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1.
The nanocrystalline SrFe12 O 19 materials were prepared by a sol-gel auto-combustion method using different fuels such as citric acid, dextrose, aniline, and hexamine. The combustion product obtained from all the fuels except from that of aniline show a single phase of SrFe12 O 19 materials upon annealing at 1000 °C/2 h. The combustion product obtained from aniline as fuel shows SrFe12 O 19 as the main phase with α-Fe2 O 3 as impurity. No notable change in lattice parameters is observed due to variation in fuels for SrFe12 O 19 materials. With a little change in the NIR relative reflectance (72–85 %) on fuels, the different SrFe12 O 19 materials display high NIR reflectance in the wavelength range, 1500–2500 nm. The photoluminescence emission spectra of SrFe12 O 19 materials reveal a broad emission peak at ~350 nm which is reminiscent to the Ba-based hexaferrite, BaFe12 O 19. The FESEM images expose quite dissimilar morphology for the various fuels used in the synthesis of SrFe12 O 19 materials. Hysteresis loops for all the nanocrystalline SrFe12 O 19 materials observed under the applied field of ±1.5 T at room temperature exhibit hard ferromagnetic property. The SrFe12 O 19 materials produced from glycine and aniline as fuels exhibit highest and lowest M s values of 61.3 and 50.5 emu/g, respectively.  相似文献   

2.
We present an extensive study of the magnetic properties of a novel La0.5Ba0.5MnO3 perovskite material prepared by the hydrothermal method. The explored sample was structurally studied by the x-ray diffraction (XRD) method which confirms the formation of a pure cubic phase of a perovskite structure with Pm3m space group. The magnetic properties were probed by employing temperature M (T) and external magnetic field M (μoH) dependence of magnetization measurements. A magnetic phase transition from ferromagnetic to paramagnetic phase occurs at 339 K in this sample. The maximum magnetic entropy change (\(\left | {{\Delta } S}_{M}^{\max } \right |\)) took a value of 1.4 J kg??1 K??1 at the applied magnetic field of 4.0 T for the explored sample and has also been found to occur at Curie temperature (TC). This large entropy change might be instigated from the abrupt reduction of magnetization at TC. The magnetocaloric effect (MCE) is maximum at TC as represented by M (μoH) isotherms. The relative cooling power (RCP) is 243.2 J kg??1 at μoH =?4.0 T. Moreover, the critical properties near TC have been probed from magnetic data. The critical exponents δ, β, and γ with values 3.82, 0.42, and 1.2 are close to the values predicted by the 3D Ising model. Additionally, the authenticity of the critical exponents has been confirmed by the scaling equation of state and all data fall on two separate branches, one for T < TC and the other for T > TC, signifying that the critical exponents obtained in this work are accurate.  相似文献   

3.
The sintering temperature for the production of Y3Ba5Cu8O18 (Y-358) preform powders synthesized in sol-gel spontaneous combustion technique was optimized. A large single-grain bulk Y-358 crystal was fabricated employing a top-seeded melt-growth technique utilizing the optimally sintered preform powders (i.e., at 900 °C for 12 h). Structural, microstructural, elemental, and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscopy, and SQUID, respectively. The structural characterization indicated that the sample is highly textured in (00l) direction. The Y-358 phase fractions were estimated in both preform powders and bulk sample using Rietveld refinement. The onset of superconducting transition is observed at 92.5 K, and the curve is very sharp indicative of the high quality of the produced bulk sample. The field dependence of critical current density (Jc) was determined at 77 K, and the self-field Jc was found to be ~26 kA/cm2. A magnetic field of 0.27 T was trapped by the sample at 77 K.  相似文献   

4.
The critical behavior of perovskite manganite La0.67Ba0.33Mn0.95Fe0.05O3 at the ferromagnetic–paramagnetic has been analyzed. The results show that the sample exhibited the second-order magnetic phase transition. The estimated critical exponents derived from the magnetic data using various such as modified d’Arrott plot Kouvel–Fisher method and critical magnetization M(T C, H). The critical exponents values for the La0.67Ba0.33Mn0.95Fe0.05O3 are close to those expected from the mean field model β = 0.504 ± 0.01 with T C = 275661 ± 0.447 (from the temperature dependence of the spontaneous magnetization below T C ), γ = 1.013 ± 0.017 with T C = 276132 ± 0.452 (from the temperature dependence of the inverse initial susceptibility above T C ), and δ = 3.0403 ± 0.0003. Moreover, the critical exponents also obey the single scaling equation of M(H, ε) = |ε| β f ±(H/|ε| β+γ ).  相似文献   

5.
X-ray diffraction data are presented for combustion products in the Al-W-N system. New, nonequilibrium intermetallic compounds have been identified, their diffraction patterns have been indexed, and their unit-cell parameters have been determined. The phases α-and β-WAl4 are shown to exist in three isomorphous forms, differing in unit-cell centering. The phases α′-, α″-, and α?-WAl4 are monoclinic, with a 0 = 5.272 Å, b 0 = 17.770 Å, c 0 = 5.218 Å, β = 100.10°; point groups C12/c1, A12/n1, I12/a1, respectively. The phases β′-, β″-, and β?-WAl4 are monoclinic, with a 0 = 5.465 Å, b 0 = 12.814 Å, c 0 = 5.428 Å, β = 105.92°; point groups A112/m, B112/m, I112/m, respectively. The compounds WAl2 and W3Al7, identified each in two isomorphous forms, differ in cell metrics (doubling) but possess the same point group: P222. WAl 2 : orthorhombic, a 0 = 5.793 Å, b 0 = 3.740 Å, c 0 = 6.852 Å. WAl 2 : orthorhombic, a 0 = 11.586 Å, b 0 = 3.740 Å, c 0 = 6.852 Å. W3Al 7 : orthorhombic, Pmm2, a 0 = 6.225 Å, b 0 = 4.806 Å, c 0 = 4.437 Å. W3Al 7 : orthorhombic, Pmm2, a 0 = 12.500 Å, b 0 = 4.806 Å, c 0 = 8.874 Å. The new phase WAl3: triclinic, P1, a 0 = 8.642 Å, b 0 = 10.872 Å, c 0 = 5.478 Å, α = 104.02°, β = 64.90°, γ = 107.15°.  相似文献   

6.
We study the magnetic field vs. temperature (HT) and pressure vs. temperature (PT) phase diagrams of the T c ≈ 5.5 K superconducting phase in Pd x Bi2Te3 (x ≈ 1) using electrical resistivity versus temperature measurements at various applied magnetic fields (H) and magnetic susceptibility versus temperature measurements at various applied magnetic fields (H) and pressure (P). The HT phase diagram has an initial upward curvature as observed in some unconventional superconductors. The critical field extrapolated to T = 0 K is H c (0) ≈ 6–10 kOe. The T c is suppressed approximately linearly with pressure at a rate d T c /d P ≈ ?0.28 K/GPa.  相似文献   

7.
In this work, we studied in detail the magnetic and magnetocaloric properties of the La0.7Ca0.2Ba0.1MnO3 compound according to the phenomenological model. Based on this model, the magnetocaloric parameters such as the maximum of the magnetic entropy change ΔS M and the relative cooling power (RCP) have been determined from the magnetization data as a function of temperature at several magnetic fields. The theoretical predictions are found to closely agree with the experimental measurements, which make our sample a suitable candidate for refrigeration near room temperature. In addition, field dependences of \({{\Delta } S}_{\mathrm {M}}^{\max }\) and RCP can be expressed by the power laws \({\Delta S}_{\mathrm {M}}^{\max }\approx a\)(μ 0 H) n and RCP ≈b(μ 0 H) m , where a and b are coefficients and n and m are the field exponents, respectively. Moreover, phenomenological universal curves of entropy change confirm the second-order phase transition.  相似文献   

8.
The magnetic properties and the critical behavior in Sr1.5Nd0.5MnO4 have been investigated by magnetization measurements. The magnetic data indicate that the compound exhibits a second-order phase transition. The estimated critical exponents derived from the magnetic data using various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical magnetization isotherms M (T C, H). The critical exponent values for this compound was found to match well with those predicted for the mean-field model (δ = 2.212 ± 0.124, γ = 0.975 ± 0.018, and β = 0.502 ± 0.012) at T C = 228.59 ± 0.17. The critical exponent γ is slightly inferior than predicted from the mean-field model. Such a difference may be due, within the context of the quenched disorder and essentially the presence of the Griffiths phase. The temperature variation in the effective exponent (γ eff) is similar to those for disordered ferromagnets.  相似文献   

9.
We report the results of magnetic, magnetocaloric properties, and critical behavior investigation of the double-layered perovskite manganite La1.4(Sr0.95Ca0.05)1.6Mn2O7. The compounds exhibits a paramagnetic (PM) to ferromagnetic (FM) transition at the Curie temperature T C = 248 K, a Neel transition at T N = 180 K, and a spin glass behavior below 150 K. To probe the magnetic interactions responsible for the magnetic transitions, we performed a critical exponent analysis in the vicinity of the FM–PM transition range. Magnetic entropy change (??S M) was estimated from isothermal magnetization data. The critical exponents β and γ, determined by analyzing the Arrott plots, are found to be T C = 248 K, β = 0.594, γ = 1.048, and δ = 2.764. These values for the critical exponents are close to the mean-field values. In order to estimate the spontaneous magnetization M S(T) at a given temperature, we use a process based on the analysis, in the mean-field theory, of the magnetic entropy change (??S M) versus the magnetization data. An excellent agreement is found between the spontaneous magnetization determined from the entropy change [(??S M) vs. M 2] and the classical extrapolation from the Arrott curves (µ0H/M vs. M 2), thus confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in this system and in other compounds as well.  相似文献   

10.
Amorphous Co75Er17B8 ribbons were prepared by the melt spinning technique, and their magnetic properties were studied. Mean field theory was used to describe the temperature dependence of magnetization. High-field magnetization studies performed in magnetic fields up to 15 T have revealed a magnetic behavior typical of a non-collinear magnetic structure of Er and Co sublattices. The simulated magnetization curves show the existence of two critical fields at H cri1 =?9.5 T and H cri2 =?94.2 T, corresponding to collinear ferrimagnet, and collinear field-forced ferromagnetic behaviors. The high value of H cri2 highlights the strong antiferromagnetic interaction between Er and Co sublattices. From the non-collinear regime, the inter-subnetwork molecular field coefficients of the ferrimagnetic alloy were accurately evaluated. In addition, it is shown that the region of canted moments can be satisfactorily described by a phase diagram in the H-T plane.  相似文献   

11.
A systematic study on the effect of monovalent cation doping on structural, magnetic, and magnetocaloric properties of Pr0.85 A 0.15MnO3 (A = Ag and K) samples synthesized by a sol-gel method has been carried out. The crystal structure and morphology have been worked by X-ray diffraction (XRD) and scanning electron microscopy (SEM) imaging measurements. The XRD results indicate that both samples have orthorhombic structure. Magnetization versus temperature measurements show that our samples display a ferromagnetic-to-paramagnetic phase transition with increasing temperature. The ferromagnetic-to-paramagnetic phase transition temperature (T C) values were found as 74 and 116 K for Pr0.85Ag0.15MnO3 and Pr0.85 K 0.15MnO3, respectively. The magnetic entropy changes were evaluated from isothermal magnetization curves measured at various temperatures near T C by steps of 4 K. The values of the magnetic entropy change were determined as 0.99 and 1.39 J kg ?1 K ?1 for Pr0.85Ag0.15MnO3 and Pr0.85 K 0.15MnO3 under external field changes of 10 kOe, respectively.  相似文献   

12.
Magnetic entropy change (?ΔS M ) of Nd0.67 Ba0.33Mn0.98Fe0.02O3 perovskite have been analyzed by means of theoretical models. An excellent agreement has been found between the (ΔSM) values estimated by Landau theory and those obtained using the classical Maxwell relation. In order to estimate the spontaneous magnetization M s pont(T), we used the mean-field theory to analyses the (ΔSM) vs. M 2 data. The obtained M s pont(T) values are in good agreement with those found from the classical extrapolation from the Arrott plots(H/M vs. M 2), confirming that the magnetic entropy is a valid approach to estimate the spontaneous magnetization in our system. At a relatively low magnetic field, a phenomenological model has been used to estimate the values of the magnetic entropy change. The results are in good agreement with those obtained from the experimental data using Maxwell relation.  相似文献   

13.
La0.45Dy0.05Ca0.5Mn0.9V0.1O3, prepared by solid-state route, was characterized using x-ray diffraction at room temperature. The Rietveld refinement shows that the sample crystallizes in orthorhombic structure with Pbnm space group. A secondary phase LaVO4 has been also detected. The temperature dependence of the magnetization was investigated to determine the characteristics of the magnetic transition. The sample exhibits a paramagnetic-ferromagnetic transition (PM-FM) at T C = 81 ± 0.7 K when temperature decreases. The study of the inverse of susceptibility reveals the presence of ferromagnetic clusters in the paramagnetic region. A metamagnetic transition was observed from the M(H) curves and the magnetic entropy change was calculated from magnetization curves at different temperatures in order to evaluate the magnetocaloric effect.  相似文献   

14.
Fluctuations in the conductivity of Ba0.72K0.28Fe2As2 single crystal are studied systematically by resistance measurements as a function of temperature and magnetic field. A clear Maki?Thompson and Aslamakov?Larkin (MT–AL) two- to three-dimensional (2D–3D) crossover is found on the excess conductivity (Δσ) curves as the temperature approaches the superconducting critical temperature, T c. 3D fluctuations in superconductivity are realized near T c that are well fitted to experimental data by the 3D Aslamazov–Larkin theory. The Maki–Thompson model shows a 2D conductivity fluctuation above the 2D-3D temperature transition, T 0, which depends on magnetic field. Results show that the 2D-3D dimensional crossover moves to lower temperature with increasing magnetic field. The values of the transition temperature and the crossover in the reduced temperature, ln(ε 0), as functions of magnetic field were used to determine the coherence length and the lifetime, τ φ , of the fluctuational pairs at the temperature of 35 K. Analysis of the Ba0.72K0.28Fe2As2 single crystal gives a value of 3.76 × 10??12 s for the τ φ in the absence of magnetic field and it decreases to 2.4 × 10??12 s in magnetic field of 13 T.  相似文献   

15.
We have developed processes for the synthesis of Ni0.75Zn0.25Fe2–xLnxO4 ferrite solid solutions with the spinel structure and investigated the effect of the rare-earth elements Nd, Gd, Yb, and Lu on the chemical composition, extent, lattice parameters, and magnetic properties of the solid solutions. The results demonstrate that rare-earth solubility in the parent spinel reaches ≈2.5 at %, which leads to changes in the magnetic characteristics of the material, in particular in its saturation magnetization Ms, TC, and coercive force Hc.  相似文献   

16.
As part of a search for new spintronic materials, we have studied the magnetic properties of the CuGa0.94Mn0.06Te2 chalcopyrite solid solution in the range 2–400 K in weak and strong magnetic fields. Magnetization isotherms, σ(H), were obtained in magnetic fields of up to 3980 kA/m. σ(T) data were collected in two ways: the sample was cooled in a magnetic field or in zero field. The experimental data were analyzed by fitting to the Langevin function. The data are adequately represented by this relation in the case when the magnetic moment of the clusters is μcl = 23.4μB and the concentrations of magnetic clusters and noninteracting Mn2+ ions are n cl = 2.4 × 1025 m?3 and n pm = 5.7 × 1025 m?3, respectively. The calculated average cluster size is d cl = 33 Å, the number of Mn2+ ions per cluster is z = 21 atoms per cluster, and the magnetic moment per Mn2+ ion in the clusters is μMn = 1.1μB. This μMn value is far below the theoretical magnetic moment of the Mn2+ ion in the electronic configuration d 5(5.9μB), suggesting antiferromagnetic exchange interaction.  相似文献   

17.
The La0.78Dy0.02Ca0.2MnO3 (LDCMO) compound prepared via high-energy ball-milling (BM) presents a ferromagnetic-to-paramagnetic transition (FM-PM) and undergoes a second-order phase transition (SOFT). Based on a phenomenological model, magnetocaloric properties of the LDCMO compound have been studied. Thanks to this model, we can predict the values of the magnetic entropy change ΔS, the full width at half-maximum δ T FWHM, the relative cooling power (RCP), and the magnetic specific heat change ΔC p for our compound. The significant results under 2 T indicate that our compound could be considered as a candidate for use in magnetic refrigeration at low temperatures. In order to further understand the FM-PM transition, the associated critical behavior has been investigated by magnetization isotherms. The critical exponents estimated by the modified Arrott plot, the Kouvel–Fisher plot, and the critical isotherm technique are very close to those corresponding to the 3D-Ising standard model (β = 0.312 ± 0.07, γ = 1.28 ± 0.02, and δ = 4.80).Those results revealed a long-range ferromagnetic interaction between spins.  相似文献   

18.
The hexaferrite BaFe12 O 19 phase was synthesized through the mechanical alloying process followed by subsequent annealing. Rietveld refinements of as-milled powder annealed at 700 °C confirm the formation of the BaFe12 O 19 phase with the presence of an important amount of the α-Fe2 O 3 phase. Thus, prior mechanical milling shows much lower reaction temperature and less reaction time compared to conventional methods. Further annealing up to 900 and 1100 °C could not enable the formation of a single BaFe12 O 19 phase, reaching an optimum phase composition ratio close to BaFe12 O 19/ α-Fe2 O 3 70/30. The crystallite size was found to be in the nanoscale level but increases with increasing temperature (BaFe12 O 19 = 20–62 nm; α-Fe2 O 3 = 31–74 nm). SEM micrographs show that as the annealing temperature rises, the particles become more regular with sharp edges and hexagonal-like shapes. Magnetic measurements reveal that both M s and M r increase with annealing temperature to reach maximum values at 900 °C then remain unchanged, associated with phase composition. The coercivity H c increases upon annealing up to 700 °C to a much higher value, from 1.7 kOe for as-milled powder to 4.8 kOe. Its value then decreases, attributed to grain (particle) growth (formation of larger particles) due to high annealing temperatures: 900–1100 °C. The obtained composites show very interesting magnetic properties and can be considered for potential applications, such as hyperthermia, heavy metal and dye removal, and hard/soft magnetic composites.  相似文献   

19.
In the present work, a novel MgAl2Ti3O10 ceramic was obtained using a traditional solid-state reaction method. X-ray diffraction and energy dispersive spectrometer showed that the main MgAl2Ti3O10 phase was formed after sintered at 1300–1450 °C. With rising the sintering temperature from 1300 to 1450 °C, the bulk density (ρ), relative permittivity (ε r ) and Q?×?f value firstly increased, reached the maximum values (3.61 g/cm3, 14.9, and 26,450 GHz) and then decreased. The temperature coefficient of resonator frequency (τ f ) showed a slight change at a negative range of ??94.6 to ??83.7 ppm/°C. When the sintering temperature was 1400 °C, MgAl2Ti3O10 ceramics exhibited the best microwave dielectric properties with Q?×?f?=?26,450 GHz, ε r ?=?14.9 and τ f ?=???83.7 ppm/°C.  相似文献   

20.
Doped topological insulators (TI) Bi2?x Nd x Se3 single crystals were prepared by the self-flux method. The phase structure, magnetic properties, and electrical transport properties of the samples were studied. The X-ray diffraction (XRD) patterns of the sample indicate an incorporation of Nd into the Bi2Se3, and the crystal can be easily cleaved with silvery surface. The Bi2?x Nd x Se3 sample shows a giant magnetoresistance (MR) with different magnetic field. The positive magnetoresistance (MR) can reach 190 % at the field of 9 Tesla when the field is perpendicular to ab-plane of the crystal. In addition, at low magnetic fields, the MR exhibits a weak antilocalization (WAL) cusp.  相似文献   

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