Reentrant spin glass behavior and large initial permeability of Co0.5−xMnxZn0.5Fe2O4

A series of polycrystalline ferrites having nominal chemical composition Co_0.50−xMn_xZn_0.5Fe₂O₄ (0<x<0.4) have been synthesized by the solid-state reaction technique. The XRD analysis confirms single phase cubic spinel structure for all compositions. Lattice constant increases from 0.84195 to 0.84429 nm with the increasing Mn content and obeys Vegard's law. The average grain size increases by increasing both Mn content and sintering temperatures. Room temperature saturation magnetization increases for x=0.1 and decreases for increasing Mn content. The coercivity decreases with increasing Mn content due to the decrease of anisotropy constant. A reentrant spin glass behavior of these samples is observed from the zero field cooled magnetization measurements. The real part of the initial permeability increases by increasing both Mn content and sintering temperatures. This is due to the homogeneous grain growth and densification of the ferrites. The highest initial permeability 137 is observed for x=0.4 sintered at 1573 K on the other hand, the highest relative quality factor (2522) is obtained for the sample Co_0.2Mn_0.3Zn_0.5Fe₂O₄ sintered at 1523 K. The Mn substituted Co_0.50−xMn_xZn_0.5Fe₂O₄ ferrites showed improved magnetic properties.     

Complex permittivity, complex permeability and microwave absorption properties of ferrite-paraffin polymer composites

We have investigated the electromagnetic (EM) characteristics of Co_xMn_1−xFe₂O₄ spinel ferrite (where x=0.0, 0.5 and 1.0) nanoparticles (NPs)/paraffin nanocomposite material at 8-20 GHz. Co_xMn_1−xFe₂O₄ NPs have been synthesized by cetyltrimethylammonium assisted hydrothermal route using NaOH. A variation in complex dielectric permittivity and magnetic permeability at room temperature with frequency in the range 8-20 GHz has been studied. Particles showed phase purity and crystallinity in powder X-ray diffraction (XRD) analysis. At the same time, Co_xMn_1−xFe₂O₄ NPs demonstrated a spinel cubic structure from XRD results. A reflection loss of −46.60 dB was found at 10.5 GHz for an absorber thickness of 2 mm. Co_xMn_1−xFe₂O₄ may be attractive candidates for EM wave absorption materials.     

Preparation and magnetic properties of the conductive Co(1−x)NixFe2O4/polyaniline microsphere composites

Core-shell Co_(1−x)Ni_xFe₂O₄/polyaniline nanoparticles, where the core was Co_(1−x)Ni_xFe₂O₄ and the shell was polyaniline, were prepared by the combination of sol-gel process and in-situ polymerization methods. Nanoparticles were investigated by Fourier transform spectrometer, X-ray diffraction diffractometer, Scanning electron microscope, Differential thermal analysis and Superconductor quantum interference device. The results showed that the saturation magnetization of pure Co_(1−x)Ni_xFe₂O₄ nanoparticles were 57.57 emu/g, but Co_(1−x)Ni_xFe₂O₄/polyaniline composites were 37.36 emu/g. It was attributed to the lower content (15 wt%), smaller size and their uneven distribution of Co_(1−x)Ni_xFe₂O₄ nanoparticles in the final microsphere composites. Both Co_(1−x)Ni_xFe₂O₄ and PANI/Co_(1−x)Ni_xFe₂O₄ showed superparamagnetism.     

Structural, thermal and magnetic properties of Ni1−xMnxFe2O4 nanoferrites

In this paper, the structural, thermal and magnetic properties of Ni_1−xMn_xFe₂O₄ are presented. It is observed that high concentration of Mn²⁺ ions into NiFe₂O₄ tends to reduce the particle size. Calcination at 500 °C has resulted in the growth of Ni_1−xMn_xFe₂O₄ nanoparticles, but the calcination at 900 °C has led to the evaporation of the majorities of the polyvinyl alcohol. After calcination at 900 °C, crystallographically oriented NiMnFe₂O₄ nanoparticles are formed. These Ni_1−xMn_xFe₂O₄ nanoparticles show hysteresis behaviour upon magnetization. On the other hand, saturation magnetization (Ms) values decreases with increasing Mn content in ferrite due to the influence of Mn²⁺ ion in the sub lattice.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Effects of Mn substitution of Co and Fe in spinel CoFe2O4 thin films

Effects of Mn substitution for Co and Fe on the structural and magnetic properties of inverse-spinel CoFe₂O₄ have been investigated. Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ thin films were prepared by a sol–gel method. The observed increase of the lattice constant of Mn_xCo_1−xFe₂O₄ indicates that Mn²⁺ ions substitute the octahedral Co²⁺ sites. Conversion electron Mössbauer spectroscopy data indicate that a fraction of octahedral Co²⁺ ions exchange sites with tetrahedral Fe³⁺ ions through Mn doping. Vibrating-sample magnetometry data exhibit a large increase of saturation magnetization for both Mn_xCo_1−xFe₂O₄ and Mn_yCoFe_2−yO₄ films compared to that of the CoFe₂O₄ film. Such enhancement of magnetization can be explained in terms of a breaking of ferrimagnetic order induced by the Co²⁺ migration.     

Structural, AC, and DC magnetic properties of Zn1−xCoxFe2O4

Structural, AC and DC magnetic properties of polycrystalline Zn_1−xCo_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures (1100-1300 °C), and various dwell times (0.2-15 h) have been investigated thoroughly. The bulk density of the Zn_0.60Co_0.40Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1100 to 1250 °C, and above 1250 °C the bulk density decreases slightly. The Zn_0.80Co_0.20Fe₂O₄ samples show similar behavior of changes to that of Zn_0.60Co_0.40Fe₂O₄ samples except that the bulk density is found to be highest at 1200 °C. The DC magnetization as a function of temperature curves show that the Zn_0.60Co_0.40Fe₂O₄ sample is ferrimagnetic at room temperature while the Zn_0.80Co_0.20Fe₂O₄ sample is paramagnetic at room temperature. The T_c of Zn_0.80Co_0.20Fe₂O₄ sample is found to be 170 K from DC magnetization measurement. Separate measurement (AC magnetization), initial permeability as a function of temperature shows that the T_c of the Zn_0.60Co_0.40Fe₂O₄ sample is 353 K. Slight variation of T_c is observed depending on sintering condition. The initial permeability for the Zn_0.60Co_0.40Fe₂O₄ composition sintered at 1250 °C is found to be maximum.     

Magnetoresistive properties of Zn1−xCoxFe2O4 ferrites

The magnetic and magnetoresistive properties of spinel-type Zn_1−xCo_xFe₂O₄ (x=0, 0.2 and 0.4) ferrites are extensively investigated in this study. A large negative magnetoresistance (MR) effect is observed in Zn_1−xCo_xFe₂O₄ ferrites of spinel structure. These materials are either ferrimagnetic or paramagnetic at room temperature, and show a spin-(cluster) glass transition at low temperatures, depending on the chemical compositions. The MR curves as a function of magnetic fields, MR(H), are parabolic at all temperatures for paramagnetic polycrystalline ZnFe₂O₄. The MR for ZnFe₂O₄ at 110 K in the presence of 9 T applied magnetic field is 30%. On the other hand, MR(H) are linear for x=0.2 and 0.4 ferrimagnetic Zn_1−xCo_xFe₂O₄ samples up to 9 T. The MR effect is independent of the sintering temperatures, and can be explained with the help of the spin-dependent scattering and the Yafet–Kittel angle of Zn_1−xCo_xFe₂O₄ mixed ferrites.     

Synthesis and characterization of Ni1−xCoxFe2O4 nanoparticles

Ni_1−xCo_xFe₂O₄ (x=0.6, 0.8 and 0.9) nanoparticles have been synthesized with various crystallite sizes depending on the thermal treatments and composition (cobalt content) using the sol-gel combustion method. The size of nanoparticles has been controlled by thermal treatment. On the other hand, the magnetic property of the ferrite has been controlled by changing the heat treatment. Morphology and particle sizes of Ni_1−xCo_xFe₂O₄ have been studied using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The presence of functional group has been identified by Fourier Transform Infrared (FTIR) spectra. From TGA-DTA studies, the weight gains of Ni_1−xCo_xFe₂O₄ nanoparticles have been observed and it might be due to capping organic molecules with oxygen at temperatures above 200 °C. Magnetic properties of Ni_1−xCo_xFe₂O₄ particles have been analysed using VSM and it is found that saturation magnetization (M_s) has increased with particle size and has coercivity (H_c) increased initially and then decreased. The M_s and H_c values decreased with the increase of content of cobalt in Ni_1−xCo_xFe₂O₄.     

Structural, transport and electrochemical properties of LiNi1 − yCoyMn0.1O2 and Al, Mg and Cu-substituted LiNi0.65Co0.25Mn0.1O2 oxides

LiNi_{1 - y − z}Co_yMn_zO₂ (y = 0.25, 0.35, 0.5, 0.6; z = 0.1, 0.2), LiNi_0.63Cu_0.02Co_0.25Mn_0.1O₂, LiNi_0.65Co_0.25Mn_0.08Al_0.02O₂, LiNi_0.65Co_0.25Mn_0.08Mg_0.02O₂ and LiNi_0.65Co_0.25Mn_0.08Al_0.01Mg_0.01O₂ cathode materials were synthesized by a soft chemistry EDTA-based method. Structural and transport properties of pristine and delithiated materials (Li_xNi_0.65Co_0.25Mn_0.1O₂, Li_xNi_0.55Co_0.35Mn_0.1O₂ and LiNi_0.63Cu_0.02Co_0.25Mn_0.1O₂ oxides) are presented. In the considered group of oxides there is no correlation between electrical conductivity and the a parameter (M-M distance in the octahedra layers). The results of electrochemical performance of cathode materials are presented. The best stability during first 10 cycles was obtained for Li/Li_xNi_0.63Cu_0.02Co_0.25Mn_0.1O₂ cell due to enhanced kinetics of intercalation process.     

Electrical conduction and magnetoelectric effect of (x) BaTiO3 + (1−x) Ni0.92Co0.03Cu0.05Fe2O4 composites in ferroelectric rich region

Particulate composites with composition (x)BaTiO₃+(1−x)Ni_0.92Co_0.03Cu_0.05Fe₂O₄ in which x varies as 1, 0.85, 0.70, 0.55 and 0 (in mol%) were prepared by the conventional double sintering ceramic technique. The presence of two phases viz. ferromagnetic (Ni_0.92Co_0.03Cu_0.05Fe₂O₄) and ferroelectric (BaTiO₃) was confirmed by X-ray diffraction analysis. The dc resistivity and thermo-emf measurements were carried out with variation of temperature. The ac conductivity (σ_ac) measurements investigated in the frequency range 100 Hz to 1 MHz conclude that the conduction in these composites is due to small polarons. The variation of dielectric constant and loss tangent with frequency (20 Hz to 1 MHz) was studied. The static magnetoelectric conversion factor, i.e. dc (dE/dH)_H was measured as a function of intensity of applied magnetic field. The changes were observed in electrical properties as well as in magnetoelectric voltage coefficient as the molar ratio of the constituent phases was varied. A maximum value of magnetoelectric conversion factor of 536.06 μV/cm Oe was observed for the composite with 70% BaTiO₃+30% Ni_0.92Co_0.03Cu_0.05Fe₂O₄ at a dc magnetic field of 2.3 K Oe. The maximum magnetoelectric conversion output has been explained in terms of ferrite-ferroelectric content, applied static magnetic field and resistivity.     

The effect of Mn substitution on thermoelectric properties of Ca3MnxCo4−xO9 at low temperatures

The effects of partial substitution of Mn for Co on the thermoelectric properties of Ca₃Mn_xCo_4−xO₉ (x=0, 0.03, 0.9), prepared by sol-gel process, were investigated at the temperatures from 380 K down to 5 K. The results indicate that the substitution of Mn for Co results in increase in thermopower at temperatures >∼80 K, and substantial (23-31% at 300 K) decrease in lattice thermal conductivity in the whole temperature range investigated. The temperature behavior of ZT suggests that Ca₃Mn_xCo_4−xO₉ with light Mn substitution would be a promising candidate for high-temperature thermoelectric applications.     

Li0.5Fe2.5−xMnxO4 ferrite sintered from microwave-induced combustion

Li_0.5Fe_2.5−xMn_xO₄ (0≦x≦1.0) powders with small and uniformly sized particles were successfully synthesized by microwave-induced combustion, using lithium nitrate, ferric nitrate, manganese nitrate and carbohydrazide as the starting materials. The process takes only a few minutes to obtain as-received Mn-substituted lithium ferrite powders. The resultant powders annealed at 650 ^°C for 2 h and were investigated by thermogravimeter/differential thermal analyzer (TG/DTA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), and thermomagnetic analysis (TMA). The results revealed that the Mn content were strongly influenced the magnetic properties and Curie temperature of Mn-substituted lithium ferrite powder. As for sintered Li_0.5Fe_2.5−xMn_xO₄ specimens, substituting an appropriate amount of Mn for Fe in the Li_0.5Fe_2.5−xMn_xO₄ specimens markedly improved the complex permeability and loss tangent.     

Structural and magnetic properties of MnxCo1−xFe2O4 ferrite nanoparticles

We report on the structural and magnetic properties of nanoparticles of Mn_xCo_1−xFe₂O₄ (x=0.1, 0.5) ferrites produced by the glycothermal reaction. From the analysis of XRD spectra and TEM micrographs, particle sizes of the samples have been found to be about 8 nm (for x=0.1) and 13 nm (for x=0.5). The samples were characterized by DC magnetization in the temperature range 5-380 K and in magnetic fields of up to 40 kOe using a SQUID magnetometer. Mössbauer spectroscopy results show that the sample with higher Mn content has enhanced hyperfine fields after thermal annealing at 700 °C. There is a corresponding small reduction in hyperfine fields for the sample with lower Mn content. The variations of saturation magnetization, remnant magnetization and coercive fields as functions of temperature are also presented. Our results show evidence of superparamagnetic behaviour associated with the nanosized particles. Particle sizes appear to be critical in explaining the observed properties.     

Influence of frequency, temperature and composition on electrical properties of polycrystalline Co0.5CdxFe2.5−xO4 ferrites

Polycrystalline ferrites with general formula Co_0.5Cd_xFe_2.5−xO₄ (0.0?x?0.5) were prepared by sol-gel method. The dielectric properties ε′, ε″, loss tangent tan δ and ac conductivity σ_ac have been studied as a function of frequency, temperature and composition. The experimental results indicate that ε′, ε″, tan δ and σ_ac decrease as the frequency increases; whereas they increase as the temperature increases. These parameters are found to increase by increasing the concentration of Cd content up to x=0.2, after which they start to decrease with further increase in concentration of Cd ion. The dielectric properties and ac conductivity in studied samples have been explained on the basis of space charge polarization according to Maxwell and Wagner's two-layer model and the hoping between adjacent Fe²⁺ and Fe³⁺ as well as the hole hopping between Co³⁺and Co²⁺ ions at B-sites. The values of activation energies E_f for conduction process are determined from Arrhenius plots, and the variations in these activation energies as a function of Cd content are discussed. The complex impedance analysis is used to separate the grain and grain boundary of the system Co_0.5Cd_xFe_2.5−xO₄. The variations of both grain boundary and grain resistances with temperature and composition are evaluated in the frequency range 42 Hz-5 MHz.     

Magnetic characterization of Bi(Fe1 − xMnx)O3

Bi(Fe_1 − xMn_x)O₃ ceramics (x up to 0.3) were prepared by rapid sintering. Weak ferromagnetism with two magnetic anomalies at low temperatures was observed for Bi(Fe_0.95Mn_0.05)O₃ and Bi(Fe_0.9Mn_0.1)O₃. From temperature-dependent magnetic relaxation measurements, the anomalies at 20 K and 100 K are related to the freezing of cluster spin glass.     

Enhancement of initial permeability due to Mn substitution in polycrystalline Ni0.50−xMnxZn0.50Fe2O4

The structural and magnetic properties of Mn substituted Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.00, 0.10 and 0.20) sintered at various temperatures have been investigated thoroughly. The lattice parameter, average grain size and initial permeability increase with Mn substitution. Both bulk density and initial permeability increase with increasing sintering temperature from 1250 to 1300 °C and above 1300 °C they decrease. The Ni_0.30Mn_0.20Zn_0.50Fe₂O₄ sintered at 1300 °C shows the highest relative quality factor and highest initial permeability among the studied samples. The initial permeability strongly depends on average grain size and intragranular porosity. From the magnetization as a function of applied magnetic field, M(H), it is clear that at room temperature all samples are in ferrimagnetic state. The number of Bohr magneton, n(μ_B), and Neel temperature, T_N, decrease with increasing Mn substitution. It is found that Mn substitution in Ni_0.50−xMn_xZn_0.50Fe₂O₄ (where x=0.20) decreases the Neel temperature by 25% but increases the initial permeability by 76%. Possible explanation for the observed characteristics of microstructure, initial permeability, DC magnetization and Neel temperature of the studied samples are discussed.     

The synthesis and the magnetic properties of Gd-doped FexCo1−x/CoyFe3−yO4 micro-octahedrons composites

Gd³⁺-substituted micro-octahedron composites (Fe_xCo_1−x/Co_yGd_zFe_3−y−zO₄) in which the Fe-Co alloy has either a bcc or fcc structure and the oxide is a spinel phase were fabricated by the hydrothermal method. The X-ray diffraction (XRD) patterns indicate that the as-synthesized Gd³⁺-substituted micro-octahedron composites are well crystallized. Scanning electron microscopy (SEM) images show that the final product consists of larger numbers of micro-octahedrons with the size ranging from 1.3 to 5 μm, and the size of products are increased with increasing the concentration of KOH. The effect of the Co²⁺/Fe²⁺ ratio (0?Co²⁺/Fe²⁺?1) and substitution Fe³⁺ ions by Gd³⁺ ions on structure, magnetic properties of the micro-octahedrons composites were investigated, and a possible growth mechanism is suggested to explain the formation of micro-octahedrons composites. The magnetic properties of the structure show the maximal saturation magnetization (107 emu/g) and the maximal coercivity (1192 Oe) detected by a vibrating sample magnetometer.     

Preparation of substituted barium ferrite BaFe12−x(Ti0.5Co0.5)xO19 by citrate precursor method and compositional dependence of their magnetic properties

As a possible candidate for the left-handed metamaterial with negative permeability, a series of Ti, Co-substituted M-type barium hexaferrite BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ (x=0, 1, 2, 3, 4 and 5) was prepared by citrate precursor method. The formation processes of the substituted barium hexaferrite compounds from the precursors were followed by the measurements of powder X-ray diffraction (XRD), Infrared absorption spectra (FT-IR), and thermogravimetry and differential thermal analysis (TG/DTA) coupled with mass spectroscopy (MS). In the case of the non-substituted sample, the formation of the barium hexaferrite is regulated by the thermal decomposition of BaCO₃ and the solid-state reactions of BaO and Fe₂O₃ in the temperature range from 800 to 1100 K. The formation temperature of the substituted BaFe_12−x(Ti_0.5Co_0.5)_xO₁₉ is higher than that for the non-substituted sample and increases with the value of x, due to the effects of carbonate ions incorporated by the partial substitution of Fe³⁺ by (Ti_0.5Co_0.5)³⁺. On heating up to ca. 1200 K, all the substituted samples transform into the magenetoplumbite phase as is the non-substituted sample. The compositional dependence of the magnetic properties of the substituted barium hexaferrite was investigated by the magnetization measurement. The decrease in the magnetic anisotropy was confirmed by the change in the magnetization curve and coercivity H_C with the composition x. A negative permeability spectrum was observed in the BaFe₉(Ti_0.5Co_0.5)₃O₁₉ in the frequency range from 2 to 4 GHz.     

Crystal structure and magnetic properties of Mn substituted ludwigite Co3O2BO3

The needle shape single crystals Co_3−x Mn_xO₂BO₃ with ludwigite structure have been prepared. According to the X-ray diffraction data the preferable character of distinct crystallographic positions occupation by Mn ions is established. Magnetization field and temperature dependencies are measured. Paramagnetic Curie temperature value Θ=−100 K points out the predominance of antiferromagnetic interactions. Spin-glass magnetic ordering takes the onset at T_N=41 K. The crystallographic and magnetic properties of Co₃O₂BO₃:Mn are compared with the same for the isostructural analogs Co₃O₂BO₃ and CoO₂BO₃:Fe.