Sol–gel auto combustion synthesis of CoFe2O4/1-methyl-2-pyrrolidone nanocomposite: Its magnetic characterization

A magnetic nanocomposite was generated by the sol–gel auto-combustion method in the presence of 1-methyl-2-pyrrolidone, a functional solvent. The temperature-dependent magnetic properties of the CoFe₂O₄ nanoparticles have been extensively studied in the temperature range of 10–400 K and magnetic fields up to 80 kOe. Zero field cooled (ZFC) and field cooled (FC) curves indicate that the blocking temperature (T_B) of the CoFe₂O₄ nanoparticles is above 400 K. It was found from M–H curves that the low temperature saturation magnetization values are higher than bulk value of CoFe₂O₄. The saturation magnetization (M_s), remanence magnetization (M_r), reduced remanent magnetization (M_r/M_s) and coercive field (H_c) values decrease with increasing temperature. The M_r/M_s value of 0.75 at 10 K indicates that the CoFe₂O₄ nanoparticles used in this work have, as expected, cubic magnetocrystalline anisotropy according to the Stoner–Wohlfarth model. T^1/2 dependence of the coercive field was observed in the temperature range of 10–400 K according to Kneller's law. The extrapolated T_B and the zero-temperature coercive field values calculated according to Kneller's law are almost 427 K and 13.2 kOe, respectively. The room temperature H_c value is higher than that of the previously reported room temperature bulk values. The effective magnetic anisotropy constant (K_eff) was calculated as about 0.23×10⁶ erg/cm³ which is lower than that of the bulk value obtained due to disordered surface spins.     

Photon transmission method for studying void‐closure kinetics during coalescence of hard latex particles

The contribution of viscous flow to void‐closure processes during film formation with hard latex particles was studied. Film optical clarity was used to follow the progress of this event. The latex films were prepared from poly (methyl methacrylate) (PMMA) particles and annealed in 10 min time intervals above the glass transition (T_g) temperature. Scanning electron microscopy (SEM) was used to detect the variation in the physical structure of the annealed films. To mimic the latex film‐formation process, Monte Carlo simulations were performed for photon transmission through the latex film and the number of transmitted and scattered photons are calculated as a function of the mean free path. A relation for transmitted light intensity, I_tr versus void closure (time)^1/2 (t^1/2) was derived by using the Vogel–Fulcher viscosity equation. The viscosity constant, B, was produced using this I_tr(t^1/2) relation at various temperatures and found to be 12.8 × 10³ K. It is shown that Monte Carlo results justified the I_tr(t^1/2) relation. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 981–988, 1999     

Ferroelectric phase transitions studied by dielectric and light scattering spectroscopies on Ba1−xCaxTiO3 single crystals (x = 0.12, 0.20)

The ferroelectric phase transition of Ca-substituted barium titanate single crystals, Ba_1?xCa_xTiO₃ (BCTO-x), with x = 0.12 and 0.20 grown by optical floating zone method were studied by dielectric and Brillouin scattering methods. The X-ray diffraction investigation confirmed the structural phase transition from cubic to tetragonal phase in BCTO-x. The dielectric constant exhibited a sharp peak at the ferroelectric phase transition temperature (T_c) and a deviation from the Curie–Weiss law on approaching T_c from the high-temperature side. Brillouin scattering results showed that, similar to the acoustic properties of BCTO-0.20, dynamic precursor polar clusters are formed in the paraelectric phase of BCTO-0.12 single crystals which interact with acoustic waves via electrostrictive coupling resulting in large acoustic anomalies near T_c. However, the temperature dependence of the dielectric constant indicated that diffuseness of the ferroelectric phase transition of BCTO-x became enhanced with Ca content, which was attributed to compositional disorder induced by random substitution of Ba cations by Ca ions.     

Correlation between glass transition temperature and molecular mass in non-polymeric and polymer glass formers

It is shown that the glass transition temperature T_g of molecular (non-polymeric) glass formers correlates with molecular mass M as T_g(M) ∝ M^α, α = 0.51 ± 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T_g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T_g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T_g(M) dependence in polymers begin to deviate from the universal T_g(M) correlation of molecular glasses and eventually saturates at some polymer specific T_g∞ value. We conclude that at least a substantial part of T_g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T_g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T_g(M) dependence common with molecular glasses.     

Void closure by viscous flow during coalescence of hard latex particles

Steady state fluorescence (SSF) technique was used to study the void closure process during coalescence of hard latex particles. Latex films were prepared by annealing pyrene (P_y)-labelled poly(methyl methacrylate) particles above the glass transition temperature. During the annealing processes, the optical clarity of the film increased considerably. Direct fluorescence emission from excited pyrene was monitored as a function of annealing temperature to detect these changes. Scanning electron microscopy in conjunction with Monte Carlo simulations of photon diffusion in latex film were used to interpret the fluorescence results. Void Closure temperature (T_c) and time (t_c) were measured at the point at which the fluorescence emission intensity becomes maximum. This was associated with the longest optical path of a photon in latex film. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 651–659, 1997     

Thermally- and solvent-induced crystallization kinetics of syndiotactic polystyrene viewed from time-resolved measurements of infrared spectra at the various temperatures (1) estimation of glass transition temperature shifted by solvent absorption

syndiotactic Polystyrene (sPS) glass crystallizes into the α form when it is heated above the glass transition temperature (T_g, about 100 °C). sPS can be crystallized also into the δ form in the solvent atmosphere at room temperature. In order to trace the structural evolution process, the time-resolved infrared spectral measurements have been performed in the isothermal crystallization from the glass to α form and in the solvent-induced crystallization from the glass to δ form at the various temperatures. Absorbance of crystallization-sensitive infrared bands was plotted against time, from which the crystallization kinetics were analyzed on the basis of Avrami equation: X(t)=1−exp[−(kt)ⁿ] where X is a normalized crystallinity, n is an index, k is a rate constant, and t is a time. The isothermal crystallization was investigated also by carrying out the temperature jump experiment of DSC thermograms, giving almost the same results as the infrared spectral measurements. The Avrami index n was 2-5 depending on the crystallization temperature (T_c). The k was also dependent on the T_c, about 10⁻¹-10⁻⁴ s⁻¹ and could be fitted reasonably by the equation of crystallization kinetics. An extrapolation of the k vs T_c plot to the negligibly small k value allowed us to predict the temperature at which no crystallization should occur, ca. 100 °C, in good agreement with the observed T_g value. On the other hand, the solvent-induced crystallization was investigated for the first time at the various temperatures from 50 to 9 °C by the time-resolved measurement of infrared spectra. Compared with the experiment at room temperature, the crystallization was highly accelerated at 40-50 °C, while the crystallization rate was reduced remarkably at such a low temperature as 9 °C. The time dependence of infrared absorbance was analyzed for the crystallization-sensitive bands on the basis of Avrami equation as the first approximation, although the crystallization mechanism was more complicated than the isothermal crystallization case. The logarithm of the k value was found to change almost linearly with temperature and an extrapolation to infinitesimally small k value gave a T_g of about −15 °C. That is to say, the glass transition temperature was estimated to shift remarkably from 100 to −15 °C by absorbing solvent molecules or by a plasticizing effect.     

Temperature- and frequency-dependent hysteresis scaling behaviour and phase transitions in a [001]c 0.73Pb(Mg1/3Nb2/3)O3-0.27PbTiO3 single crystal

The dielectric properties and bipolar polarization-electric field (P-E) and strain-electric field (S-E) dynamic hysteresis of a relaxor [001]_c 0.73Pb(Mg_1/3Nb_2/3)O₃-0.27PbTiO₃ (PMN-0.27PT) single crystal were investigated to reveal more details of the temperature-induced phase transitions. Different linear scaling relations for ferroelectric hysteresis area <A>, coercive field E_c, saturation polarization P_s and remnant polarization P_r versus temperature τ were measured in different temperature regions. For each measurement frequency f, all hysteresis parameters were found to decrease linearly with temperature in the temperature range of the single rhombohedral (R) phase or tetragonal (T) phase, and the rate of decrease in the T phase was observed to be much larger than the corresponding rate in the R phase. In the temperature range near the R-T phase transition, the exponent α in the power law <A>∝f ^α for the R phase was found to be smaller than that for the T phase, and the magnitude of α depended strongly on temperature when the crystal was in the R-T coexisting phase state. Our experimental and theoretical results indicate that the difference in the activation energy and dipole moment in the R and T phases may lead to the observed discrepancy for the P-E and S-E hysteresis behaviour in different temperature regions.     

Influence of Ag doping on electrical and magnetic properties of La0.625(Ca0.315Sr0.06)MnO3 ceramics

Polycrystalline Ag-doped [La_0.625(Ca_0.315Sr_0.06)MnO₃]_1-x:Ag_x (LCSMO) ceramics with (x = 0, 0.03, 0.05, 0.10, 0.15, and 0.20) were prepared by sol-gel method, and their structures and properties were characterized. X-ray diffraction results indicated that all bulk samples had single phase with orthorhombic phase (space group of Pbnm) without impurities. With the increase of Ag doping content, the resistivity of the samples decreased, while the remanent magnetization and coercive field increased. The metal to insulator transition temperature (T_p), temperature coefficient of resistance (TCR) and Curie temperature (T_c) for x = 0.20 were determined as 300 K, 9.38% (292.6 K) and 291.86 K respectively. The highest MR value of 28.36% (295.03 K) was obtained at x = 0.15. XPS data revealed that substitution between A-site ions and Ag⁺ could increase the ratio of Mn⁴⁺ ion. Double exchange effect (DE) enhanced by changing Mn–O bond distance, Mn–O–Mn bond angle, and increasing Mn⁴⁺ ion concentration. These features promoted the transfer of itinerant electron between Mn³⁺ and Mn⁴⁺ ions. However, the magnetization obtained at x = 0.20 was less than that at x = 0, as diamagnetic Ag released magnetism of the samples. The results suggested that the LCSMO polycrystalline ceramics could be used as a candidate to prepare room temperature infrared detectors, magnetic sensors or magneto-electric devices, and so on.     

Synthesis and magnetic properties of monodisperse CoFe2O4 nanoparticles coated by SiO2

The monodisperse CoFe₂O₄ nanoparticles were synthesized by a modified chemical coprecipitation method. Coating SiO₂ on the surface of the CoFe₂O₄ nanoparticles was carried out to keep single domain particles non-interacting with cubic magnetocrystalline anisotropy. The Curie temperatures (T_c) of the monodisperse CoFe₂O₄ nanoparticles can be accurately measured because the SiO₂ shells prevented the aggregation and growth of nanoparticles at high temperature. The magnetic properties of the CoFe₂O₄@SiO₂ nanoparticles with core-shell structure in a wide temperature range (300～950?K) were investigated. It is remarkable that the coercive field (H_c) of CoFe₂O₄ nanoparticles increased from about 760?Oe to 1806?Oe after being coated with SiO₂, which increased by 137.6% compared to the uncoated samples at 300?K. The saturation magnetization (M_s) of the CoFe₂O₄@SiO₂ nanoparticles is 34.59?emu/g, which is about 52% of the naked CoFe₂O₄ nanoparticles value (66.51?emu/g) at 300?K. The hysteresis loops of the CoFe₂O₄@SiO₂ nanoparticles showed an orderly magnetic behavior at high temperature, such as the M_s, remanence magnetization (M_r) and H_c decreased as temperature increasing, being equal to zero near T_c. This is a good indication that the CoFe₂O₄@SiO₂ nanoparticles are suitable for a wide variety of technological applications at high temperature.     

Room temperature multiferroic behavior and magnetoelectric coupling in (K,Na)NbO3-based ceramics

Polycrystalline (K_0.48Na_0.52)_0.95Li_0.05(Nb_0.95Sb_0.05)_1-xFe_xO₃ (0?≤?x?≤?0.030) ceramics are synthesized by the conventional solid state reaction method. The samples with 0.010?≤?x?≤?0.030 are found to possess good ferroelectricity, weak ferromagnetism and large magnetodielectric effect simultaneously at room temperature. Temperature dependence of magnetization and dielectric properties show that there are both magnetic and magnetodielectric anomalies in the vicinity of the ferroelectric rhombohedral-orthorhombic (T_R-O) and orthorhombic-tetragonal (T_O-T) phase transition temperatures. Furthermore, magnetodielectric effect as large as 16% is observed at 0.9?T near T_R-O when x?=?0.030. According to these results, a strong spin-lattice coupling is considered to exist in these samples. In the structure transition process, the spin interactions between Fe ions will alter for the severe distortion of the FeO₆ octahedron. Moreover, since there are more possible polarization vectors due to polymorphic phase transition (PPT), the local polarization of FeO₆ will also be greatly affected when the external magnetic field changes the spin interactions.     

Temperature-induced strain mediated magnetization changes in NiFe2O4/BaTiO3 heterostructure

Herein, we report the temperature-induced magnetization changes of NiFe₂O₄ thin film, which is coated over a ferroelectric BaTiO₃ ceramic substrate. The solid-state reaction method was adopted for the preparation of ferroelectric BaTiO₃ (BT) substrate, whereas NiFe₂O₄ (NFO) film was deposited by spin-coating method. Rietveld refinement revealed that BT substrate has a tetragonal (P4mm) crystal system along with a minor orthorhombic phase (Amm2) at room temperature. The GIXRD analysis confirms the phase purity of NFO/BT heterostructure. Polarization hysteresis with respect to electric field (P-E loop) and the temperature-dependent dielectric measurement of BT substrate demonstrate its typical ferroelectric and phase transition behavior, respectively. Magnetization hysteresis loops were recorded for the NFO/BT heterostructure at 150, 240 and 300?K. A significant increase in the remnant magnetization (M_R) and coercive field (H_C) of NFO film are noticed while cooling the heterostructure below 300?K. Variation in the magnetization of NFO film corresponds to the change in the structural phase transition (Amm2 at 240?K and R3c at 150?K) of BT while cooling below RT. The interfacial strain mediated coupling is the primary mechanism attributed to the temperature-induced changes in the magnetization of NFO/BT heterostructure.     

An experimental study on on-line optimizing control of free radical bulk polymerization in a rheometer-reactor assembly under conditions of power failure

An experimental study on the on-line optimizing control of a sample free radical bulk polymerization system, namely, methyl methacrylate (MMA), is carried out in a rheometer-reactor assembly. Two initiator loadings and three cases involving external disturbances (power failure) are studied. The disturbances are assumed to be of two kinds: one that leads to a sudden increase in the temperature of the reaction mass (cooling water pump failure) over the planned temperature history, T(t), and one leading to a sudden drop in the temperature (heater failure). The temperature and the viscosity, η, histories are used to describe the ‘state’ (conversion, x_m, and weight-average molecular weight, M_w) of the polymerizing mass. The polymerization is first carried out under an off-line computed optimal temperature history, T_op(t), obtained using the adapted jumping gene version of the elitist genetic algorithm (GA-II-aJG). A planned disturbance is introduced after the start of polymerization and continues for a pre-specified duration. A new optimal temperature history, T_reop(t), is calculated on-line (in about 3 min of real time) using GA-II-aJG. This is implemented as soon as the disturbance is rectified. Experimental values of x_m(t), M_w(t) and η(t) are also measured. These are observed to be in good agreement with model predictions for all the cases. It is found that the information on the viscosity of the reaction mass can be used effectively for on-line optimizing control. This can help ‘save’ the batch (give a product having the desired values of the average molecular weights) optimally, in as short a reaction time as possible. The effect of re-tuning of the model parameters using experimental data on the temperature, T_exp(t), and the viscosity, η(t), is also demonstrated.     

Elemental substitution at Tl-site of TlSr2CaCu2O7 superconductor: A review

This review is on the effects of elemental substitutions at the Tl-site of TlSr₂CaCu₂O₇ (Tl-1212) superconductor. The objective of this paper was to determine the elements that enhance or suppress the superconducting transition temperature, T_c of (Tl_1-xM_x)Sr₂CaCu₂O₇ phase. The elements included in this review were M = Pb, Bi, Cr, V, Re, Zr, In, Er, Gd, Na, K, Rb and Se. The relations between transition temperature and the ionic radius, coordination number, valence state and optimization of the hole concentration of the selected elements were studied. The preparation methods, sintering temperature, Tl-1212 phase and structural stability were also discussed. All alkali metals (K, Na and Rb) suppressed the transition temperature of the Tl-1212 phase. The transition temperature generally decreased linearly with ionic radius except for Cr which showed the highest T_c although its ionic radius is smaller.     

Physical and magnetic properties of Mn–Zr doped La–Sr ferrite prepared by the auto-combustion route

This is the first report ever on (Mn²⁺–Zr⁴⁺) doped M-type lanthanum strontium hexaferrite with general formula, Sr_0.85La_0.15(MnZr)_xFe_12−2xO₁₉ where x=0.0, 0.25, 0.50, 0.75, and 1.0, prepared by citrate auto-combustion method. These ferrites were characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDX) and Vibrating sample magnetometer (VSM). X-ray diffraction patterns show the formation of high purity hexaferrite phase without other secondary phases for all the synthesized samples. It was observed from magnetic hysteresis data that the coercive force is reduced from 5692.5 Oe to 1669.2 Oe with increase in doping contents but the net magnetization of the samples varies slightly from 60.6 to 55.2 emu/gm. High saturation magnetization (M_s), low coercivity (H_c) and remanence magnetization (M_r) values of these materials make them particularly suitable for data recording.     

Abnormal Curie temperature behavior and enhanced strain property by controlling substitution site of Ce ions in BaTiO3 ceramics

Dopants have a great effect on the phase transition behavior and the properties of the ferroelectrics. Here we report an abnormal Curie temperature (T_c) behavior and enhanced strain property by controlling the doping site of Ce ions in the BaTiO₃ ceramics. The Ce doped A-site and B-site BaTiO₃ ceramics (BT-xCe-A and BT-xCe-B, x=2, 4, 6, 8 mol%) were prepared by a conventional solid state reaction method through a different sintering temperature. The Raman test and the XPS results give evidence that Ce is successfully incorporated into Ba-site as Ce³⁺ in the BT-xCe-A samples, and into Ti-site as Ce⁴⁺ in the BT-xCe-B samples. Different doping sites have distinct phase transition behavior. Compared with the BT-xCe-A ceramics, the BT-xCe-B ceramics show higher T_c, and the T_c show abnormal increasing behavior with the increase of the Ce content. In the Ce doped BaTiO₃ system, this phenomenon has not been reported before. The origin of the higher T_c and its increasing behavior is discussed from the viewpoint of the larger local strain field generated by the Ce⁴⁺ ions entering into B-site. Besides, the BT-xCe-B ceramics show a stronger diffuse phase transition behavior. The reason is considered that the Ce substituting B-site leads to a multiphase coexistence, which induces more frustration states for the polarization according to the random defect field theory. Due to such distinct phase transition behavior, the BT-xCe-B ceramics show the enhanced maximum polarization (P_max) and enhanced strain properties compared with the BT-xCe-A ceramics. This work may provide a promising way to design high performance materials by controlling the substituting site of the dopant in other lead-free systems.     

The magnetic,thermal transport properties,magnetothermal effect and critical behavior of Co3Sn2S2 single crystal

Through the magnetic/thermal transport measurements combined with the analyses of magnetocaloric effect and critical behavior of Co₃Sn₂S₂ single crystal, the main results we obtained are as follows: in the case of the magnetic field H//c-axis, Co₃Sn₂S₂ exhibits the phase-separation state below T_c in the low-field region (H < 500 Oe). T_c increases slightly from 174 to 177 K with an increase in H from 100 to 10 kOe. The second-order magnetic phase transition near T_c and the itinerant ferromagnetism below T_c are identified. The magnetization below T_c matches well with the three-dimensional Ising model, instead of the mean-field model. In the case of H//ab, T_c changes between 175 and 178 K with varying H. Noticeably, M above T_c exhibits a small positive value, instead of the null M as commonly expected in the paramagnetic region. An extra phase transition below 166 K is observed. The magnetic transition near T_c seems not to be the second-order phase transition. All results show a significant characteristic of anisotropic magnetic phase transition for the Co₃Sn₂S₂ single crystal. They support mutually those in previous reports, moreover, some new phenomena are also observed. They also provide the experimental evidences for the deep insight into the magnetic phase–transition behavior of Co₃Sn₂S₂.     

Magnetic properties of La0.7Sr0.3MnO3 under the pressure and the transport property under the magnetic field

Polycrystalline La_0.7Sr_0.3MnO₃ sample (LSMO) was synthesized by the solid phase reaction; it exhibits the paramagnetic-ferromagnetic transition at T_c = 362 K at the ambient pressure; it is paramagnetic metallic state above T_c and the ferromagnetic metallic state below T_c. It was observed that the pressure effect depends on the temperature range: (a) In the paramagnetic region, the magnetization M hardly changes with the pressure P, that is, ΔM≈0. There exist the antiferromagnetic (AFM) coupled ferromagnetic clusters in the paramagnetic region, and the pressure enhances the AFM coupling. (b) In the temperature range around T_c, the pressure increases M, that is, ΔM > 0, with the concomitant increase in T_c; the average pressure coefficient dT_c/dP is 5.40 K/GPa at P = .74 GPa, much smaller than 15.47 and 15.90 K/GPa for La_0.7Ca_0.3MnO₃ and La_0.9Ca_0.1MnO₃, respectively, due to the different distortion degree of MnO₆ octahedra in Ca and Sr doped manganites. (c) In the temperature region below T_c, the pressure reduces M, that is, ΔM < 0. M is determined by the competition between the Mn³⁺-O-Mn⁴⁺ double exchange and the interparticle dipolar interaction. The pressure enhances the interparticle dipolar interaction, leading to a significant decrease in magnetization. The resistivity of LSMO exhibits the metallic behavior in the temperature range of 5 K~370 K; it decreases as the applied magnetic field H increases from 0 to 7 T, that is, the magnetoresistance effect which is more significant around T_c. The fitting to the low-temperature resistivity shows that the applied magnetic field reduces the scattering from the grain boundary, electron, phonon, and magnon, especially reduces the electron-electron scattering.     

The thermochromic characteristics of Zn-doped VO2 that were prepared by the hydrothermal and post-annealing process and their polyurethane composite films

In this paper, Zn-doped VO₂ nanoparticles have been successfully fabricated by a two-step hydrothermal-annealing process, and the thermally induced visible light transmittance enhancement of Zn-doped VO₂ has been studied for the first time. It is found that Zn-doped VO₂ not only exhibits excellent solar modulation ability (ΔT_sol = 15.27%) but also can reduce the phase transition temperature and increase the visible light transmittance after the heat-induced phase transition (ΔT_lum=+5.78%). Moreover, with the increase of Zn doping concentration, the phase transition temperature (T_c) and phase transition hysteresis (ΔT) both decrease. It is shown that the Zn-doped VO₂-PU films not only have good solar light modulation ability and properties of improving visible light transmission after phase transition, but also have good durability. The research result is of great significance for improving the visible light transmittance after phase transition and realizing the practical application of VO₂ in the field of smart windows.     

Structural and magnetic properties of LaVO3 - Absence of anomalous diamagnetism

The novel magnetic and structural properties of LaVO₃ have been studied in a comprehensive manner using X-ray and neutron powder diffraction, thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, electrical transport, and dc magnetization. The lattice parameters have been found to be larger than previously reported, indicating the possibility of a novel phase. For vanadium ions, the valence state is found to be in accordance with the expected value (+3), the effect of larger lattice parameters is also present in the X-ray absorption spectroscopy measurement. Magnetic transition due to the antiferromagnetic ordering of the vanadium sublattice is observed at 142 K (T_N). It undergoes a transition to a monoclinic crystal structure below 140 K (T_t). Negative magnetization (anomalous diamagnetism), which has been reported in earlier studies on the magnetic properties of LaVO₃, is not observed in the present study. In the M ? H isotherms, the ferromagnetic component of vanadium spins is absent. Below T_N, the estimated magnetic structure of vanadium sublattice from powder neutron diffraction measurements is C-type antiferromagnetic, which persists in the entire low temperature region. The observed novel structural phase gives rise to pure C-type antiferromagnetic arrangement without weak ferromagnetic component. Theoretical calculations have been carried out to understand the nature of the magnetic ground state.     

Influence of Co content on the characterization and magnetic properties of magnetite

Preparation of nanosized Co_xFe_3−xO₄; 0.05 ≤ x ≤ 0.20 particles from metal nitrates solution through citrate–precursor method was performed. XRD pattern of all prepared systems showed single phase with cubic spinel structure. The crystallite size was determined from TEM and found to agree with that calculated from Sherrer's equation (60–76 nm) The magnetic constants such as molar magnetic susceptibility (χ_M), Curie temperature (T_C) and saturation magnetization (M_S) were measured and the results indicated that, at x = 0.2 the values of χ_M, M_S, remanent magnetization (M_r,) and coercive field (H_c) are 23 emu/g mol, 77.62 emu/g, 33.17 emu/g and 574.5 Oe, respectively_.