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.     

Preference orientation and magnetic properties of CoxFe3−xO4 nanocrystalline thin films on quartz substrate

Nanocrystalline spinel ferrite thin films of Co_xFe_3−xO₄ (x=0.3$x=0.3$, 0.5, 0.8, and 1.0) have been prepared by RF sputtering on quartz substrate without a buffer layer at room temperature and annealed at the temperature range from 200 to 600 °C in air. The as-sputtered films exhibit the preferred orientation and the high magnetization and coercivity. After annealing, the preferred orientations become poor, but the magnetization and coercivity increase. The sample with a magnetization of 455 emu/cm³, a coercivity of 2.8 kOe, a remanence ratio of 0.72, and a maximum energy product of 2.4 MGOe has been obtained. The influence of Co ions and annealing temperature on the magnetic properties has been discussed.     

Microstructure characterization and magnetic behavior of NiAlxFe2−xO4 and Ni1−yMnyAl0.2Fe1.8O4 ferrites

NiAl_xFe_2−xO₄ and Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites were prepared by the conventional ceramic method and were characterized by X-ray diffraction, scanning electron microscopy, and magnetic measurements. The single spinel phase was confirmed for all prepared samples. A proper explanation of data is possible if the Al³⁺ ions are assumed to replace Fe³⁺ ions in the A and B sites simultaneously for NiAl_xFe_2−xO₄ ferrites, and if the Mn²⁺ ions are assumed to replace Ni²⁺ ions in the B sites for Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites. Microstructural factors play an important role in the magnetic behavior of Ni_1−yMn_yAl_0.2Fe_1.8O₄ ferrites with large Mn²⁺ content.     

Structural characterization and magnetization of Mg0.7Zn0.3SmxFe2−xO4 ferrites

Mg_0.7Zn_0.3Sm_xFe_2−xO₄ ferrites were prepared by the solid-state reaction method and were characterized by X-ray diffraction and magnetization measurements. A single spinel phase was obtained in the range 0.00?x?0.03$0.00?x?0.03$. The lattice parameter was found to increase at x=0.01$x=0.01$ and then decreases up to x=0.03$x=0.03$, which may indicate a distortion in the spinel lattice. The saturation magnetization was found to decrease with the increase in x up to 0.04, due to the replacement of the Fe³⁺ ions by the Sm³⁺ ions.     

The structure and magnetic properties of Zn1−xNixFe2O4 ferrite nanoparticles prepared by sol–gel auto-combustion

Zn_1−xNi_xFe₂O₄ ferrite nanoparticles were prepared by sol–gel auto-combustion and then annealed at 700 °C for 4 h. The results of differential thermal analysis indicate that the thermal decomposition temperature is about 210 °C and Ni–Zn ferrite nanoparticles could be synthesized in the self-propagating combustion process. The microstructure and magnetic properties were investigated by means of X-ray diffraction, scanning electron microscope, and Vibrating sample magnetometer. It is observed that all the spherical nanoparticles with an average grain size of about 35 nm are of pure spinel cubic structure. The crystal lattice constant declines gradually with increasing x from 0.8435 nm (x=0.20) to 0.8352 nm (x=1.00). Different from the composition of Zn_0.5Ni_0.5Fe₂O₄ for the bulk, the maximum M_s is found in the composition of Zn_0.3Ni_0.7Fe₂O₄ for nanoparticles. The H_c of samples is much larger than the bulk ferrites and increases with the enlarging x. The results of Zn_0.3Ni_0.7Fe₂O₄ annealed at different temperatures indicate that the maximum M_s (83.2 emu/g) appears in the sample annealed at 900 °C. The H_c of Zn_0.3Ni_0.7Fe₂O₄ firstly increases slightly as the grain size increases, and presents a maximum value of 115 Oe when the grains grow up to about 30 nm, and then declines rapidly with the grains further growing. The critical diameter (under the critical diameter, the grain is of single domain) of Zn_0.3Ni_0.7Fe₂O₄ nanoparticles is found to be about 30 nm.     

Structural,electrical transport,and magnetic properties of Ni1−xZnxFe2O4

Structural, electrical, and magnetic properties of Ni_1−xZn_xFe₂O₄ (x=0.2, 0.4) samples sintered at various temperatures have been investigated thoroughly. The bulk density of the Ni_0.8Zn_0.2Fe₂O₄ samples increases as the sintering temperature (T_s) increases from 1200 to 1300 °C and above 1300 °C the bulk density decreases slightly. The Ni_0.6Zn_0.4Fe₂O₄ samples show similar behavior of changes to that of Ni_0.8Zn_0.2Fe₂O₄ samples, except that the bulk density is found to be the highest at 1350 °C. The DC electrical resistivity, ρ(T)$\rho (T)$, decreases as the temperature increases indicating that the samples have semiconductor-like behavior. As the Zn content increases, the Curie temperature (T_c), resistivity, and the activation energy decrease while the magnetization, initial permeability, and the relative quality factor (Q) increases. A Hopkinson peak is obtained near T_c in the real part of the initial permeability vs. temperature curves. The ferrite with higher permeability has a relatively lower resonance frequency. The initial permeability and magnetization of the samples has been found to correlate with density, average grain sizes. Possible explanation for the observed structural, magnetic, and changes of resistivity behavior with various Zn content are discussed.     

Structural,magnetic and electrical properties of Co1−xZnxFe2O4 synthesized by co-precipitation method

Nanoparticles of Co_1−xZn_xFe₂O₄ with stoichiometric proportion (x) varying from 0.0 to 0.6 were prepared by the chemical co-precipitation method. The samples were sintered at 600 °C for 2 h and were characterized by X-ray diffraction (XRD), low field AC magnetic susceptibility, DC electrical resistivity and dielectric constant measurements. From the analysis of XRD patterns, the nanocrystalline ferrite had been obtained at pH=12.5–13 and reaction time of 45 min. The particle size was calculated from the most intense peak (3 1 1) using the Scherrer formula. The size of precipitated particles lies within the range 12–16 nm, obtained at reaction temperature of 70 °C. The Curie temperature was obtained from AC magnetic susceptibility measurements in the range 77–850 K. It is observed that Curie temperature decreases with the increase of Zn concentration. DC electrical resistivity measurements were carried out by two-probe method from 370 to 580 K. Temperature-dependent DC electrical resistivity decreases with increase in temperature ensuring the semiconductor nature of the samples. DC electrical resistivity results are discussed in terms of polaron hopping model. Activation energy calculated from the DC electrical resistivity versus temperature for all the samples ranges from 0.658 to 0.849 eV. The drift mobility increases by increasing temperature due to decrease in DC electrical resisitivity. The dielectric constants are studied as a function of frequency in the range 100 Hz–1 MHz at room temperature. The dielectric constant decreases with increasing frequency for all the samples and follow the Maxwell–Wagner's interfacial polarization.     

Effect of indium incorporation in Zn1−xCoxO thin films

In the present paper, the preliminary investigations of a series of ZnO thin films co-doped with indium and cobalt with an objective to elucidate the correlation, if any, between the carrier concentration and the induced room temperature ferromagnetism (RTFM), are presented. The single-phasic (Zn_99.5In_0.5)_1−xCo_xO thin films are deposited by spray pyrolysis. The substitution of Zn²⁺ by Co²⁺ has been established by optical transmission analysis of these films. The films are ferromagnetic at room temperature; and the magnetization has higher value for indium and cobalt co-doped thin film as compared with Zn0₉₀Co_0.1O thin film (having no indium).     

Magnetic properties of nanocrystalline Co1−xZnxFe2O4 prepared by forced hydrolysis method

Nanocrystalline zinc-substituted cobalt ferrite powders, Co_1−xZn_xFe₂O₄ (x=0, 0.2, 0.4), were for the first time prepared by forced hydrolysis method. Magnetic and structural properties in these specimens were investigated. The average crystallite size is about 3.0 nm. When the zinc substitution increases from x=0 to x=0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g and the coercive field decreases from 1.22 to 0.71 T. All samples are superparamagnetic at room temperature and ferrimagnetic at temperatures below the blocking temperature. The high value of the saturation magnetization and the very thin thickness of the disorder surface layer of all samples suggests that this forced hydrolysis method is suitable not only for preparing two metal element systems but also for three or more ones.     

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.     

Structural and magnetic properties of Co1−xZnxFe2O4 nanoparticles by co-precipitation method

Co_1−xZn_xFe₂O₄ nanoparticles were prepared by co-precipitation method with x varying from 0 to 1.0. The powder samples were characterized by X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and Fourier transform infrared spectroscopy (FTIR). The average crystallite sizes of the particles were determined from XRD. X-ray analysis showed that the samples were cubic spinel. The average crystallite size (D_aveXR) of the particles precipitated was found to vary from 6.92 to 12.02 nm decreasing with the increase in zinc substitution. The lattice constant (a_o) increased with the increase in zinc substitution. The specific saturation magnetization (M_S) of the particles was measured at room temperature. The magnetic parameters such as M_S, H_c, and M_r were found to decrease with the increase in zinc substitution. FTIR spectra of the Co_1−xZn_xFe₂O₄ with x varying from 0 to 1.0 in the range 400–4000 cm⁻¹ were reported. The spinel structure and the crystalline water adsorption of Co_1−xZn_xFe₂O₄ nanoparticles were studied by using FTIR.     

A comparative study of the electrical and the magnetic properties and Mössbauer studies of normal and hot pressed MgxMn1−xFe2O4 ferrites

Modifying the proportion of the base composition by substituting with suitable dopants and improving the preparation conditions is expected to change the performance of ferrites. In the present study, Mg_xMn_1−xFe₂O₄ series of ferrites were prepared by, the conventional ceramic technique and the hot-pressed ceramic technique. Hot pressing of Mg–Mn ferrites results in an improvement of their magnetic and micro-structural properties as it controls simultaneously grain growth and porosity. Hot pressing of the Mg–Mn ferrites, however, results in a deterioration of their DC resistivity. The cation distribution has been studied by X-ray analysis and magnetisation. The variation of the saturation magnetisation and Curie temperature with increasing concentration of the Mg²⁺ ions can be explained on the basis of cation distribution and Neel's two sub-lattice models. The observed Mössbauer spectra show two hyperfine split sextets with an absence of magnetic relaxation indicating an absence of domain wall oscillations. The variations of the internal magnetic field have been qualitatively explained by taking into account, the predominant super-transferred hyperfine interactions (STHI).     

Magnetic viscosity-coercivity relation for Fe1−xCox fine particles

The variation of the applied field results in a subsequent change of magnetization with time. There is a relationship between the coercivity (H_c), as the equilibrium characteristic of the system, and its magnetic stability (1/S), as a parameter characterizing the time dependence. 1/S as a function of H_c has been measured and studied for different Fe_1−xCo_x samples. We synthesized several samples with different values of x by applying various magnetic fields during the grains’ growth, and observed a linear relationship between 1/S and H_c.     

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.     

Analysis of Raman scattering of Ga1−xMnxAs dilute magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x=1.4-3.0%) grown on (1 0 0) GaAs substrates by molecular beam epitaxy were characterized using Raman spectroscopy. As Mn is introduced into GaAs, a marked increase in disorder in the material occurs, as indicated by the growth of the disorder-allowed transverse-optical Raman line. Another important result is that as the Mn concentration in Ga_1−xMn_xAs increases further beyond ca. 2%, Raman-active coupled-plasmon-longitudinal-optical phonon modes arise, which signals the increasing presence of holes, and thus provides a useful tool for determining their concentration. Using the depletion-layer approach from the Raman spectroscopy data, we determined the carrier concentration for samples with x=2.2% and 3.0% was to be 7.2×10¹⁹ and 8.3×10²⁰ cm⁻³, respectively.     

Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route

Magnetic nanoparticles of cobalt ferrite have been synthesized by wet chemical method using stable ferric and cobalt salts with oleic acid as the surfactant. X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) confirmed the formation of single-phase cobalt ferrite nanoparticles in the range 15–48 nm depending on the annealing temperature and time. The size of the particles increases with annealing temperature and time while the coercivity goes through a maximum, peaking at around 28 nm. A very large coercivity (10.5 kOe) is observed on cooling down to 77 K while typical blocking effects are observed below about 260 K. The high field moment is observed to be small for smaller particles and approaches the bulk value for large particles.     

Transport and magnetotransport properties of Mn-doped InxGa1−xAs/GaAs quantum well structures

We report on the transport, magnetotransport and magnetic properties of In_0.17Ga_0.83As quantum well in GaAs δ-doped by Mn. At low temperatures, the anomalous Hall effect was observed which detects the spin-polarized carriers. Negative magnetoresistance was found at low temperatures, which became positive at high temperature.     

Effect of domain transformation on the magnetic properties of CuxNi1−xFe2O4 ferrites

We report the effect of crossover from multi- to single-domain particles on the magnetic properties of Cu_xNi_1−xFe₂O₄ (x=0 and 0.5) oxides due to grain size reduction to nanometer scale. The Mössbauer spectra for milled nanosized powder show a combination of ordered and super-paramagnetic behavior. The coercive field (H_C) is found to increase with reduction in grain size (G) according to the equation H_C=a_m+b_m/G for multi-domain particles. For single-domain particles H_C decreases with G according to H_C=a_s−b_s/G².     

Structural and magnetic studies on mechanosynthesized BaFe12−xMnxO19

Barium hexaferrite powders with manganese substitution were prepared by mechanosynthesis. The structural and magnetic properties were characterized by X-ray diffractometer and vibration sample magnetometer, respectively. XRD patterns were refined by Rietveld method. Preferential site occupation of manganese ion was investigated by room temperature (RT) Mössbauer measurements. XRD results showed a single-phase barium hexaferrite with some residual hematite. Crystallite size was observed to decrease with substitution amount. Lower saturation magnetization and increased coercivity is observed in substituted samples. RT Mössbauer measurements showed that manganese ions preferentially occupy 12k, 4f₂, and 2a sites.     

Magnetic behavior of MnAs precipitates in Ga1−xMnxAs diluted magnetic semiconductor

Ferromagnetic Ga_1−xMn_xAs layers (where x≈4.7–5.5%) were grown on (1 0 0) GaAs substrates by molecular beam epitaxy. These p-type (Ga,Mn)As films were revealed to have a ferromagnetic structure and ferromagnetism is observed up to a Curie temperature of 318 K, which is ascribed to the presence of MnAs secondary magnetic phases within the film. It is highly likely that the phase segregation occurs due to the high Mn cell temperature around 890–920 °C, as it is well established that GaMnAs is unstable at such a high temperature. The MnAs precipitate in the samples with x≈4.7–5.5% has a Curie temperature T_c≈318 K, which was characterized from field-cooled and zero-field-cooled magnetization curves.