Effect of Gd doping on magnetic, electric and dielectric properties of MgGdxFe2−xO4 ferrites processed by solid state reaction technique

MgGd_xFe_2−xO₄ (x = 0.0, 0.05, 0.1 and 0.15) ferrites, with improved dc resistivity, initial permeability, saturation magnetization, and extremely low relative loss factor, have been synthesized by solid state reaction technique. The microstructures, electric, dielectric and magnetic properties have been investigated by means of X-ray diffraction, Keithley 2611 system, impedance analyzer and VSM respectively. The addition of Gadolinium in Mg ferrite has been shown to play a crucial role in enhancing the electric, dielectric and magnetic properties. The dc resistivity is increased by two orders of magnitude as compared to Mg ferrite. Saturation magnetization has been increased by two times and remnant magnetization has been increased by more than three times due to the doping of Gd³⁺ ions in Mg ferrite. The relative loss factor was found to have very low values and is of the order of 10⁻⁴-10⁻⁵ in the frequency range 0.1-30 MHz. The variations of electric, dielectric and magnetic properties of the samples have been studied as a function of frequency and Gd³⁺ ions concentration measured at room temperature. High resistivity and improved magnetic properties can be correlated with better compositional stoichiometry and the replacement of Fe³⁺ ions by Gd³⁺ ions. The mechanisms responsible to these results have been discussed in this paper.     

Effect of ZrO2 addition on the microstructure and electromagnetic properties of YIG

In this work, we reported the microstructure and electromagnetic properties of a series of zirconium substituted yttrium iron garnet ferrites (YCaZrIG) with iron deficiency composition of Y_3−xCa_xZr_xFe_4.93−xO₁₂ (x = 0.1, 0.2, 0.3, and 0.4, with electrostatic balance by Ca²⁺ substituted for Y³⁺ ions) prepared by a solid-reaction method. The addition of ZrO₂ shows no obvious influence on the phase, density and dielectric constant of YIG ferrites. When Zr addition x ≤ 0.3, the substitution of Zr⁴⁺ for Fe³⁺ decreases the amount of Fe ions, increases the lattice parameter and enhances the grain growth of garnet phase. The solubility of zirconium in YCaZrIG ferrite was found to be approximately 0.3, above which excess ZrO₂ would lead to the precipitation of a second phase inside the YCaZrIG ferrite. This would inhibit the grain growth of garnet phase and cause an increase in the dielectric loss and coercivity. The observed reduce for saturation magnetization when x = 0.4 is possibly due to antiparallel alignment of magnetic moment of Fe³⁺ in the d site caused by the decrease of a-d exchange interaction. Additionally, we got the optimum electromagnetic properties in the samples with x = 0.3: ?_r = 14.1, tan δ_e = 2.5 × 10⁻⁴, H_c = 47 A/m, 4πM_s = 1936 × 10⁻⁴T, ΔH = 7.1 KA/m.     

Synthesis and functional properties of the Ni1−xMnxFe2O4 ferrites

Nanocrystalline Ni_1−xMn_xFe₂O₄ (x = 0; 0.17; 0.34; 0.5) ferrite powders were successfully synthesized using the sol-gel combustion method, by using nitrates as cations source and citric acid (C₆H₈O₇) as combustion/chelating agent. The reaction advancement was observed by means of IR absorption spectroscopy, by monitoring two characteristic bands for the spinel compounds at about 600 cm⁻¹ and 400 cm⁻¹, respectively. The as-synthesized powders were characterized by IR spectroscopy, X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The magnetic study shows that the saturation magnetization decreases with increasing the Mn addition, as result of the particle size reduction. The dielectric properties were measured as a function of frequency in the range of 10 Hz to 1 MHz. The real part of permittivity has values of ∼88 at 1 kHz and ∼7 at 1 Hz for x = 0. An increasing dielectric permittivity with increasing the amount of Mn is observed. For all the investigated compositions, both the real and imaginary parts of permittivity decrease with frequency.     

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 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⁺.     

Studies on the magnetic, magnetostrictive and electrical properties of sol-gel synthesized Zn doped nickel ferrite

Zinc doped nickel ferrite i.e., Ni_1−xZn_xFe₂O₄ (0 ≤ x ≤ 0.6) have been prepared by using sol-gel method. X-ray diffraction of these samples shows the presence of single-phase cubic spinel structure. The room temperature magnetic measurements showed that saturation magnetization (M_s) increases with the substitution of Zn²⁺ ions up to x = 0.4 and thereafter it begins to decrease, whereas magnetostriction (λ) value decreases with the addition of Zn²⁺ in the Ni-Zn ferrite. Dielectric permittivity (?′), dielectric loss tangent (tan δ) and AC conductivity (σ_AC) for all the prepared samples have been studied as a function of frequency and composition in the range from 0.05 Hz to 10 MHz at room temperature. It has been observed that initially ?′, tan δ and σ_AC decreases with the substitution of Zn²⁺ up to x = 0.4 and then increases with the further addition of Zn²⁺ ions. Variation in the slope parameter s with zinc contents indicates the presence of different type of conduction mechanism in different compositions. The dielectric loss curves exhibit relaxation peaks which shift with the addition of Zn contents. The results have been explained on the basis of space charge polarization according to Maxwell-Wagner's two-layer model and the hopping of charges between Fe²⁺ and Fe³⁺ as well as between Ni³⁺ and Ni²⁺ ions at the octahedral sites.     

Effect of chromium substitution on structural,electrical and magnetic properties of NiZn ferrites

Ni_0.5Zn_0.5Fe_2−xCr_xO₄ (0≤x≤0.5) ferrites were successfully prepared by conventional solid state reaction method to investigate the effect of chromium substitution on the structural, electrical and magnetic properties. X-ray powder diffraction results demonstrate that all the prepared samples are well crystallized single-phase spinel structures without secondary phase. As chromium concentration increases, the lattice parameter and crystallite size gradually decrease. The magnetic measurement indicates that saturation magnetization is substantially suppressed by Cr³⁺ doping, changing from 73.5 A·m²/kg at x=0 to 46.3 A·m²/kg at x=0.5. While the room-temperature electrical resistivity is more than four orders of magnitude enhanced by Cr³⁺ substitution, reaching up to 1.1×10⁸ Ω·cm at x=0.5. The dielectric constant monotonously decreases with rising frequency for these ferrites, showing a normal dielectric dispersion behavior. The compositional dependence of dielectric constant is inverse with that of electrical resistivity, which originates from the reduced Fe²⁺/Fe³⁺ electric dipole number by doping, indicating inherent correlation between polarization and conduction mechanism in ferrite.     

Synthesis and morphological study of chromium substituted Zn-Mn ferrites nanostructures via sol-gel method

Nanocrystalline ZnMn_1−xCr_xFeO₄ (1.0 ≥ x ≥ 0) ferrites were prepared by sol-gel route. X-ray diffraction (XRD) method was used to confirm the formation of single phase cubic spinel lattice for all the composition. The lattice parameter (a) shows a decreasing trend with the increase in Cr content. In all the studied compositions, spherical crystalline nanoparticles of about 30 nm size were observed by transmission electron microscopy (TEM) technique. The elemental analysis as obtained from EDAX is in close agreement with the expected composition from the stoichiometry of reactant solutions used. Infrared spectroscopic studies revealed two main absorption bands in the range of 400-800 cm⁻¹ arising due to tetrahedral (A) and octahedral (B) stretching vibrations. On substitution of Cr³⁺ in Mn³⁺ site, the stretching frequency of the octahedral site increases smoothly but that of the tetrahedral site is seen to be unaltered. The detailed results of XRD, SAED and infrared spectroscopy have been discussed so as to bring out the role of chromium substitution in determining structural properties of Zn-Mn ferrites.     

Part II. Large scale applications of NixMn0.8−xMg0.2Fe2O4; 0.1 ≤ x ≤ 0.35 using laser irradiation

Ni_xMn_0.8−xMg_0.2Fe₂O₄; 0.1 ≤ x ≤ 0.35 was prepared by standard ceramic technique at sintering temperature 1200 °C using heating / cooling rate 4 °C/min. The samples were irradiated by Nd YAG pulsed laser with energy of the pulse 250 mJ. X-ray diffractograms reveal cubic spinel structure for all the samples before and after laser irradiation. After laser irradiation, better crystallinity was obtained in a form of an increase in the calculated crystal size. This increase was discussed as due to the change in the valence of some ions like Fe³⁺, Ni²⁺ and Mn²⁺. The conductivity of all the investigated samples decreases after laser irradiation and becomes temperature independent for a wider range than that before irradiation. This was ascribed to electron rearrangement after laser irradiation. Accordingly, these ferrites are recommended to be useful in electronic devices.     

Spectroscopic and magnetic investigation of NiCo nanoferrites

Spectroscopic and magnetic characterization of Ni_1−xCo_xFe₂O₄ (0.0 ≤ x ≤ 0.5) nanoparticles is presented. The infrared spectra are measured in the frequency range 700-350 cm⁻¹. Two prominent bands are observed, low frequency band at about 400 cm⁻¹ and high frequency band at about 600 cm⁻¹ assigned to octahedral and tetrahedral sites, respectively. The force constants K_o and K_t corresponding to octahedral and tetrahedral sites, respectively are also calculated from FTIR spectra. The effect of co-concentration on the magnetic properties has been investigated using a vibrating sample magnetometer (VSM). After appropriate treatments, it is found that both magnetic saturation (M_s) and coercivities (H_c) increase with co-concentration.     

Dielectric properties of Cu substituted Ni0.5−xZn0.3Mg0.2Fe2O4 ferrites

Dielectric properties of Cu substituted Ni-Zn-Mg ferrite samples having the general formula Ni_0.5−xCu_xZn_0.3Mg_0.2Fe₂O₄ (where x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) synthesized by Pramanik method are reported. The single phase formation of the ferrites was confirmed by XRD technique. The lattice parameter is found to increase with increase in Cu content. Average grain size, obtained from SEM micrographs, is found to increase with increase in Cu content. Dielectric parameters were measured as a function of frequency at room temperature as well as at higher temperatures. The variation in dielectric constant (?′) with temperature at four different fixed frequencies viz. 1 kHz, 10 kHz, 100 kHz, and 1 MHz was also studied. The room temperature dielectric constant (?′) and dielectric loss (tan δ) are found to decrease with increase in frequency. The ac conductivity (σ_ac) is found to increase with increase in the frequency.     

Dielectric and magnetic behavior of BaCd2−xSrxFe16O27 W-type hexagonal ferrites

We have prepared BaCd_2−xSr_xFe₁₆O₂₇ (x = 0, 0.5, 1, 1.5 and 2.0) W-type hexagonal ferrites by standard ceramic method. In this work, the structural, dielectric and magnetic properties have been studied of the prepared samples. The XRD analysis of the samples reveals single phase behavior sintered at 1400 °C for 6 h. The saturation magnetization (M_s) shows increasing behavior with the increasing concentration of Sr²⁺. While the coercivity (H_c) decreases rapidly up to 409 G for x = 1.5 and then increases for (x > 1.5) due to the preference of Cd²⁺ for tetrahedral sites and the number of spin-down magnetic ions. The real and imaginary parts of the dielectric constant (?′,?″) and dielectric loss tangent (tan δ) are determined in the frequency range 0.1-10⁷ Hz. It is observed that both the real and imaginary parts of the dielectric constant and tan δ decrease with the increasing concentration of Sr²⁺, which is due to the contribution of Cd²⁺ ions in addition to Fe³⁺ and Fe²⁺ ions to interfacial polarization.     

Structural, dielectric and magnetic properties of Nd-doped Co2Z-type hexaferrites

Z-type hexaferrites doped with Nd³⁺, Ba_3−xNd_xCo₂Fe₂₄O₄₁ (x = 0, 0.05, 0.10, 0.15, and 0.25), were prepared by solid-state reaction. The effect of the Nd³⁺ ions substitution for Ba²⁺ ions on the microstructure and electromagnetic properties of the samples was investigated. The results reveal that an important modification of microstructure, complex permeability, complex permittivity, and static magnetic properties can be obtained by introducing a relatively small amount of Nd³⁺ instead of Ba²⁺. SEM image shows that the grains of the ferrites doped with Nd³⁺ were smaller, more perfect and homogeneous than that of the pure ferrite. The real part (?′) of complex permittivity and imaginary part (?″) increase at first, and then decrease with increasing Nd content. At low frequency, the imaginary part μ″ of complex permeability decreases with Nd content and then increases when frequency is above 7.0 GHz. The magnetization (M_s) and the coercivity (H_c) are 79.38 emu g⁻¹ and 36.94 Oe for Ba_2.75Nd_0.25Co₂Fe₂₄O₄₁. The data of magnetism show that the ferrite doped with Nd³⁺ ions is a better soft magnetic material due to the higher magnetization and lower coercivity.     

Synthesis of nanocrystalline nickel-zinc ferrite (Ni0.8Zn0.2Fe2O4) thin films by chemical bath deposition method

The nickel-zinc ferrite (Ni_0.8Zn_0.2Fe₂O₄) thin films have been successfully deposited on stainless steel substrates using a chemical bath deposition method from alkaline bath. The films were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), static water contact angle and cyclic voltammetry measurements. The X-ray diffraction pattern shows that deposited Ni_0.8Zn_0.2Fe₂O₄ thin films were oriented along (3 1 1) plane. The FTIR spectra showed strong absorption peaks around 600 cm⁻¹ which are typical for cubic spinel crystal structure. SEM study revealed compact flakes like morphology having thickness ∼1.8 μm after air annealing. The annealed films were super hydrophilic in nature having a static water contact angle (θ) of 5°.The electrochemical supercapacitor study of Ni_0.8Zn_0.2Fe₂O₄ thin films has been carried out in 6 M KOH electrolyte.The values of interfacial and specific capacitances obtained were 0.0285 F cm⁻² and 19 F g⁻¹, respectively.     

Magnetic properties and large magnetocaloric effect in Gd-Ni amorphous ribbons for magnetic refrigeration applications in intermediate temperature range

Amorphous Gd_68−xNi_32+x (x = −3, 0, 3) ribbons were prepared by melt-spinning method. The crystallization onset temperatures T_x1 for Gd_68−xNi_32+x amorphous ribbons with x = −3, 0, and 3 are 561, 568, and 562 K, respectively. All the samples undergo the second-order magnetic transition at temperatures between ∼122 (x = −3 and 3) and 124 K (x = 0). The Curie temperature T_C does not change with the composition significantly. The maximum isothermal magnetic entropy changes (−ΔS_M)_max of Gd₇₁Ni₂₉, Gd₆₈Ni₃₂, and Gd₆₅Ni₃₅ amorphous ribbons for a magnetic field change of 0-5 T were 9.0, 8.0, and 6.9 J kg⁻¹ K⁻¹, respectively. Large values of the refrigerant capacity (RC) were obtained in these ribbons. For example, Gd₇₁Ni₂₉ amorphous ribbon has a maximum RC value of 724 J kg⁻¹. Large magnetic entropy change and RC values together with high stability enable the Gd₇₁Ni₂₉ amorphous alloy a competitive candidate among the magnetic refrigeration materials working at temperatures near 120 K.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting Mg with Ni

The microwave dielectric properties of La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La(Mg_0.5−xNi_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method at various sintering temperatures. The X-ray diffraction patterns of the La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.71 g/cm³, dielectric constant (?_r) of 20.19, quality factor (Q × f) of 74,600 GHz, and temperature coefficient of resonant frequency (τ_f) of −85 ppm/°C were obtained for La(Mg_0.4Ni_0.1Sn_0.5)O₃ ceramics that were sintered at 1550 °C for 4 h.     

Thermoelectric properties of p-type skutterudites YbxFe3.5Ni0.5Sb12 (0.8 ? x ? 1)

p-Type skutterudites, with nominal compositions Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1), have been synthesized by induction melting with subsequent annealing, and their thermoelectric properties evaluated from 3.5 to 745 K to assess their suitability for thermoelectric-based waste heat recovery applications. We report results for the synthesis and measurements of Seebeck coefficient (S), electrical resistivity (ρ), thermal conductivity (κ), Hall coefficient (R_H) and effective mass (m^*/m₀) of Yb_xFe_3.5Ni_0.5Sb₁₂ (0.8 ? x ? 1). Powder X-ray diffraction and electron probe microanalysis show that this system has a narrow filling fraction range of x ∼ 0.84-0.86 for Yb in the crystallographic voids. All samples show positive R_H for the entire temperature range studied, with carrier concentrations ranging from 9.6 × 10²⁰ to 2.8 × 10²¹ cm⁻³ at room temperature. Relatively high values of S result in high power factors up to 17 μW cm⁻¹ K⁻² at room temperature. However, large values of κ and a sharp reduction in the S at high temperature due to bipolar conduction prevent the attainment of high thermoelectric figure of merit.     

Dielectric properties of polycrystalline Cu-Zn ferrites at microwave frequencies

The real dielectric constant ?′ and complex dielectric constant ?″ of Cu_1−xZn_xFe₂O₄ have been measured at room temperature in the high frequency range 1 MHz to 1.8 GHz. At low frequencies the dielectric loss is found to be constant up to 1.4 GHz and there is a sudden rise at 1.5 GHz. A qualitative explanation is given for the composition, frequency dependence of the dielectric constant and dielectric loss of Cu_1−xZn_x Fe₂O₄. These are correlated with the W-H plot which gives the information about change in the average crystal size and strain of the samples. The micro-morphological features of the samples were obtained by Scanning Electron Microscopy (SEM). The micrograph shows that the increase of the Zn content in Cu ferrite increases the grain size.     

Preparation of Ni0.5Zn0.5Fe2O4/SiO2 nanocomposites and their adsorption of bovine serum albumin

The magnetic nanocomposites of (1 − x)Ni_0.5Zn_0.5Fe₂O₄/xSiO₂ (x = 0-0.2) were synthesized by the citrate-gel process and their absorption behavior of bovine serum albumin (BSA) was investigated by UV spectroscopy at room temperature. The gel precursor and resultant nanocomposites were characterized by FTIR, XRD, TEM and BET techniques. The results show that the single ferrite phase of Ni_0.5Zn_0.5Fe₂O₄ is formed at 400 °C, with high saturation magnetization and small coercivity. A porous, amorphous silica layer is located at the ferrite nanograin boundaries, with the silica content increasing from 0 to 0.20, the average grain size of Ni_0.5Zn_0.5Fe₂O₄ calcined at 400 °C reduced from about 18-8 nm. Consequently, the specific surface area of the nanocomposites ascends clearly with the increase of silica content, which is largely contributed by the increase in the thickness of the porous silica layer. The Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites demonstrate a better adsorption capability than the bare Ni_0.5Zn_0.5Fe₂O₄ nanoparticles for BSA. With the increase of the silica content from 0 to 0.05 and the specific surface area from about 49-57 m²/g, the BSA adsorption capability of the Ni_0.5Zn_0.5Fe₂O₄/SiO₂ nanocomposites calcined at 400 °C improve dramatically from 22 to 49 mg/g. However, with a further increase of the silica content from 0.05 to 0.2, the specific surface area increase from about 57-120 m²/g, the BSA adsorption for the nanocomposites remains around 49 mg/g, owing to the pores in the porous silica layer which are too small to let the BSA protein molecules in.     

304L stainless steel oxidation in carbon dioxide: An XPS study

From the early beginning of the oxidation of 304L stainless steel in carbon dioxide at 1273 K (1 min, for a weight gain of 0.02 mg cm⁻²), the surface of the alloy was entirely covered by oxides: magnetite Fe₃O₄, chromia Cr₂O₃ and traces of wüstite Fe_1−xO. Later on, for weight gains approaching 1 mg cm⁻², magnetite remained at the outer interface, with traces of hematite (Fe₂O₃), above a thick layer of wüstite Fe_1−xO. Magnetite and wüstite may favour adhesion of thermal plasma protective coatings such as alumina.