Electron-Spin-Resonance Study of Polycrystalline Zn1−x Cr x O Ceramics

The structural and magnetic properties of polycrystalline ceramics of Zn_1?x Cr _x O (x=0.01–0.10) annealed at 900 and 1200 °C were systematically investigated by means of X-ray diffractometer, electron spin resonance (ESR) spectroscopy, and a superconducting quantum interference device magnetometer. A coexistence of two structural phases of wurtzite-ZnO and spinel-ZnCr₂O₄ was found even in the samples with the lowest Cr-doping concentration of 1 at.%. Our experimental results have indicated that Cr ions are incorporated into the Zn site of the ZnO host lattice, and act as paramagnetic centers. Higher annealing temperature enhances the formation tendency of ZnCr₂O₄, and the proportion of Cr²⁺ relative to Cr³⁺ in ZnO. This results in the broadening of ESR spectral line. Dipole exchange interactions due to Cr³⁺–Cr³⁺, Cr³⁺–Cr²⁺, and Cr²⁺–Cr²⁺ pairs are assigned to be responsible for the ESR signals and paramagnetic behavior of Zn_1?x Cr _x O samples.     

Annealing Induced Enhancement in Magnetic Properties of Co0.5Zn0.5Fe2O4 Nanoparticles

In this paper, cobalt zinc ferrite (Co_0.5Zn_0.5Fe₂O₄) nanoparticles (NPs) have been prepared using chemical co-precipitation method. In order to investigate the annealing induced effects on their various physical properties, the prepared samples have been annealed at 500 °C, 650 °C and 1000 °C and then compared with as-prepared sample. X-ray diffraction (XRD) patterns of as-prepared and annealed samples at various temperatures exhibit single phase spinel structure. Enhancement in crystallinity and crystallite size is observed with the increase in annealing temperature. The annealing has also greatly influence the morphology and grain size of prepared NPs. The Co_0.5Zn_0.5Fe₂O₄ NPs have shown remarkable enhancement in magnetic moment with increase in annealing temperature. The bandgap energies of Co_0.5Zn_0.5Fe₂O₄ NPs have been measured via UV Spectrometer and observed to decrease with annealing temperature. FTIR spectra of the samples reveal the presence of both high frequency and low-frequency bands due to tetrahedral and octahedral sites, which corroborate well with the XRD results. The observed characteristics of cobalt zinc ferrite NPs as a function of annealing temperature are the rising contender for many data storage and nanodevice applications. Finally, the genotoxicity of prepared nanoferrites has been checked via comet assay.     

Spinel ferrites of the quaternary system Cu-Ni-Fe-O: Synthesis and characterization

The decomposition of the freeze dried Cu(II)-Ni(II)-Fe(III) formate precursors at 1000°C in air yields complex oxides Cu_xNi_1−xFe₂O_4±δ (0 ≤ x ≤ 1) with a cubic spinel structure. For x < 0.7, single phase spinels are formed at 1000°C. However, for 0.7 ≤ x ≤ 1, Copper oxide (CuO) is identified as a second phase and the formation of a pure spinel phase requires an increase of the iron content in the mixture. For example, Cu_0.81Ni_0.1Fe_2.09O₄ is a single phase at 1000°C/air. Other single spinel phases Cu_0.5+yNi_0.5−y−zFe_2+zO_4±δ, 0 ≤ (y + z) ≤ 0.5, in the phase triangle Cu_0.5Ni_0.5Fe₂O₄–CuFe₂O₄–Cu_0.5Fe_2.5O₄ have been synthesized under special p(O₂)/T—synthesis conditions. The increase of the iron content requires an increase of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. The oxygen exchange between Cu_0.9Fe_2.1O_4.02 and the reducing gaseous phases shows that the non stoichiometry δ of copper ferrite is only about ±0.03. Significant changes in the oxygen content lead to the separation in different phases. The electrical and magnetic properties of copper ferrite samples depend on their chemical composition and preparation conditions.     

Controllable preparation of large-area arrays of Al-substituted CoCuNi ferrite rods with improvement of saturation magnetization and initial permeability

Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ (x = 0, 0.07, 0.14, and 0.21) rods of large-area arrays are synthesized by a solvothermal method, followed by calcination in air. The samples are characterized by powder X-ray diffraction, FT-IR spectra, scanning electron microscope, and vibrating sample magnetometer. The effect of diamagnetic Al³⁺ ion substitution and calcination temperature on the structure, morphology, and magnetic properties of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ has been investigated. The results indicate that high-crystallized cubic Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ rods of large-area arrays are obtained when the precursors are calcined at 750 °C in air for 3 h. The crystallite size of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ increases with the increase in Al³⁺ content, attributed to the decrease in lattice strain in Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ with the increase in Al³⁺ content. The lattice parameters of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄ slightly increase with the increase in Al³⁺ content. This is due to the transformation from cubic NiFe₂O₄ phase to cubic CoFe₂O₄ phase after doping Al³⁺ ion. Al³⁺ substitution can improve the magnetic properties of Co_0.5Cu_0.3Ni_0.2Al _x Fe_2?x O₄. Co_0.5Cu_0.3Ni_0.2Al_0.14Fe_1.86O₄, calcined at 950 °C, has the highest specific saturation magnetization (86.36 ± 2.25 emu/g) and magnetic moment (3.586 ± 0.093 μ _B ). Co_0.5Cu_0.3Ni_0.2Al_0.21Fe_1.79O₄, calcined at 950 °C, has the highest initial permeability (17.216 ± 0.448). The results are explained by Neel’s two sublattices.     

Effect of Zn<Superscript>2+</Superscript> doping on the structural,magnetic and dielectric properties of MnFe<Subscript>2</Subscript>O<Subscript>4</Subscript> prepared by the sol–gel method

Mn_1?xZn_xFe₂O₄ (x?=?0.2–0.8) ferrite samples were successfully prepared by the sol–gel method. X-ray diffraction study reveals that single cubic spinel phase was formed in Mn_1?xZn_xFe₂O₄ samples. The SEM micrographs revealed that the microstructures change significantly with different Zn²⁺ doping concentration and sintering temperature while the grain size grow up to 9.48 μm for Mn_0.6Zn_0.4Fe₂O₄ sample sintered at 1100 °C. Further, the dielectric and magnetic measurements indicated that both Zn²⁺ doping and sintering temperature could affect both electrical and magnetic parameters such as dielectric constant and saturation magnetization in a great manner. The Mn_0.6Zn_0.4Fe₂O₄ sample sintered at 1100 °C for 8 h is found to show the largest M _s value (77.30 emu/g) in this work. These results indicate that Zn²⁺ doping or sintering temperature can adjust the microstructures, dielectric and magnetic properties of Mn_1?xZn_xFe₂O₄ ferrites.     

Structure and magnetic properties of NixZn1?xFe2O4 thin films prepared through electrodeposition method

Nanocrystalline nickel–zinc ferrites Ni _x Zn_1−x Fe₂O₄ thin films have been studied and synthesized via electrodeposition–anodization process. Electrodeposited (NiZn)Fe₂ alloys were obtained from non-aqueous ethylene glycol sulphate bath. The formed alloys were electrochemically oxidized (anodized) in aqueous (1 M KOH) solution, at room temperature, to the corresponding hydroxides. The parameters controlling the current efficiency of the electrodeposition of (NiZn)Fe₂ alloys such as the bath composition and the current density were studied and optimized. The anodized (NiZn)Fe₂ alloy films were annealed in air at different temperatures ranging from 850 to 1000 °C for different times from 1 to 4 h. The change in the crystal structure, crystallite size, microstructure, and magnetic properties of the produced ferrites were investigated using X-ray diffraction patterns (XRD), scanning electron microscope (SEM) and vibrating sample magnetometer (VSM). The results revealed the formation of Ni–Zn ferrites thin films were formed. The crystallite sizes of the produced films were in the range between 32 and 81 nm. High saturation magnetization of 48.81 emu/g was achieved for Ni_0.5Zn_0.5Fe₂O₄ thin film produced after annealing the alloy at 850 °C for 4 h. The annealing process of the oxidized alloy anodization process was found to be first order reaction. The activation energy of the crystallization of Ni–Zn ferrite was found to be 62 KJ/mol.     

Synthesis of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite and study of its magnetic behavior at different temperatures

Ni_0.5Zn_0.5Fe₂O₄ ferrite nanocrystals with average diameter in the range of 1–2 nm have been synthesized by reverse microemulsion. X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) are used to characterize the structural, morphological and magnetic properties. X-ray analysis showed that the nanocrystals possess cubic spinel structure. The absence of hysteresis, negligible remanence and coercivity at 300 K indicate the superparamagnetic character and single domain in the nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ ferrite materials. The nanocrystalline Ni_0.5Zn_0.5Fe₂O₄ ferrite were annealed at 600 °C. As a result of heat treatment the average particle size increases from 2 nm to 5 nm and the corresponding magnetization values have increased to 21.69 emu/g at 300 K. However, at low temperature of 100 K, the annealed samples show hysteresis loop which is the characteristic of a superparamagnetic to ferromagnetic transition. In addition, a comparative study of the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄ ferrite nanocrystals obtained from reverse microemulsion has been carried out with those obtained from the general chemical co-precipitation route.     

Effect of La–Zn Substitution on the Structure and Magnetic Properties of Low Temperature Co-Fired M-Type Barium Ferrite

Ba(LaZn) _x Fe_12?2x O₁₉ (0≤x≤0.5) powders with Bi₂O₃ as an additive was synthesized by a sintered route at 900 °C or 950 °C. The structure and magnetic properties of La–Zn substituted M-type barium ferrites were also investigated. When 0≤x≤0.5, only one crystal phase existed in the sample, and the morphology of the grains were shown to be gradually irregular. The little amount of La³⁺ ions and Zn²⁺ ions changed the equilibrium of Fe²⁺ and Fe³⁺ at the 2a site, which increased the Fe³⁺–O–Fe²⁺ superexchange interaction strength, and the saturation magnetization (Ms) of the samples was also improved. Meanwhile, the substitution of La³⁺ and Zn²⁺ ions and the grains’ size bought great effects on the magnetocrystalline anisotropy field. As a result, with sintering at 950 °C for 6 h, the max Ms value of the samples with x=0.1 was 67.26 emu/g, and the minimum coercivity (H _c ) value was 1718.89 Oe with x=0.3, respectively.     

Structural,dielectric, ac conductivity and electromagnetic shielding properties of polyaniline/Ni<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> composites

A conducting polymer, polyaniline (PANI)/Ni_0.5Zn_0.5Fe₂O₄ composites with high dielectric absorbing properties and electromagnetic shielding effectiveness at low frequencies were successfully synthesized through a simple in situ emulsion polymerization. PANI was doped with hydrochloric acid to improve its electrical properties and interactions with ferrite particles. PANI/Ni_0.5Zn_0.5Fe₂O₄ composites were characterized by X-ray diffraction analysis, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analysis. Frequency dependence of dielectric and ac conductivity (σ_ac) studies have been undertaken on the PANI/Ni_0.5Zn_0.5Fe₂O₄ composites in the frequency range 50 Hz–5 MHz. The electrical conduction mechanism in the PANI/Ni_0.5Zn_0.5Fe₂O₄ is found to be in accordance with the electron hopping model. Further, frequency dependence of electromagnetic interference (EMI) shielding effectiveness (SE) is studied. The EMI shielding effectiveness is found to decrease with an increase in the frequency. The maximum value 55.14 dB of SE at 50 Hz was obtained at room temperature for PANI/Ni_0.5Zn_0.5Fe₂O₄ composites in the 50 Hz–5 MHz frequency range. PANI/Ni_0.5Zn_0.5Fe₂O₄ composites were demonstrated as a promising functional material for the absorbing of electromagnetic waves at low frequencies because of a large amount of dipole polarizations in the polymer backbone and at the interfaces of the Ni–Zn ferrite particles and PANI matrix.     

Microstructure and magnetic transition in Cr-substituted Mg–Zn spinel ferrite powders prepared via hydrothermal method

Mg–Zn ferrite powder specimens with the nominal composition Mg_0.5Zn_0.5Cr _x Fe_2?x O₄ (x = 0.0–1.0 with steps of 0.2) were synthesized via hydrothermal method. It is found that all the specimens exhibit a typical spinel structure, and the lattice parameter increases slightly with x, which confirms the substitution of Cr³⁺ for Fe³⁺. The average crystallite size first increases but then decreases with x, which is not the same as the results in previous reports on spinels. The particles in specimens are the aggregate of small nanoscale crystallites, and roughly aggregate more extensively with the increasing Cr content. With the increase of x, the saturation magnetization decreases rapidly, and it becomes more and more difficult for the specimens to magnetize to saturation. The increase of coercivity from 0.6 (x = 0.0) to 32.3 kA m^?1 (x = 1.0) shows a transition from a typical soft magnetic behavior to a hard magnetic behavior with the increase of Cr content, which was not reported before.     

Study the structure stability of NiFe2−x Cr x O4 (x = 0, 0.08) during H2/CO2 cycle reaction

H₂/CO₂ cycle reaction activities of spinel structure NiFe_2−x Cr _x O₄ (x = 0, 0.08) prepared by co-precipitation were determined. The results showed that pure NiFe₂O₄ had almost lost its CO₂ decomposition activities after 15 cycles, while Cr³⁺ doped NiFe₂O₄ still had about 40% of the initial reaction activity value after 50 reaction cycles. The magnetic properties of samples annealed at 350 °C indicated that M _s and M _r decreased from 32.49 to 26.04 emu/g and 9.39 to 7.31 emu/g, respectively, but H _c increased from 230 to 1800 Oe with the increasing of Cr³⁺content. XRD Rietveld analysis showed that there appeared 23.15% Fe _y Ni_1−y (0<y < 1) and no Fe₃C in pure NiFe₂O₄ system after the first H₂/CO₂ cycle reaction. With the increasing of cycle times, the phase abundance of NiFe₂O₄ decreased rapidly. At the same time, Fe₃C appeared and its content increased fast. After 15 cycles, the phase abundance of NiFe₂O₄ is less than 5%wt, but those of Fe _y Ni_1−y (0 < y < 1) and Fe₃C enhanced to 48.15 %wt and 46.92 %wt, respectively. However, the cycle reaction life of NiFe_2−x Cr _x O₄ (x = 0.08) was much longer than NiFe₂O₄. The spinel structure stability was improved dramatically because of the existing of Cr³⁺ in the cell of NiFe₂O₄. After 50 cycles, the phase abundance of NiFe₂O₄ still had 20 %wt.     

Nanocrystalline spinel Ni0.6Zn0.4Fe2O4: A novel material for H2S sensing

Nanocrystalline powders of Ni_1?xZn_xFe₂O₄ (0 ≤ x ≤ 0.5) mixed ferrites, with cubic spinel structure and average crystallite size ranging from 28 to 42 nm, were synthesized by the ethylene glycol mediated citrate sol–gel method. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). The response of prepared Ni_1?xZn_xFe₂O₄ mixed ferrites to different reducing gases (liquefied petroleum gas, hydrogen sulfide, ethanol gas and ammonia) was investigated. The sensor response largely depends on the composition, temperature and the test gas species. The Zn content has a significant influence on the gas-sensing properties of Ni_1?xZn_xFe₂O₄. Especially, Ni_0.6Zn_0.4Fe₂O₄ composition exhibited high response with better selectivity to 50 ppm H₂S gas at 225 °C. Incorporation of palladium (Pd) further improved the response, selectivity and response time of Ni_0.6Zn_0.4Fe₂O₄ to H₂S with the shift in the operating temperature towards lower value by 50 °C. The enhanced H₂S sensing properties can mainly be attributed to the selectivity to oxidation of H₂S and noble metal additive sensitization. Furthermore, the sensor exhibited a fast response and a good recovery.     

Mössbauer studies of thiospinels VI. The system FexZn1−xCr2S4

The spinel system Fe_xZn_1?xCr₂S₄ with 0 ≦ x ≦ 1 has been prepared in polycristalline form. All samples are p type semiconductors. I.R. spectra have been measured between 200 cm^?1 and 600 cm^?1. Room temperature ⁵⁷Fe-Mössbauer spectra show the typical behaviour of tetrahedral site Fe(II) surrounded by different octahedral site neighbours. In comparison with Mössbauer spectra of the spinel system Fe_1+xCr_2?xS₄ the distribution, 0.01 < y < 0.02, is derived.     

The Transition from Paramagnetic to Ferromagnetic States as Influenced by Evolving Microstructure of Ni0.5Zn0.5Fe2O4

In this paper the effect of sintering temperature on Ni_0.5Zn_0.5Fe₂O₄ is examined closely. The evolution of toward magnetically ordered materials was to be tracked with the parallel evolving microstructure subjected to sintering temperatures in an ascending order. The starting powder of Ni_0.5Zn_0.5Fe₂O₄ was prepared via mechanical alloying and later molded into toroidal samples. After each sintering, we observed the resulting changes in the materials. The XRD data showed a single phase being formed as early as 600 °C and the peak intensity was increasing with the sintering temperature indicating an increase in the degree of crystallinity. The BH hysteresis loops showed the evolution from paramagnetism to moderate ferromagnetism to strong ferromagnetism with microstructural changes. For lower sintering temperatures, the samples showed paramagnetic behavior dominating the samples. As sintering temperature increased, paramagnetic states decreased and, at 900 °C, a moderately ferromagnetic state appeared. Sintering at 1000 °C produced a strongly ferromagnetic state giving a well-formed sigmoid-shape hysteresis loop.     

Effect of Cr substitution on microwave absorption of BaFe12O19

The chromium substituted barium hexaferrites were prepared by self-propagating combustion method. The crystalline structure, complex permittivity, complex permeability, and hyperfine parameters of BaFe_12−xCr_xO₁₉ (x varies from 0.2 to 1.0 in a step of 0.2) were measured with X-ray diffraction, vector network analyzer and Mössbauer spectroscopy. At 850 °C, only a part of Cr³⁺ ions are permitted to enter the lattice of barium ferrite. With further calcination at 1000 °C, all Cr³⁺ ions enter the lattice. After substitution, the complex permittivity is increased. The Cr³⁺ ions substitute for the Fe³⁺ ions on the 2a site, and simultaneously lead to a quadrupole splitting on the 4f₁ site. These changes decrease the anisotropy field, which are accordant with the spectra of μ″. In the crystalline cells of the substituted barium ferrites, some Fe²⁺ ions are formed. It results in a bigger dielectric loss.     

Comparative Studies on the Structure and Magnetic Properties of Ni–Zn Ferrite Powders Prepared by Glycine-Nitrate Auto-combustion Process and Solid State Reaction Method

Ni–Zn ferrite compositions (Ni_1?x Zn _x Fe₂O₄) are well known due to their remarkable soft magnetic properties, which potentially have a broad range of applications in many areas. In this study, Ni–Zn ferrite with the chemical formula of Ni_0.64Zn_0.36Fe₂O₄ was prepared by the glycine-nitrate autocombustion process (GNP) and solid state reaction method (SSRM). In order to achieve a desirable particle size, the SSRM powders were milled for 3 h at a milling rate of 200 rpm. The structure and magnetic properties of the ferrite powders, which were synthesized by both methods, were characterized and their properties were compared. The results indicate that a significant amount (～?90 wt.%) of nanocrystalline Ni_0.64Zn_0.36Fe₂O₄ ferrite with the average crystallite size of 47 nm, particle size of 200 nm, saturation magnetization of 73 emu/g and coercivity of 54 Oe has been formed by means of the glycine-nitrate process. The results also show that not only the saturation magnetization of the GNP ferrite powder is relatively similar to that of the milled SSRM powders, but also it is synthesized at a much shorter duration than that of the solid state reaction method.     

Growth mechanism and room temperature ferromagnetism property of the Zn1−xCrxS nanobelts

In this paper, the wurtzite-type Zn_1?xCr_xS nanobelts (NBs) were obtained by a simple hydrothermal process with the ethanol amine as the oriented-assembly agent at 180 °C. The results showed that the Cr³⁺ ions substituted for the Zn²⁺ sites in the host zinc sulfide (ZnS), and the maximum concentration of the Cr³⁺ ions in the ZnS NBs was 4.80 %. The Zn_1?xCr_xS NBs were constructed by the nanowires, and the growth mechanism of the NBs was investigated in detail. The Zn_1?xCr_xS NBs showed the ferromagnetism property at room temperature and the magnetic saturation value was increased as the Cr³⁺ doped ratio increased.     

Oxidation of ferritic steels dispersion strengthened with TiO2 and Y2O3 oxides in lead melt

We study the structure and composition of scales formed during the contact of Fe–13Cr–2Motype ferritic steels hardened with oxides TiO₂ and Y₂O₃ with oxygen-containing (10⁻³ mass% O) lead melt at 550°C for 1000 h. It is established that a Fe₃ O₄ – Fe (Fe_{1 − x} , Cr _x )₂ O₄ two-layer scale forms. Its upper layer (Fe₃ O₄) grows in the direction of the melt, and the internal layer (Fe (Fe_{1 – x} , Cr _x )₂ O₄) grows in the direction to the matrix. Oxide particles favor an increase in the porosity of the internal sublayer of the scale. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 38 – 44, September–October, 2008.     

Magnetic Properties of Mg x Mn1?x Fe2O4 Nanoferrites

We report the magnetic properties of Mg _x Mn_1?x Fe₂O₄ (0??x??1.0) nanosize compounds with particle sizes between 8?nm and 15?nm. The evolutions of the properties as a function of composition have been investigated by X-ray diffraction, M?ssbauer, and SQUID magnetometry. Pure cubic spinel could be obtained under a low reaction temperature of 200°C in all the samples. Impurity phases have been observed in compounds annealed at above 900°C. Magnetic relaxation is observed in samples with particles of about 8?nm. The spectra with particle sizes larger than 10?nm could be fitted with at least two sextets attributed to Fe³⁺ ions on tetrahedral (A) and octahedral?(B) sites. The magnetization measurement indicates superparamagnetic behavior in nanosized compounds.     

Synthesis and characterization of zinc and calcium nanoferrites

In this work, Zn_(1?x)Ca_xFe₂O₄ nanoparticles (x = 0, 0.5 and 1) have been synthesized by sol–gel method followed by heat treatment at a temperature within the range of 300–700 °C. The samples with appropriate saturation magnetization (Ms), low coercivity and remanence were Zn₀Ca₁Fe₂O₄ treated at 300 °C (Ms ~ 25 emu/g), Zn₀Ca₁Fe₂O₄ treated at 400 °C (Ms ~ 40 emu/g) and Zn_0.50Ca_0.50Fe₂O₄ treated at 400 °C (Ms ~ 31 emu/g). These samples were analyzed by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and transmission electron microscopy. The heating ability of selected nanoparticles was evaluated under a magnetic field using a solid state induction heating equipment. The obtained nanoferrites showed a particle size within the range of 13–14 nm. The Zn₀Ca₁Fe₂O₄ treated at 400 °C was able to heat the nanoferrite particles/water suspension (10 mg/2 ml) at a temperature of 44 °C under the selected magnetic field (10.2 kA/m and frequency 362 kHz). Additionally, in vitro bioactivity assessment was performed by immersing samples in a simulated body fluid for different periods of time at physiological conditions of pH and temperature. The samples showed an appropriate bioactivity. These nanoferrites are highly potential materials for hyperthermia treatment.