Chemical synthesis, structural and magnetic properties of nano-structured Co-Zn-Fe-Cr ferrite

Nanoparticles of Co_1−xZn_xFe_2−xCr_xO₄ (x = 0.0-0.5) ferrites were prepared by chemical co-precipitation technique using metal sulphates. The structural and magnetic properties were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and AC susceptibility measurements. X-ray diffraction patterns indicate that the samples possess single phase cubic spinel structure. The lattice constant initially increases for x ≤ 0.3 and thereafter for x > 0.3 it decreases with increasing x. The saturation magnetization (Ms), magneton number (n_B) and coercivity (Hc) decreases with increasing Cr-Zn content x. Curie temperature deduced from AC susceptibility data decreases with increasing x.     

Effect of cooling rate on evolution of superconducting phases during decomposition and recrystallization of (Bi,Pb)-2223 core in Ag-sheathed tape

The reformation of (Bi,Pb)-2223 from the liquid or melt is very important for a melting process of (Bi,Pb)-2223 tape. By combination of quenching experiment with X-ray diffraction (XRD) analysis, the effect of cooling rate on the evolution of three superconducting phases in the (Bi,Pb)-2223 core of Ag-sheathed tape was investigated. The results show that (Bi,Pb)-2223 reformation from the melt seems to experience different routes during slowly cooling at different rates. One is that (Bi,Pb)-2223 phase reformed directly from the melt, and no Bi-2212 participate in this process. The other is that (Bi,Pb)-2223 is converted from the intermediate product, Bi-2212, which formed from the melt during the first cooling stage. Due to the inherent sluggish formation kinetics of (Bi,Pb)-2223 from Bi-2212, only partial (Bi,Pb)-2223 can finally be reformed with the second route.     

Structural, magnetic and electrical properties of Al substituted Ni-Zn ferrite nanoparticles

Nanocrystalline ferrite materials having the general formula Ni_0.7Zn_0.3Fe_2−xAl_xO₄ (0.0 ≤ x ≤ 0.5) have been synthesized by citrate-gel auto combustion method and characterized using X-ray diffraction (XRD), energy dispersive X-ray (EDX), field emission scanning electron microscopy (FE-SEM), dc magnetization, dielectric and impedance spectroscopy measurements. XRD studies confirm that all the samples show single phase cubic spinel structure. The crystallite size of Ni_0.7Zn_0.3Fe_2−xAl_xO₄ (0.0 ≤ x ≤ 0.5) nanoparticles calculated using the Debye-Scherrer formula was found in the range of 13-17 nm. The value of lattice parameter ‘a’ is found to decrease with increasing Al³⁺ content. EDX patterns confirm the compositional formation of the synthesized samples. FE-SEM micrographs show that all the samples have nano-crystalline behavior and particles show spherical shape. The variation of dielectric properties ?′,?″, and tan δ with frequency shows the dispersion behavior which is explained in the light of Maxwell-Wagner type of interfacial polarization in accordance with the Koop's phenomenological theory. The dc magnetization studies infer that magnetic moment of Ni_0.7Zn_0.3Fe_2−xAl_xO₄ (0.0 ≤ x ≤ 0.5) nanoparticles was found to decrease with Al doping. Impedance spectroscopy techniques have been used to investigate the effect of grain and grain boundary on the electrical properties of the synthesized compounds.     

Effect of Th doping on superconductivity in CePt3Si

Alloys from the solid solution Ce_1−xTh_xPt₃Si (x = 0.0, 0.02, 0.04, 0.08, 0.1, 0.2 and 1.0) were prepared by arc-melting. X-ray Rietveld powder analyses revealed that alloys in the compositional range 0 ≤ x ≤ 0.2 crystallize with the CePt₃B-type with a random distribution of Ce and Th atoms in positions 1(b) (1/2, 1/2, z) of the noncentrosymmetric space group P4mm. Th-doping results in a rapid suppression of the superconductivity. The alloy with x = 0.02 shows the onset of superconducting state at T_c = 400 mK, while that with x = 0.04 remains in normal metallic state at least down to 70 mK.     

Phase evolution, crystal structure and dielectric behavior of (1 − x)Nd(Zn0.5Ti0.5)O3 + xBi(Zn0.5Ti0.5)O3 compound ceramics

The phase evolution, crystal structure and dielectric properties of (1 − x)Nd(Zn_0.5Ti_0.5)O₃ + xBi(Zn_0.5Ti_0.5)O₃ compound ceramics (0 ≤ x ≤ 1.0, abbreviated as (1 − x)NZT-xBZT hereafter) were investigated. A pure perovskite phase was formed in the composition range of 0 ≤ x ≤ 0.05. The B-site Zn²⁺/Ti⁴⁺ 1:1 long range ordering (LRO) structure was detected by both XRD and Raman spectra in x ≤ 0.05 samples. However, this LRO structure became gradually degraded with an increase in x. The dielectric behaviors of the compound ceramic at various frequencies were investigated and correlated to its chemical composition and crystal structure. A gradually compensated τ_f value was obtained in (1 − x)NZT-xBZT microwave dielectrics at x = 0.03, which was mainly due to the dilution of dielectric constant in terms of Claussius-Mossotti differential equation.     

Relationship between structure and properties in high-temperature Bi(Al0.5Fe0.5)O3-PbTiO3 piezoelectric ceramics

Ceramic samples of xBi(Al_0.5Fe_0.5)O₃-(1 − x)PbTiO₃ (BAF-PT, x = 0.05-0.5) solid solutions were fabricated using the conventional solid state reaction method. X-ray diffraction analysis revealed that all compositions can form single perovskite phase with tetragonal symmetry. The relationship between the tetragonal lattice parameters, tetragonality c/a, cell volume, and ferro-piezoelectric characterization as a function of x was systematically investigated. The BAF modification can effectively improve the poling condition at a proper BAF content. A combination of piezoelectric constant of d₃₃ (50-60 pC/N), electromechanical planar coupling coefficients of k_p (20.3-22.5%), and high Curie temperature T_c (>478 °C) suggested that BAF-PT could be a good candidate for high-temperature piezoelectric applications.     

Large magnetic entropy change near room temperature in La0.7(Ca0.27Ag0.03)MnO3 perovskite

In this paper, the magnetic properties and magnetocaloric effect (MCE) of La_0.7(Ca_1−xAg_x)_0.3MnO₃ (x = 0, 0.1, 0.2, 0.7, and 1) powder samples are reported. Our polycrystalline compounds were synthesized using the solid state reaction method at high temperature. Magnetization measurements versus temperature showed that all our samples exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, T_C, has been found to increase from ∼250 K for x = 0-270 K for x = 1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La_0.7(Ca_0.27Ag_0.03)MnO₃ composition, the maxima of the magnetic entropy changes from the applied magnetic field (ΔS_M) at 2 and 5 T are about 4.5 and 7.75 J/kg K, respectively, at the Curie temperature of ∼263 K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271 J/kg for the applied fields of 2 and 5 T, respectively. Due to the large ΔS_M, large RCP, and high Curie temperature, La_0.7(Ca_0.27Ag_0.03)MnO₃ is promising for application in potential magnetic refrigeration near room temperature.     

Synthesis, structural and electrical characterization of Sb substituted spinel nickel ferrite (NiSbxFe2−xO4) nanoparticles by reverse micelle technique

Spinel nickel ferrite with nominal composition NiFe_2−xSb_xO₄ (where x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1) has been synthesized by the reverse microemulsion method. The samples synthesized were characterized by XRD, FTIR, TGA/DTGA, SEM, AFM, Mossbauer Spectroscopy and DC electrical resistivity measurements. The XRD analysis confirmed the formation of single spinel phase and the crystallite size was found to be in the range of 8-38 nm. The particle size of the synthesized samples was also confirmed by the AFM and SEM which was found in the range of 5-45 nm and 10-45 nm, respectively and this size is small enough for obtaining the suitable signal-to-noise ratio in high density recording media. The Mossbauer spectra of samples showed two well-resolved Zeeman patterns corresponding to A and B sites and also observed a doublet at higher substitution. The DC electrical resistivity showed an interesting behavior with temperature and observed a metal-to-semiconductor transition temperature (T_M-S) which suggests that the material can be applied for switching applications. The resistivity increases with the increase in Sb-content and it suggests that the material can be fruitfully used for applications in microwave devices.     

X-ray diffraction and dielectric studies across morphotropic phase boundary in (1 − x) [Pb(Mg0.5W0.5)O3]-xPbTiO3 ceramics

We present here the results of comprehensive X-ray diffraction and dielectric studies on several compositions of (1 − x)[Pb(Mg_0.5W_0.5)O₃]-xPbTiO₃ (PMW-xPT) solid solution across the morphotropic phase boundary. Rietveld analysis of the powder X-ray diffraction data reveals cubic (space group Fm3m) structure of PMW-xPT ceramics for the compositions with x ≤ 0.42, tetragonal (space group P4mm) structure for the compositions with x ≥ 0.72 and coexistence of the tetragonal and cubic phases for the intermediate compositions (0.46 ≤ x ≤ 0.68). Temperature dependence of the dielectric permittivity above room temperature exhibits diffuse nature of phase transitions for the compositions in the cubic and two phase region while the compositions with tetragonal structure at room temperature exhibit sharp ferroelectric to paraelectric phase transition. The PMW-xPT compositions with coexistence of tetragonal and cubic phases at room temperature exhibit two anomalies in the temperature dependence of the dielectric permittivity above room temperature. Using results of structural and dielectric studies a partial phase diagram of PMW-xPT ceramics is also presented.     

Ferromagnetism in ZnTe:Cr film grown on Si(1 0 0)

Zn_1−xCr_xTe (x = 0.0 and 0.05) films were grown on Si(1 0 0) substrate by using thermal evaporation method. The structure of the films was investigated by X-ray diffraction and it showed the formation of ZnCrTe phase with an amorphous background, which indicated poor crystallinity. Composition analysis by XPS disclosed the presence of antiferromagnetic Cr₂O₃ and Cr precipitates. Magnetic domains were observed by using magnetic force microscopy at ambient temperature and the result showed anisotropic domains with an average size of 3.5 nm. Magnetic field dependence of magnetic moment measurements showed obvious hysteresis loop with a coercive field of 121 Oe at 300 K. Temperature dependence of magnetic moment showed short-range ferromagnetic order. The Curie temperature was estimated to be 354.5 K.     

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.     

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.     

Magnetic properties of Sn-Mg substituted strontium hexaferrite nanoparticles synthesized via coprecipitation method

Nanoparticles of Sn-Mg substituted strontium hexaferrite with the composition of SrFe_12−x(Sn_0.5 Mg_0.5)_xO₁₉ (x = 0.0-1.0) were synthesized by chemical coprecipitation method. Deionized water/ethanol (50/50) was used as the solvent. The single phase strontium hexaferrites were obtained at pH 13 and Fe³⁺/Sr²⁺ molar ratio of 9 after calcination at 800 °C. The mean particle size of samples was decreased from 82 to 56 nm with increasing the Sn-Mg content from x = 0.0 to x = 0.8. The effect of Sn-Mg substitution on magnetic properties of hexaferrites was studied using vibrating-sample magnetometer. It was found that increasing the Sn-Mg from x = 0.0 to x = 0.8 reduced the coercivity from 4728.9 to 1455.5 Oe and increased the saturation magnetization from 51.34 to 65.49 emu/g. A vector network analyzer was used to investigate the microwave absorption properties. According to microwave measurements, doped strontium hexaferrite composites had much more effective electromagnetic absorption properties than undoped strontium hexaferrite composite.     

Magnetic and magnetocaloric properties of ternary Gd-Co-Al bulk metallic glasses

Bulk metallic glasses (BMGs) with compositions of Gd₅₅Co_xAl_45−x (15 ≤ x ≤ 30) and Gd₆₀Co_yAl_40−y (15 ≤ y ≤ 30) were synthesized by an injection casting technique. Temperature dependence of magnetization of the BMGs indicates that their Curie temperatures can be tailored between 96 and 143 K by varying Gd and Co concentration. The magnetic entropy changes of the BMGs are greater than 9.0 J/kg K except for the Gd₅₅Co₃₀Al₁₅ glass that exhibits a reduced magnetization due to its large Co content. The relative cooling powers of the BMGs are greater than those of any other crystalline compounds and decrease with the increasing Co content.     

Rhombohedral-monoclinic phase transition related to grain orientation in Zr-rich Pb(ZrxTi1−x)O3 thin films

Pb(Zr_xTi_1−x)O₃ thin films with mixed orientations of (1 1 1) and (1 0 0) were prepared on Pt/Ti/SiO₂/Si substrate by sol-gel technique. The compositions of PZT thin films are chosen as x = 0.55 and x = 0.58. Both of the compositions are in the rhombohedral phase region of the Pb(Zr_xTi_1−x)O₃ phase diagram, but the former is near the monoclinic phase existence region, and the latter is far from the monoclinic phase existence region. Rhombohedral-monoclinic phase transitions are reported in both of the thin films. The results show that the phase transition is related to the grain orientation. Phase transitions in the films are clearly identified: rhombohedral phase transforms to M_B phase in (1 1 1)-oriented grains, and rhombohedral phase transforms to M_A phase in (1 0 0)-oriented grains. The remnant polarization is determined by the content of (1 1 1)-oriented grains. It is shown that the remnant polarization is greater in the film with higher content of (1 1 1)-oriented grains.     

Electrical, magnetic, and Hall properties of AlxCoCrFeNi high-entropy alloys

This investigation explores the electrical and magnetic properties of as-cast, -homogenized, and -deformed Al_xCoCrFeNi (C-x, H-x, and D-x, respectively) alloys at various temperatures from 4.2 to 300 K. Experimental results reveal that carrier density of the alloys is of 10^22-23 cm⁻³. H-x has a carrier mobility of 0.40-2.61 cm² V⁻¹ s⁻¹. The residual electrical resistivity of the alloys varies from 100 to 220 μΩ cm. The temperature coefficient of resistivity (TCR) of H-2.00 is small (82.5 ppm/K). Therefore, defects in the lattice dominate electrical transportation. Some compositions exhibit Kondo-like behavior. At 300 K, H-0.50, H-1.25, and H-2.00 are ferromagnetic, while H-0.00, H-0.25, and H-0.75 are paramagnetic. Al and AlNi-rich phases reduce the ferromagnetism of single FCC and single BCC H-x, respectively. Spin glass behavior of some compositions is also observed. Alloys H-x are of the hole-like carrier type, and ferromagnetic H-x exhibits an anomalous Hall effect (AHE).     

Phase, structural, and magnetocaloric properties of high temperature annealed LaFe11.6Si1.4BX

The phase relation, microstructural, hysteresis, Curie temperature, and magnetocaloric effects of LaFe_11.6Si_1.4B_x (x = 0.1, 0.2, 0.3, 0.4, and 0.5) prepared by arc-melting and then annealed at 1373 K (1.5 h) + 1523 K (5 h) were investigated. It was found that the main phase is NaZn₁₃-type phase, the impurity phases include α-Fe, Fe₂B, and small amount of La₅Si₃. The boron atom can dissolve into the crystal lattice of LaFe_11.6Si_1.4B_x to form interstitial solid solution, but the content of solid solution is not up to x = 0.5. For LaFe_11.6Si_1.4B_x (x = 0.1, 0.3, and 0.5) compounds, the Curie temperature T_C increases from 190.6 to 198.3 K with the increasing of B content from x = 0.1 to 0.5. The first order magnetic transition behavior becomes weaker and magnetic entropy change ΔS_M (T, H) drops with the increasing of B content, respectively. However, ΔS_M (T, H) still remains a large value, 11.18 J/kg K, when x reaches to 0.5 at 0-2 T. An attractive feature is that both thermal and magnetic hysteresis can be reduced remarkably by introducing B. The maximum magnetic hysteresis loss near T_C drops from 22.52 to 4.95 J/kg when the content of B increases from x = 0.1 to 0.5.     

Effects of lanthanum doping on the preferred orientation, phase structure and electrical properties of sol-gel derived Pb1-3x/2Lax (Zr0.6Ti0.4)O3 thin films

Pb_1-3x/2La_x (Zr_0.6Ti_0.4)O₃ thin films (0 ≤ x ≤ 0.08) were prepared on the Pt (1 1 1)/Ti/SiO₂/Si (1 0 0) substrates by a sol-gel method. The morphology, preferred orientation, phase structure, dielectric and ferroelectric properties of the films have been investigated. Our results show that lanthanum doping is favorable to enhance crystalline and obtain (1 0 0)-preferred orientation of the films. Meanwhile, it is suggested that the films undergo a structure change from “rhombohedral” phase to monoclinic phase as the lanthanum-doped content is increased to x ≈ 0.05. Results of dielectric properties and ferroelectric properties indicate that lanthanum doping contributes to improve film dielectric constant and dielectric loss while it brings about a striking decrease in remnant polarization value. Possible explanations for the variations of electrical properties have been discussed in terms of preferred orientation, phase structure and large lattice distortion.     

Sintering behaviors, magnetic and electric properties of Bi-Zn co-doped Co2Y ferrites

The Bi and Zn substitution effects on the sintering behaviors, magnetic and electric properties of hexagonal ferrites with a composition of 2(Ba_1−xBi_xO)·2(Zn_yCo_0.8−yCu_0.2O)·6(Fe_2−x/3Zn_x/3O₃) were investigated. The results showed that the addition of Bi and Zn can significantly promote Co₂Y densification. The Y phase may be triggered to decompose into M and spinel phases at high sintering temperatures (above 1050 °C) for samples with excess Bi (x = 0.2) substitution, which resulted in densification and magnetic properties degradation. Co₂Y ferrites with x = 0.1 and y = 0.4 sintered at 1050 °C show a relative density of 94%, a high initial permeability of 4.5, a quality factor (Q) of 50.