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.     

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.     

Monodispersed nanocrystalline Co1–xZnxFe2O4 particles by forced hydrolysis: Synthesis and characterization

Zinc-substituted cobalt ferrites, Co_1–xZn_xFe₂O₄, were for the first time successfully prepared by forced hydrolysis method. The obtained materials are single phase, monodispersed nanocrystalline with an average grain size of about 3 nm. These materials are superparamagnetic at room temperature and ferrimagnetic at temperature lower than the blocking temperature. When the zinc substitution increases from x=0 to 0.4, at 4.2 K, the saturation magnetization increases from 72.1 to 99.7 emu/g. The high saturation magnetization of these samples suggests that this method is suitable for preparing high-quality nanocrystalline magnetic ferrites for practical applications.     

Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

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.     

Permeability-frequency spectra of (Fe1−xcox)73.5Cu1Nb3Si13.5B9 nanocrystalline alloys under different magnetic fields

Under various amplitude of AC magnetic fields domain wall motion is the main mechanism in the magnetization process. This includes domain wall bulging and domain wall displacing. In this paper complex permeability-frequency spectra of (Fe_1−xCo_x)_73.5Cu₁Nb₃Si_13.5B₉ (x=0,0.5$x=0,0.5$) nanocrystalline alloys were measured as a function of the AC magnetic field, ranging from 0.001 to 0.04 Oe. Obvious changes have been found in complex permeability spectra for alloy x=0$x=0$ with the change of the amplitude of AC magnetic field, but variation of AC magnetic field has little effect on complex permeability spectra for alloy x=0.5$x=0.5$. This is attributed to the increased pinning field after substitution of Fe with Co in Fe_73.5Cu₁Nb₃Si_13.5B₉ nanaocrystalline alloy.     

Soft magnetic properties and giant magneto-impedance effect of Fe73.5−xCrxSi13.5B9Nb3Au1 (x=1–5) alloys

In this paper, the effect of microstructural and surface morphological developments on the soft magnetic properties and giant magneto-impedance (GMI) effect of Fe_73.5−xCr_xSi_13.5B₉Nb₃Au₁ (x=1, 2, 3, 4, 5) alloys was investigated. It was found that the Cr addition causes slight decrease in the mean grain size of α-Fe(Si) grains. AFM results indicated a large variation of surface morphology of density and size of protrusions along the ribbon plane due to structural changes caused by thermal treatments with increasing Cr content. Ultrasoft magnetic properties such as the increase of magnetic permeability and the decrease of coercivity were observed in the samples annealed at 540 °C for 30 min. Accordingly, the GMI effect was also observed in the annealed samples.     

Synthesis and magnetic characterization of cobalt-substituted ferrite (CoxFe3−xO4) nanoparticles

Cobalt-substituted ferrite nanoparticles were synthesized with a narrow size distribution using reverse micelles formed in the system water/AOT/isooctane. Fe:Co ratios of 3:1, 4:1, and 5:1 were used in the synthesis, obtaining cobalt-substituted ferrites (Co_xFe_3−xO₄) and some indication of γ-Fe₃O₄ when 4:1 and 5:1 Fe:Co ratios were used. Inductively coupled plasma mass spectroscopy (ICP-MS) verified the presence of cobalt in all samples. Fourier transform infrared (FTIR) showed bands at ∼560 and ∼400 cm⁻¹, characteristic of the metal–oxygen bond in ferrites. Transmission electron microscopy showed that the number median diameter of the particles was ∼3 nm with a geometric deviation of ∼0.2. X-ray diffraction (XRD) confirmed the inverse spinel structure typical of ferrites with a lattice parameter of a=8.388 Å for Co_0.61Fe_0.39O₄, which is near that of CoFe₂O₄ (a=8.394 Å). Magnetic properties were determined using a superconducting quantum interference device (SQUID). Coercivities higher than 8 kOe were observed at 5 K, whereas at 300 K the particles showed superparamagnetic behavior. The anisotropy constant was determined based on the Debye model for a magnetic dipole in an oscillating field and an expression relating χ′ and the temperature of the in-phase susceptibility peak. Anisotropy constant values in the order of ∼10⁶ erg/cm³ were determined using the Debye model, whereas anisotropy constants in the order of ∼10⁷ erg/cm³ were calculated assuming Ωτ=1 at the temperature peak of the in-phase component of the susceptibility curve as commonly done in the literature. Our analysis demonstrates that the assumption Ωτ=1 at the temperature peak of χ′ is rigorously incorrect.     

Solid state reaction synthesis of NiFe2O4 nanoparticles

Ni-ferrite (NiFe₂O₄) nanoparticles have been synthesized via a solid state reaction process. Ni and Fe bi-metallic nanoparticles in the form of Ni₃₃Fe₆₇ alloy nanopowder are first synthesized by simultaneous evaporation of the required amounts of pure Ni and Fe metals followed by rapid condensation of the evaporated metal flux into solid state by means of an inert gas, helium, using the process of inert gas condensation (IGC). In order to form the NiFe₂O₄ structure, as-synthesized samples (Ni₃₃Fe₆₇) are annealed for 12 h in ambient conditions at different annealing temperatures. Structural analyses show that NiFe₂O₄ starts to form at around 450 °C and gets progressively well defined with increasing annealing temperatures yielding particle with size ranging between 15 and 50 nm. Besides successfully forming NiFe₂O₄, NiO/Fe₃O₄ core/shell nanoparticles have also been synthesized by adjusting the annealing conditions. Three different structures, Ni₃₃Fe₆₇, NiO/Fe₃O₄, and NiFe₂O₄, obtained in this study are compared with respect to their structural and magnetic properties.     

Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles

CoFe₂O₄ ferrite nanoparticles were prepared by a modified chemical coprecipitation route. Structural and magnetic properties were systematically investigated. X-ray diffraction results showed that the sample was in single phase with the space group . The results of field-emission scanning electronic microscopy showed that the grains appeared spherical with diameters ranging from 20 to 30 nm. The composition determined by energy-dispersive spectroscopy was stoichiometry of CoFe₂O₄. The Curie temperature in the process of increasing temperature was slightly higher than that in the process of decreasing temperature. This can be understood by the fact that heating changed Co²⁺ ion redistribution in tetrahedral and in octahedral sites. The coercivity of the synthesized CoFe₂O₄ samples was lower than the theoretical values, which could be explained by the mono-domain structure and a transformation from ferrimagnetic to superparamagnetic state.     

Exchange bias and vertical shift in CoFe2O4 nanoparticles

Magnetic properties of core–shell cobalt ferrite nanoparticles prepared by co-precipitation route in the range 15–48 nm have been studied. It is shown that the coercivity follows non-monotonic size dependence and exhibits a peak at around 26 nm. Field-cooled magnetization exhibited both horizontal (exchange bias) and vertical shifts. The exchange bias is understood as originating at the interface between a surface region (with structural and spin disorder) and a core ferrimagnetic region. The dependence of the exchange bias and vertical shift on the particle size and cooling field is found to have significant differences. These differences are explained in the light of recent results that suggest that there is a variation of the pinning strength amongst the interface spins and the vertical shift is affected by the more strongly pinned uncompensated spins.     

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.     

Effects of precursor types of Fe and Ni components on the properties of NiFe2O4 powders prepared by spray pyrolysis

The effects of the precursor types of Ni and Fe components on the morphology, mean size, and magnetic property of NiFe₂O₄ powders prepared by spray pyrolysis from the spray solution, with citric acid were studied. The precursor powders with hollow and thin wall structure turned to the nano-sized NiFe₂O₄ powders after post-treatment at a temperature of 800 °C. The nickel ferrite powders obtained from the spray solution with ferric chloride had nanometer sizes and narrow size distributions irrespective of the types of nickel precursor. The nickel ferrite powders obtained from the spray solution with ferric nitrate and nickel chloride also had nanometer size and narrow size distribution. The saturation magnetizations of the NiFe₂O₄ powders changed from 37 to 42 emu/g according to the types of the Fe and Ni precursors. The saturation magnetizations of the NiFe₂O₄ powders increased with increasing the Brunauer-Emmett-Teller (BET) surface areas of the powders.     

Synthesis and anomalous magnetic properties of CoCr2O4 nanocrystallites with lattice distortion

Nanocrystalline Cobalt chromite (CoCr₂O₄) ceramic has been synthesized under a mild condition, rather than by a high-temperature sintering (e.g. >1673 K, in general). A shifted hysteresis loop with an exchange-bias field of 35.7 kA/m and a high coercivity of 627.9 kA/m at 4.2 K was achieved under the cooling field of 2.39×10⁶ A/m. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) results reveal that a strong lattice distortion and a large amount of surface defects exist in CoCr₂O₄ nanocrystallites (NCs). The anomalous magnetic properties, such as bias field and large coercivity, are attributed not only to the nanosize effect but also to the lattice distortion and crystal defects.     

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.     

Morphology and magnetic properties of Co0.8Fe2.2O4 films prepared by the chemical solution deposition

Co_0.8Fe_2.2O₄ ferrite thin films have been prepared on Si(0 0 1) substrates by the chemical solution deposition. Structural characteristics indicate all films are single phase with spinel structure and the space group and the mean grain size increases from 8 to 30 nm with the increase of annealing temperature. The magnetic properties of Co_0.8Fe_2.2O₄ thin films are highly dependent on annealing temperature. The sample annealed at 800 °C possesses high saturation magnetization, moderate coercivity and squareness ratio, making it a promising application candidate in high-density record and magneto-optical materials.     

Synthesis and magnetic properties of Co–Zn magnetic fluid

Co_1−xZn_xFe₂O₄ (with x varying from 0 to 0.7) nanoparticles to be used for ferrofluid preparation were prepared by chemical co-precipitation method. The fine particles were suitably dispersed in transformer oil using oleic acid as the surfactant. The magnetization (M_s) and the size of the particles were measured at room temperature. The magnetization (M_s) was found to decrease with the increase in zinc substitution. The magnetic particle size (D_m) of the fluid was found to vary from 11.19 to 4.25 nm decreasing with the increase in zinc substitution.     

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 and magnetic properties of SrFe12O19 hexaferrite synthesized by a modified chemical co-precipitation method

M-type strontium hexaferrite (SrFe₁₂O₁₉) particles had been prepared by a modified chemical co-precipitation route. Structural and magnetic properties were systematically investigated. Rietveld refinement of X-ray powder diffraction results showed that the sample was single-phase with the space group of P6₃/mmc and cell parameter values of a=5.8751 Å and c=23.0395 Å. The results of field-emission scanning electronic microscopy showed that the grains were regular hexagonal platelets with sizes from 2 to 4 μm. The composition determined by energy dispersive spectroscopy is the stoichiometry of SrFe₁₂O₁₉. The ferrimagnetic to paramagnetic transition was sharp with Curie temperature T_C=737 K, which further confirmed that the samples were single phase. However, it was found that the coercivity, saturation magnetization and the squareness ratio of the synthesized SrFe₁₂O₁₉ samples were lower than the theoretical values, which could be explained by the multi-domain structure and the increase of the demagnetizing factor.     

Temperature dependence of magnetic properties of NiFe2O4 nanoparticles embeded in SiO2 matrix

Spinel ferrite NiFe₂O₄ nanoparticles (?25 nm) in SiO₂ matrix were prepared by sol–gel method. The phase and average crystallite size of the samples were determined by X-ray diffraction method and the particle size distributions were studied by a transmission electron microscope. Magnetic properties of the samples were investigated with different ferrite particle sizes and at various temperatures down to 10 K. Superparamagnetic properties were observed at room temperature when the particle size is less than 10 nm.In superparamagnetic state, the field dependence of magnetization follows Langevin function which was originally developed for paramagnetism. The effective anisotropy constant K_eff is found to increase significantly with the decrease in particle volume and an order of magnitude higher than that of the bulk samples when the particle size is below 5 nm due to the dominance of surface anisotropy. In case of nanosized systems, the effect of size reduction on the law of approach to saturation has also been studied in detail.