Low field magnetoresistance properties of (La0.75Sr0.25)1.05Mn0.95O3 polycrystalline thin films on a-SiO2/Si substrates prepared by ex-situ solid phase crystallization

We report the low field magnetoresistance (LFMR) properties of (La_0.75Sr_0.25)_1.05Mn_0.95O₃(LSMO) films on a-SiO₂/Si substrates, prepared by ex-situ solid phase crystallization of amorphous films deposited by dc-magnetron sputtering at room temperature. The average grain size of the LSMO films was gradually increased with increasing annealing temperature (T _an ) and film thickness. High T _an also caused the growth of an amorphous inter-diffusion layer between a-SiO₂ and LSMO. The highest LFMR values of 16 and 1.0 % were achieved at 100 K, 1.2 kOe and 300 K, 0.5 kOe, respectively, from an LSMO film of 200 nm thickness annealed at 900 °C. In accordance with a modified brick layer model, grain boundary areal resistance gradually increased with increasing T _an and decreasing film thickness due to the penetration of the amorphous inter-diffused phase into the LSMO grain boundary. Improved LFMR values are attributable to modification of the LSMO grain boundary into a more effective spin-dependent scattering center.     

X-ray diffraction study of Ba0.985Na0.015Ti0.985Nb0.015O3, Ba0.6Na0.4Ti0.6Nb0.4O3 and Ba0.3Na0.7Ti0.3Nb0.7O3 compositions

The Ba_0.985Na_0.015Ti_0.985Nb_0.015O₃, Ba_0.6Na_0.4Ti_0.6Nb_0.4O₃ and Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ compositions of the (1 − x) BaTiO₃–xNaNbO₃ (BTNNx) system have been studied by X-ray diffraction and by measurements of dielectric properties. The specimens with composition BTNN (x = 0.015, 0.40 and 0.70) have been refined by the JANA program from X-ray powder diffraction data. Ceramic samples with composition (1 − x) BaTiO₃ + xNaNbO₃ (where x = 0.015, 0.40 and 0.70) were prepared by calcinations from appropriate mixture of BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅. The calcined powder was sintered at temperature range 1200–1400 °C. As the composition x increased from 0.015 (and 0.70), the ferroelectric ceramics (x = 0.015, FE) with tetragonal phase changed to the ferroelectric relaxors (RFE, x = 0.40). RFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to 180 K) due to a frustration between RFE and FE state. These ceramics present the classical ferroelectric character when 0 ≤ x < 0.075 and 0.55 < x ≤ 1 and relaxor character when 0.075 ≤ x ≤ 0.55.     

Structures, electrical and thermal expansion properties of Sr-doped La2Mo2O9 oxide-ion conductors

The oxide-ion conductors (La_1−xSr_x)₂Mo₂O_9−δ (x = 0.01–0.08) were prepared by means of a conventional solid-state reaction. The effects of Sr doping for La site on the structures, electrical and thermal expansion properties of the oxide-ion conductor La₂Mo₂O₉ were investigated using X-ray diffraction, direct current four-probe method, thermal dilatometer and scanning electron microscopy, respectively. The results show that the lattice constants were first decreased, then increased, and decreased again with the increase of Sr doping content. The solid solubility of Sr in (La_1−xSr_x)₂Mo₂O_9−δ is x = 0.07. The sinterability of samples is markedly improved with the increase of Sr doping content. The sintered density of sample x > 0.07 is higher than 96% of its theoretical density. When x > 0.02, doping Sr in La₂Mo₂O₉ can inhibit the excessive growth of grains, thus increases the sintered density of samples. The structural transition temperature shifts to the low side with the increase of Sr doping content, and the phase transition is completely suppressed when the doping content reaches 0.07. The conductivity of sample increases with the increase of Sr doping content. The conductivity of sample x = 0.07 reaches a maximum of 0.078 S/cm and 0.101 S/cm at 800 °C and 850 °C, respectively. In this study, it was demonstrated that doping 7 mol% Sr for La site not only can completely suppress the structural phase transition in La₂Mo₂O₉, but also can effectively enhance electrical conductivity of samples at higher temperature.     

Magnetic study of Ti-substituted NiFe2O4 ferrite

The Ni_1+xTi_xFe_2−2xO₄ (0 ≤ x ≤ 0.1) ferrite systems prepared by a semi-chemical route, have been studied by electron paramagnetic resonance (EPR) at X-band, Mössbauer spectroscopy and magnetization measurements at various temperatures. EPR spectra of these samples comprise generally a broad and asymmetric EPR signal. The variation of g_eff and peak-to-peak line width ΔH_pp, with Ti concentration and temperature are attributed to the variation of dipole–dipole interaction and the superexchange interaction. Mössbauer spectra comprise two sets of sextet attributed to Fe³⁺ at two distinct sites-A and -B. Ti⁴⁺ ions are concluded to occupy the octahedral B-sites. Magnetic moment is found to decrease with the increase of Ti⁴⁺ concentration. The effective magnetic field H_eff at the A-sites also follows a similar trend. The reason is attributed to the canted structure of spins in the Ti-doped samples. An anomalous behavior at x = 0.015 is observed in the properties studied here and some sort of phase change is believed to occur at 473 K in these ferrites.     

Formation of nanoparticles and nanorods via UV irradiation of AgNO3 solutions

The synthesis of silver nanoparticles via UV irradiation of AgNO₃ solutions was controlled by using UV–vis absorption spectra and TEM (transmission electron microscope) images. The UV–vis absorption method is good enough for the general control of synthesis process, and TEM images give us information about size of formed species. For investigated solutions of silver nitrate in ethanol and water, we observed formation of large nanoparticles (size about 100 nm) and nanorods (100 nm in length). Moreover, there was effort to confirm evidence of formation of these particles by using TOF mass spectrometer. Due to laser desorption/ionization process there is only evidence of small silver nanoparticles Ag_x, x ≤ 4 (clusters), and variety of silver compounds Ag_xN_yO_z (x ≤ 5, y ≤ 2, z ≤ 3).     

Structural, magnetic and electrical transport properties for a series of La1−xSrxFe1−xMnxO3 (0.3 ≤ x ≤ 0.7) compounds

The structural, electrical transport and magnetic properties have been studied for compounds: La_1−xSr_xFe_1−xMn_xO₃ (0.3 ≤ x ≤ 0.7). The lattice parameter, a, first decreases with x, and followed by an increase when Sr²⁺ and Mn⁴⁺ was continuously doped. The cell parameters, b and c, slightly decrease with coupled substitution of Sr²⁺ for La³⁺ and Mn⁴⁺ for Fe³⁺. In the paramagnetic temperature range, formation of magnetic clusters is suggested; the sizes of clusters decrease with x up to 0.5, following that they increase sharply with continuing doping. The electrical behaviors of all specimens demonstrate insulators and the electrical resistivity increases with content of Mn⁴⁺ and Sr²⁺ ions doped. A variable range hopping model is suitable to describe electrical transport process for the compounds at low temperature. At high temperature the electrical transport process can be described by bipolaron model for all compounds.     

Crystallization kinetics of new compound of V2O5–PbO–Li2O–Fe2O3 glass using differential thermal analysis

Different glasses with nominal compositions of (70 − x)V₂O₅–xPbO–20Li₂O–10Fe₂O₃ (0 ≤ x ≤ 12.5 mol.%) were successfully obtained by the melt quenching technique. Crystallization kinetics of these glasses was studied under non-isothermal conditions using the formal theory of transformations for heterogeneous nucleation. The procedure was applied to the experimental data obtained by differential thermal analysis, using several measurements at different heating rates. In addition, from the heating rate dependence of the glass transition temperature, the glass transition activation energy was derived. The crystallization results are analyzed and both the activation energy of crystallization process and the crystallization mechanism are characterized in terms of Gao–Wang method. The phases at which the glass crystallizes after the thermal process have been identified by X-ray diffraction. The diffractogram of the transformed material indicates the presence of microcrystallites of LiV₁₅O_35.5, Pb₃(VO₄)₂ and PbO₂ beside the remaining an additional amorphous matrix.     

Dielectric and magnetic properties of 0.7Bi(Fe1−xCrx)O3–0.1BaTiO3–0.2PbTiO3 solid solutions

0.7Bi(Fe_1−xCr_x)O₃–0.1BaTiO₃–0.2PbTiO₃ (x = 0, 0.1, 0.2, 0.3) solid solutions were prepared by the traditional ceramic process. X-ray diffraction results revealed that the samples with x = 0–0.3 showed pure perovskite structure. Frequency and temperature dependences of dielectric constants and dielectric loss of the samples were investigated. Both dielectric constant and the loss tangent increased at given frequencies (100 Hz–1 MHz), while the Curie temperature of the solid solutions decreased with increasing Cr content. Room temperature magnetic hysteresis loops indicated that an appropriate amount of Cr could improve magnetization of the solid solutions.     

Structural, magnetic and electrical properties of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 with 0 ≤ x ≤ 0.30

The structural, magnetic and electrical properties of (La_0.70−xNd_x)Sr_0.30Mn_0.70Cr_0.30O₃ perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinement revealed that these compounds crystallize in a rhombohedral perovskite structure when x = 0, 0.10 and 0.20, while for x = 0.30 the structure becomes orthorhombic (Pbnm). It was found that the substitution of La by Nd reduces the Curie temperature (T_C). The FC, ZFC, M(H) and AC susceptibility measurements show typical canted-antiferromagnetism for the Nd-doped samples, in which a ferromagnetic component coexists with predominant antiferromagnetic interactions. The values of the magnetization (M(H)) decrease very slightly when increasing the Nd content, compared to the undoped sample (M_S values at 5 T and 2 K are, respectively, 47.9, 47.3 and 47.5 emu/g for x = 0.10, 0.20 and 0.30, compared to 48.2 emu/g for x = 0), indicating that the Nd³⁺ contribution is negligible compared to the total moment of the ferromagnetic (Mn/Cr) network. The resistivity increases by several orders of magnitude with Nd-doping and the semi-conducting behaviour persists in the whole temperature range. The interaction between Mn⁴⁺–O–Cr³⁺and Cr³⁺–O–Cr³⁺ is responsible for the semi-conducting state.     

Magnetic and structural properties of the chromium-based Mn1−xCdxCr2S4 thiospinel

The (Mn_1−xCd_x)Cr₂S₄ phases (0 ≤ x ≤ 0.6) have been synthesized from the corresponding elements at 1123 K. These samples were characterized by powder X-ray diffraction (XRD) and magnetic susceptibility. The (Mn_1−xCd_x)Cr₂S₄ compounds crystallize in the space group Fd-3m with cell parameters a = 10.101(6) Å, 10.139(3) Å, 10.165(2) Å, and 10.192(1) Å for x = 0, 0.2, 0.4 and 0.6, respectively. An overall ferrimagnetic behavior is observed for all samples. The ferromagnetic component increases rapidly when manganese is substituted by non-magnetic cadmium, as shown by ZFC/FC measurements. At the same time, the value of the magnetization M₅₀ at 50 kOe, deduced from M(H) loops, also increases with increasing cadmium content because the antiferromagnetic alignment between chromium and manganese spins is progressively lost, leading toward well aligned moments pointing into the same direction. These results are explained by a rearrangement of the chromium spins when Mn located at the tetrahedral sites, is substituted by Cd.     

A polyethylene glycol-assisted route to synthesize Pb(Ni1/3Nb2/3)O3-PbTiO3 in pure perovskite phase

Ferroelectric relaxors (1 − x)Pb(Ni_1/3Nb_2/3)O_3−xPbTiO₃ (PNN-PT) with a composition (x = 0.36) near the morphotropic phase boundary (MPB) were prepared by a polyethylene glycol (PEG)-assisted solid-state reaction route. PEG with a molecular weight of 200 was introduced during the ball milling process of the raw oxide powders. XRD and TG/DSC results demonstrated that the interaction between PbO and PEG favors the transformation of lead-rich pyrochlore to lead-deficient pyrochlore, thus facilitating the formation of perovskite phase. Consequently, pure perovskite powders were synthesized at a relatively low temperature of 850 °C. Ceramics fabricated with the PEG-assisted route show a room temperature dielectric constant of 4987 and a maximum dielectric constant (at T_max) of 24,307 at a frequency of 1 kHz. The piezoelectric constant d₃₃ measured was 460 pC/N.     

Effect of potassium doping on the structural, magnetic and magnetocaloric properties of La0.7Sr0.3−xKxMnO3 perovskite manganites

We investigate the effect of potassium doping on the structural, magnetic and magnetocaloric properties of La_0.7Sr_0.3−xK_xMnO₃ (x = 0.05, 0.1, 0.15 and 0.2) powder samples. Our polycrystalline compounds were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted rhombohedral system with space group. With increasing potassium content, the unit cell volume exhibits a broad maximum around x = 0.15. Magnetization measurements versus temperature showed that all our samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to decrease from 365 K for x = 0 to 328 K for x = 0.2 as well as the saturated magnetization M_sp which shifts from 3.68 μ_B/Mn for x = 0 to 3.05 μ_B/Mn for x = 0.2. The critical exponent γ defined as M_sp (T) = M_sp(0)[1−(T/T_C)]^γ is found to remain almost constant and equal to 0.33 for all our samples. The maximum of magnetic entropy changes |ΔS_max| of La_0.7Sr_0.3−xK_xMnO₃ for x = 0.05 and 0.15 is found to be respectively, 1.37 and 1.2 J kg⁻¹ K⁻¹ under a magnetic field change of 1 T.     

Sample thickness dependence of giant magnetoimpedance under low fields for La0.67Sr0.33Mn0.98Co0.02O3 manganites

The La_0.67Sr_0.33Mn_0.98Co_0.02O₃ plate sintered at 1250 °C shows a low field magnetoresistance (LFMR) effect under fields H < 4.6 kOe. However, the DC magnetoresistance ΔR/R₀ is very small, only −1.84% under H = 4.6 kOe. The character of helical growth was observed on grain surface of La_0.67Sr_0.33Mn_0.98Co_0.02O₃ phase. The LFMR is connected with the interface of grain or grain boundary, while the giant magnetoimpedance under low fields for La_0.67Sr_0.33Mn_0.98Co_0.02O₃ sintered plates strongly depends upon the plate thickness. With an increase of sample thickness, the magnetoimpedance increases and the frequency, where the maximum magnetoimpedance occurs, shifts to low frequencies. A giant magnetoimpedance of −15.6% and a large AC magnetoresistance of −30.5% could be obtained under a very small field H = 600 Oe for the plate with a thickness of 3 mm.     

The magnetic phase diagram of DyFe12−xMox (1.00≤x≤3.00)

The magnetocrystalline anisotropy and magnetic structure of DyFe_12−xMo_x (1.00≤x≤3.00) have been investigated in detail by X-ray diffraction, thermomagnetic analysis, AC magnetic susceptibility, singular point detection technique and angular-magnetization measurement. A magnetic phase diagram of DyFe_12−xMo_x (1.00≤x≤3.00) has been proposed. At room temperature, all DyFe_12−xMo_x compounds exhibit uniaxial anisotropy. At low temperature, a spin reorientation transition of axis-to-cone was observed for DyFe_12−xMo_x compounds with low Mo concentration, x<2.00. The spin reorientation temperature decreases with increasing Mo concentration. For DyFe_12−xMo_x compounds with high Mo concentration, magnetohistory effects were observed below 48 K.     

Magnetic phase diagram of CrTe1−xSbx (0.0≤x≤1.0)

Measurements of the high field magnetization of CrTe_1−xSb_x (0.0≤x≤1.0) were carried out at 4.2 K in pulsed magnetic fields up to 300 kOe. The temperature dependence of the magnetization of CrTe_1−xSb_x was measured in the temperature range from 4.2 K to 800 K. The magnetic phase diagram of CrTe_1−xSb_x (0.0≤x≤1.0) was determined, which is similar to the typical one for the mixed crystals of the layered antiferromagnetic and ferromagnetic compounds proposed by de Gennes.     

Monoclinic phases and stress-relief conditions in (1 − x)Pb(Mg1/3Nb2/3)TiO3–xPbTiO3 solid solutions

A comparative study on heterophase states in perovskite-type solid solutions of (1 − x)Pb(Mg_1/3Nb_2/3)TiO₃–xPbTiO₃ is carried out for compositions near the morphotropic phase boundary. The conditions for mechanical stress relief at elastic matching of phases are analysed at x = const in a wide temperature range. The heterophase states concerned with the presence of the intermediate monoclinic phase are interpreted using the domain state–interface diagrams calculated for x = 0.28, 0.32 and 0.34. It is shown that optimum volume fraction parameters of the domains in the monoclinic phase of the B type are varied in relatively wide ranges and promote complete stress relief with cubic–monoclinic phase coexistence. Two scenarios of stress relief at x = 0.32 are considered in connection with different heterophase states (either tetragonal–monoclinic of the B type or tetragonal–monoclinic of the C type) in a wide temperature range. Possibilities of elastic matching of two polydomain phases (tetragonal–monoclinic of the B type) with almost equal relative widths of the domains in these phases are shown for x = 0.34. The active role of domains of the monoclinic phases in stress relief and forming the planar unstrained interfaces is discussed.     

Subsolidus phase relations of the SrO–Ta2O5–CuO system at 900 °C in air

The subsolidus phase relations of the SrO–Ta₂O₅–CuO system were investigated in air. The samples were equilibrated at 900 °C. The ternary oxide Sr₃Ta₂CuO₉ compound is stable under these conditions. This phase presents a solid solution range, its actual composition being Sr₃Ta_2−xCu_1+xO_9+δ with 0.0 ≤ x ≤ 0.2. Up to about 5 at.% Cu can be incorporated in the Sr_3−xTa_1+xO_5.5+δ phase. Similarities with the SrO–Nb₂O₅–CuO system are discussed.     

New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Studies on structural and electrical properties of Co1−xNixFe1.9Mn0.1O4 ferrite

Nickel-doped iron-deficient cobalt ferrite with small amount of manganese having the chemical composition Co_1−xNi_xFe_1.9Mn_0.1O₄, with x = 0.2, 0.4, 0.6 and 0.8, were prepared by standard double sintering ceramic method. The spinel phase formation was confirmed by X-ray diffraction (XRD). The DC resistivity measurements with temperature indicate a semiconducting behavior showing a linear decrease with increasing temperature and the doping of Ni enhances the resistivity. Maximum resistivity of the order of 10⁹ Ω cm was found for composition x = 0.8. Room temperature dielectric constant measurements with frequency (100 Hz to 1 MHz), show usual dielectric dispersion. Also, the variation of room temperature AC conductivity as a function of frequency were studied and explained by using Maxwell–Wagner two-layer model. The studies on dielectric constant (′), loss tangent (tan δ) and AC conductivity (σ_AC), at four different frequencies (viz., 1, 10, 100 kHz and 1 MHz), with temperature were made.     

Effect of Yb substitution on microstructure, physical and mechanical properties of negative thermal expansion Zr1−xYbxWMoO8−x/2 (x = 0–0.05) ceramic

Cubic Zr_1−xYb_xWMoO_8−x/2 (x = 0–0.05) ceramic was first fabricated by a polymorphous precursor transition method. X-ray diffraction experiment indicates that samples with x ≤ 0.05 are single phase solid solution. The measured bulk density, microstructure, maximal compression strength and Young's modulus are obviously sensitive to Yb substitution level, while none of such sensitivity was found for the lattice parameters, negative thermal expansion coefficients and Vickers hardness. Drilling tests on Zr_0.96Yb_0.04WMoO_7.98 ceramic indicate good machinability, which is often required for quality and shape control in engineering applications.