⁺ superexchange and heterogeneity of the valence and magnetic states of manganese due to the nonuniformity of distribution of all ions and defects. The phase diagram has been constructed, which shows a strong correlation between the structural, magnetic, and magnetoresistive properties.     

Massive, thick-film, and thin-film La0.6Sr0.3Mn1.1−x Fe x O3±δ magnetoresistive ceramics: Structure and properties

Magnetoresistive ceramic and thick-and thin-film La_0.6Sr_0.3Mn_1.1?x Fe _x O_3±δ(x=0,0.04) samples are studied by X-ray diffraction, ⁵⁵Mn nuclear magnetic resonance (NMR), resistivity measurements, and magnetic measurements. Their rhombohedrally distorted ( $R\overline 3 c$ ) perovskite structure is found to contain anion and cation vacancies and nanocluster defects. Their broad asymmetric ⁵⁵Mn NMR spectra support high-frequency electron-hole exchange between Mn³⁺ and Mn⁴⁺ and the fact that their environments are different due to a high defect concentration and high structural inhomogeneity. Iron doping and an increase in the annealing temperature result in a decrease in the temperatures of the metal-semiconductor and ferromagnet-paramagnet phase transitions and an increase in the magnetoresistive effect (MRE). The low-field MRE in the low-temperature region (～100 K) in the ceramics and thick film is explained by tunneling in crystallite boundaries. An analysis of the effect of iron and the annealing temperature on the activation energy confirms the conclusion regarding a defect system of the perovskite structure and the presence of several mechanisms of activation processes.     

Structural and magnetic inhomogeneities, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.6Sr0.2Mn1.2 − x Nb x O3 ceramics

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Nb _x O₃ (x = 0–0.3) annealed at temperatures of 1260 and 1500°C have been investigated using the X-ray diffraction, electron microscopic, resistive, magnetoresistive, and magnetic (χ_ac, ⁵⁵Mn NMR) methods. It has been found that there is a correlation between the increasing unit cell parameter a of the rhombohedral R $\bar 3$ c structure and the average ionic radius with increasing niobium concentration x and annealing temperature for the case where the lattice contains anion vacancies, cation vacancies, and nanostructured clusters. The observed increase in the electrical resistivity and decrease in the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C with an increase in the niobium concentration x and the annealing temperature have been explained by the decrease in the content of the ferromagnetic phase, as well as by changes in the ratio Mn³⁺/Mn⁴⁺, the oxygen nonstoichiometry, and the concentration of defects weakening the high-frequency electronic exchange of the ions Mn³⁺ ? Mn⁴⁺. The presence of nanostructured clusters in the lattice has been confirmed by an anomalous hysteresis associated with the unidirectional exchange anisotropy of the interaction between the ferromagnetic matrix and antiferromagnetic clusters with Mn²⁺ and Nb³⁺ in distorted A-positions. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra and their computer decomposition have revealed a high-frequency electronic exchange and an inhomogeneity of the magnetic and valence states of manganese due to the nonuniform distribution of all ions and defects. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities of the imperfect perovskite structure, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between the composition, structure, resistive and magnetic properties of rare-earth manganites.     

Characterization and optimization of Ce0.6Zr0.4−x Mn x O2 (x ≤ 0.4)

In situ synthesis method is used to synthesize g-C₃N₄-P25 composite photocatalysts with different mass rations. The experiment result shows that P25 particles with diameter at range of 20–30 nm were embedded homogenously in the sheets of g-C₃N₄. Coupling g-C₃N₄ with P25 can not only improve the visible light absorption, but also improve the visible light photocatalytic activity of P25. The g-C₃N₄-P25 nanocomposite has the higher photocatalytic activity than g-C₃N₄ under visible light. The optimal g-C₃N₄ content with the highest photocatalytic activity is determined to be 84 %, which is almost 3.3 times higher than that of individual g-C₃N₄ under the visible light. The enhanced visible light photocatalytic activity could be ascribed to the formation of g-C₃N₄ and TiO₂ heteojunction, which results in an efficient separation and transfer of photo-induced charge carriers. The electron spin resonance results show that the ^·O₂ ^? radicals are main active species for g-C₃N₄ and the g-C₃N₄-P25 nanocomposites.     

Magnetic and Mössbauer spectral studies of x (NiFe2O4) + (1−x)(SrFe12O19), x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0 nanocomposites

Nano size composite of x(NiFe₂O₄)+(1?x)(SrFe₁₂O₁₉) were prepared using sol gel and aerosol route. The percentage of the components of NiFe₂O₄ and SrFe₁₂O₁₉ calculated from X-ray diffraction pattern using Rietveld analysis. The hysteresis loop for the as obtained samples exhibits no hysteresis, which may be attributed to super paramagnetic relaxation. The saturation magnetization do not show a significant change with the increase of strontium ferrites, however, the coercivity increased from 115 to 6,000 Oe. The Mössbauer spectra of these nano composites were discussed along with the magnetic moment and X-ray results.     

Structural and magnetic inhomogeneities,phase transitions, 55Mn nuclear magnetic resonance,and magnetoresistive properties of La0.6 − x Nd x Sr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La_{0.6 ? x} Nd _x Sr_0.3Mn_1.1O_3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χ_ac, ⁵⁵Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii $\bar R$ for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn²⁺ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition T _ms and ferromagnetic-paramagnetic phase transition T _C decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn³⁺-O^2?-Mn⁴⁺) ? (Mn⁴⁺-O^2?-Mn³⁺) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn³⁺/Mn⁴⁺ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχ_ac(T) and the ⁵⁵Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.     

Electrical,magnetic, and thermal properties of UCu5− x Pt x

We report measurements of the structural and electronic properties of UCu_{5??? x} Pt _x , a Pt-based analogue of the nFL system UCu_{5??? x} Pd _x , in the concentration range 0≤?x?≤?5. Forx?≤?2.5 and x?≥?4.5, the UCu_{5??? x} Pt _x system crystallizes in the AuBe₅ structure, but is mixed phase in the range 2.5?<?x?<?4.5. We observe a rapid suppression of long range antiferromagnetic order for small concentrations of Pt. The electrical resistivity, DC magnetic susceptibility, and specific heat of UCu_{5??? x} Pt _x in the concentration range 0.75?≤?x?≤?1 have temperature dependencies at low temperature (T?≤?10?K) consistent with the non-Fermi liquid behaviour found in UCu_{5??? x} Pd _x . Above x?=?1, there is a transition from non-Fermi liquid behaviour to Fermi liquid behaviour with no observation of any spin glass behaviour.     

Heat capacity and the magnetocaloric effect in Pr0.6Sr0.4Mn1–x Fe x O3 manganite

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T _C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS _M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.

     

Imperfection of the clustered perovskite structure, phase transitions, and magnetoresistive properties of ceramic La0.6Sr0.2Mn1.2 ? x Ni x O3 ± δ (x = 0–0.3)

Ceramic samples of lanthanum strontium manganite perovskites La_0.6Sr_0.2Mn_{1.2 ? x} Ni _x O_{3 ± ??} (0 ?? x ?? 0.3) have been investigated using the X-ray diffraction, magnetic (??_ac), ⁵⁵Mn NMR, resistive, and magnetoresistive methods. The specific features of the influence of the composition on the structure and properties of nonstoichiometric manganite perovskites have been established. It has been found that the rhombohedrally (R $\bar 3$ c) distorted perovskite structure contains cation and anion vacancies, as well as nanostructured clusters with Mn²⁺ ions in the A-positions. The substitution of Ni³⁺ ions (r = 0.74 ?) for Mn³⁺ ions (r = 0.785 ?) leads to a decrease in the lattice parameter a, the ferromagnetic-paramagnetic phase transition temperature T _C, and the metal-semiconductor phase transition temperature T _ms due to the disturbance of the superexchange interactions between heterovalent manganese ions Mn³⁺ and Mn⁴⁺. The observed anomalous magnetic hysteresis at 77 K has been explained by the antiferromagnetic effect of the unidirectional exchange anisotropy of the ferromagnetic matrix structure on the magnetic moments of the superstoichiometric manganese Mn²⁺ ions located in nanostructured planar clusters. An analysis of the asymmetrically broadened ⁵⁵Mn NMR spectra of the compounds has revealed a high-frequency electronic superexchange of the ions Mn³⁺ ? O^2? ? Mn⁴⁺; a local heterogeneity of their surrounding by other ions, vacancies, and clusters; and a partial localization of Mn⁴⁺ ions. The local hyperfine interaction fields on ⁵⁵Mn nuclei have been determined. The concentration dependences of the activation energy and charge hopping frequency have confirmed that the Ni ions decrease the electrical conductivity due to the weakening of the electronic superexchange Mn³⁺ ? O^2? ? Mn⁴⁺. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures T _C and T _ms, is caused by scattering at intracrystalline nanostructured heterogeneities, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between magnetic and electrical properties in rare-earth manganites.     

A new ternary carbide Dy2Mn2−xC5 (x=0.6): Preparation,crystal structure,and physical properties

The crystal structure, magnetic and electrical transport properties of the new ternary compound Dy₂Mn_2−xC₅ (x=0.6) have been investigated. According to X-ray powder diffraction the carbide crystallizes in its own structure type, space group I4/mmm, a=3.6421(2), c=15.7713(9) Å, R_B=0.062, R_p=0.134. The crystal structure contains isolated carbon atoms and C₂ dimers in square-bipyramidal holes and distorted bicapped square anti-prisms, respectively. Manganese atomic positions in the structure were found to be not fully occupied. Physical properties were studied in the temperature range down to 0.4 K. The electrical resistivity of Dy₂Mn_2−xC₅ (x=0.6) reveals its basically metallic nature with a positive temperature coefficient above about 30 K. A resistive anomaly at around 20 K indicates the appearance of an antiferromagnetic superzone boundary gap at low temperatures. A phase transition towards long range antiferromagnetic magnetic ordering below 19 K is further revealed by heat capacity and ac susceptibility data. Magnetization data refer to a non-trivial nature of the magnetic ground state which may be caused by the intrinsic structural disorder associated with random vacancies at the Mn site.     

Magnetic properties, magnetoresistance, and Raman spectra of CuV x Cr1 − x S2

The results of investigations of the magnetic and Raman spectra, magnetic properties, and magnetoresistance of vanadium-substituted chromium copper disulfides CuV _x Cr_{1 ? x} S₂ (x = 0.1) are presented.     

Effect of Nd doping on the magnetic properties of charge-ordered Bi0.6−x Nd x Ca0.4MnO3 (0.0≤x≤0.6) perovskite manganites

We have studied structure, magnetic and transport properties of polycrystalline Bi_0.6?x Nd _x Ca_0.4MnO₃ (x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6). Substitution of Nd at Bi sites induces a strong interplay between the magnetic and charge ordering. The charge-ordering temperature (T _CO) decreases with increasing x. Further, the antiferromagnetic ordering temperature (T _N) increases sharply at both extremes and remains nearly constant for x=0.2–0.4. At T<T _N a transition to a metamagnetic glass-like state is also seen. Nd doping also leads to enhancement in the magnetic moment and a concomitant decrease in resistivity up to x=0.3 and then an increase in resistivity up to x=0.5. Furthermore, Nd doping promotes an antiferromagnetic to ferromagnetic type fluctuation in the materials at room temperature, as evidenced by the change in the value of the paramagnetic Curie temperature. We find that the local lattice distortion induced by the size mismatch between the A-site cations and the 6s² character of Bi³⁺ lone pair electrons explains the observed peculiarity in magnetic and transport properties of Nd-doped Bi_0.6Ca_0.4MnO₃.     

Structural distortions and magnetisms in Fe-doped LaMn1-xFexO3 （0 〈 x ≤0.6）

X-ray absorption spectra （XAS） at Mn K-edge and Fe K-edge in LaMnl-xFexO3 show that with the increase of Fe substitution the chemical valence of Mn4＋ decreases, while the chemical valence of Fe3＋ remains unchanged. Structural distortions, such as the rotating and tilting for oxygen octahedron in the unit cell vary with iron content. A phase transition occurs at the Fe content values of 0.2~0.3. The evolutions of rotation and tilting angle of FeO6/MnO6 octahedral may be the vital factors to the structure and magnetism. We believe that the spin configuration of Fe3＋ may vary from the intermediate spin t2g4eg1 （S = 3/2） to the higher spin t2g3eg2 （S = 5/2） near the phase transition.     

Optical spectroscopy and electronic structure of compounds HoNi5 − x Al x (x = 0, 1, 2)

The optical properties of the compounds HoNi_{5 ? x} Al _x (x = 0, 1, 2) have been investigated using the ellipsometric method in the wavelength range from 0.22 to 16 μm. The electronic structure of these intermetallic compounds has been calculated in the local electron-spin density approximation with the correction for strong electronic interactions in the 4f shell of the holmium ions. The experimental dispersion dependences of optical conductivity in the region of interband light absorption have been interpreted based on the results of the calculation of the electron density of states. The plasma and relaxation frequencies of electrons have been determined.     

Lattice,electrical, and magnetic effects in lanthanum manganites La1−x Sr x MnO3 (x = 0.125, 0.150, 0.175)

The temperature dependences of the velocities of transverse and longitudinal hypersound in lanthanum strontium manganites of the composition La_1−x Sr _x MnO₃ (x = 0.125, 0.150, 0.175) have been measured at frequencies of 0.5–0.7 GHz. The structural phase transitions have been revealed, and their positions have been confirmed by data on the electrical resistance and magnetic measurements performed using the same samples. The results obtained have been analyzed in the framework of the model of competing Jahn-Teller distortions and magnetic ordering. The anomalies observed in the behavior of the velocities of longitudinal hypersound have been attributed to the local Jahn-Teller distortions, and their suppression due to the magnetic ordering has been considered a possible factor responsible for the colossal magnetoresistance.     

Structural characterization,magnetic properties and magnetocaloric effects of La0.75Sr0.25Mn1–x Cr x O3 (x = 0.15, 0.20, and 0.25)

The effect of Cr doping on the structural, magnetic and magnetocaloric properties of perovskite manganites La_0.75Sr_0.25Mn_1–x Cr _x O₃ (x = 0.15, 0.20, and 0.25) has been investigated. Crystalline structure and magnetic properties are investigated by using X-ray powder diffraction and magnetization measurements, respectively. All samples show a single phase and are found to crystallize in the distorted rhombohedral system with \( R\overline{3} \,c \) space group. A monotonous change of Curie temperature (T _C), from 314 to 253 K, is observed when content doping increases. Substantial magnetic entropy change reaching 4.20 J/kg K is revealed. Relative cooling power was estimated as well. It was found to reach 289, 323, and 386 J/kg for x = 0.15, 0.20, and 0.25, respectively. Field dependence of the magnetic entropy change showing the power law dependence \( \Delta S_{\rm M} \propto \,\,\left( {\mu_{ 0} \rm H} \right)^{n} \) is also analyzed and discussed.     

p, ρ, T,and x dependences for supercritical water-aliphatic alcohol mixtures

The p, ρ, T, and x relationships for water + aliphatic alcohol (methanol, ethanol, n-propanol) mixtures in near-critical and supercritical conditions obtained by compressibility measurements using the piezometric setup are analyzed. It is shown that the thermodynamic surface (p, ρ, T) _x or (Z, ρ, T) _x in the supercritical region can be satisfactorily described (average relative error 0.83%) by the equation of state in the form of polynomial decomposition of compressibility factor Z = p/(RT) to the series of degrees of density and temperature:      

Multiferroic properties and exchange bias in Bi1-xSrxFeO3 （x = 0-0.6） ceramics

Multiferroic properties and exchange bias(EB) in Bi1-xSrxFeO3(x = 0–0.6) ceramics synthesized by a modified Pechini method are investigated. Sr concentration dependence of structure distorting, ferroelectric properties, and dielectric properties were studied at room temperature. Appropriate Sr doping(x = 0.05–0.2) has been found to decrease the conductivity, enhance ferroelectric properties and give rise to high dielectric constant. Compared with antiferromagnetic BiFeO3 compound, BSFO-x(0 ≤ x ≤ 0.4) ceramics show weak ferromagnetism at room temperature, and their exchange bias field and vertical magnetization shift are observed and exhibit a strong dependence on the content of Sr. This observed EB effect which keeps stable in BSFO ceramics at 10 K tend to vanish at room temperature with Sr concentration over 0.4.     

Microstructure, magnetic and transport properties of magnetoresistive Cu80Fe x Ni20?x (x?=?5, 10 and 15 at.%) ribbons

The generation of copper nanoparticles in an arc furnace by the evaporation/condensation method is systematically investigated. The evaporation/condensation process is advantageous because it allows direct synthesis using pure metals as starting materials avoiding reactions of expensive and potentially poisonous precursors. In the presented system, a transferred direct current arc provides the energy for evaporation of the metal target. In order to prevent an oxidation of the particles in the process, the synthesis is conducted in an atmosphere of inert gases (purity grade 5.0). The arc stability and its effect on particle synthesis are investigated. The experiments reveal excellent long-term arc stability for at least 8?h continuous operation delivering aerosols with high reproducibility (±10?% of average particle size). The influences of the arc current and length, the flow rates of the applied gases and the injection of hydrogen in the plasma zone on the particle size distributions and the agglomerate structure are studied. The produced copper nanoparticles are characterized by scanning mobility particle sizing and scanning electron microscopy. The average particle size could be well controlled in a size range 4?C50?nm by selecting appropriate operating parameters.     

Microstructure and Mössbauer studies on magnetostrictive Tb0.3Dy0.7Fe1.95 − x Nb x (x = 0, 0.025, 0.05 and 0.075)

³⁵Cl NQR measurements were carried out between 77 and 209 K for 3-chlorothiophene, which has a glass transition in a stable crystalline state. An NQR signal with full widths of about 100 kHz at half maximum was observed in this temperature range. The spin-lattice relaxation time T ₁ was measured at the peak frequencies. The activation energy Δε _a obtained from the results of the T ₁ measurements showed a good agreement with those estimated from calorimetric measurements.