Low Value Of Coercive Field Research Articles

Structural, Optical, and Magnetic Properties of o‐TbFeO3 Nanomaterials Synthesized by the co‐Precipitation Method

AbstractFollowing the co‐precipitation method for producing terbium orthoferrite (TbFeO3), the final products were denoted as TbFeO3‐650, −750, and −850. A single‐phase of orthorhombic TbFeO3 crystals (o‐TbFeO3) was obtained with a calcination temperature of up to 850 °C, while in the range of 650–750 °C there was still a small amount of terbium and iron oxide phases. The average size of o‐TbFeO3 crystallites for the TbFeO3‐850 sample was about 76 nm and was larger than that of TbFeO3‐650 (~45 nm) and TbFeO3‐750 (~47 nm). The o‐TbFeO3 crystal structure in all samples was found to be distorted. In the applied field from −15 to +15 kOe, all of the samples had low values of coercive field (5–11 ⋅ 10−3 Oe) but high values of net magnetization (2.06–2.40 emu ⋅ g−1); the samples appeared to behave as paramagnetic materials. It was shown that the TbFeO3‐850 sample has slightly better magnetic properties than the two others. An average grain size of about 79 nm was found in the TbFeO3‐850 sample, which exhibited a morphology of sphere‐like shapes with highly agglomerated grains. The TbFeO3‐850 sample demonstrated a high absorbance at wavelengths of 400–600 nm, with a direct bandgap of about 2.68 eV.

Study of morphology and magnetism of MnFe2O4–SiO2 composites

Mixed magnetic composites of manganese ferrite and cristobalite (MnFe2O4–SiO2) were produced by the solid-state reaction method, using as starting materials in natura ores found in the state of Rio Grande do Norte – Brazil. Based on the stoichiometric proportions of the precursor ores, various composites in the form of powders and pellets were synthesized at a temperature of 1200 °C/12h under ambient atmosphere. Samples in the form of pellets were made at compression pressures of 370 and 740 MPa to investigate the effect of pressure on the physicochemical, and dielectric properties of the material. Structural, morphological and magnetic analyses were studied for the series of samples. X-ray diffractogram (XRD) results confirmed the formation of the mixed spinel phase MnFe2O4 with cubic symmetry and space group Fd-3m:1, together with peaks corresponding to SiO2–cristobalite. The Rietveld refinements identified a predominance of about 90% of the MnFe2O4 phase with crystallite variations of 130–190 nm due to the effect of compaction pressure. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and transmission electronic microscopy (TEM) mappings show clusters of triangular crystals in the pellets series. Magnetic hysteresis cycles at room temperature show a fast magnetization saturation (MS) response in a field of ∼4 kOe, with parameters dependent on the size and shape of the crystallites. The magnetically soft ferrites exhibited very low coercive field values. The hysteretic curves measured at 5 K showed a significant gain in the MS of ∼30 emu/g. Analyzes of magnetization as a function of temperature identified a blocked regime with a maximum ∼97 K (powder) and ∼170 K (pellets), and an irreversibility of the ZFC–FC curves up to room temperature. An indication of a spin-glass (SG) like state close to 20 K can also be seen in these results. Dielectric characterization of the pellets was performed from 1.0 to 8.5 GHz, and presented well-defined behavior, where real permittivity can be highly related to Rietveld refinement of XRD, and SEM/EDS results. This research suggests the feasibility of producing manganese ferrite composites directly from minerals, without the use of high-purity chemical reagents, allowing their obtainment on a large scale for commercial use.

Spin asymmetry originating from densities of states: Criterion for ferromagnetism, structures and magnetic properties of 3d metals from crystal field based DOSs

Ferromagnetism in 3d metals is re-examined in a simple band model. It is shown that the molecular field model cannot account for the low values of coercive field in ferromagnetic pure metals, and that the standard Stoner theory of band ferromagnetism incorrectly evaluates the total electronic energy and cannot predict reasonable Curie temperatures. A simple band model for magnetism is formulated, yielding a criterion for ferromagnetism even in absence of a Hubbard term, involving only the density of states (DOS) at the Fermi level, its derivative and the filling of the 3d band. By introducing a double-peaked DOS, one may explain the occurrence of ferromagnetism in bcc Fe, hcp-fcc Co and fcc Ni, the stabilization of fcc-hcp or bcc structures across all 3d elements, the occurrence of antiferromagnetism in chromium, and derive reasonable Curie temperatures. ‘Re-entrant’ ferromagnetism is predicted at ultrahigh temperatures, suggesting an alternate origin for the geomagnetic field.

Enhancement in the piezoelectric properties in lead-free BZT-xBCT dense ceramics

In this paper, we have prepared the lead-free piezoelectric BZT-xBCT (x = 0.48, 0.49, 0.50 and 0.51) polycrystalline compositions using solid-state reaction technique. Structural, microstructural and electrical properties were explored by changing the BCT concentration. Structural analysis shows the existence of morphotropic phase boundary in BZT-xBCT (x = 0.50) sample. The SEM micrographs illustrate the decrease in grain size with increase in the BCT concentration. The temperature-dependent dielectric behavior depicts high dielectric constant ~ 26,100 and low loss ~ 0.04 for BZT-0.48BCT composition at Curie temperature. The PE hysteresis loop shows the well-saturated hysteresis loops and low value of coercive field (EC). The higher values of electromechanical planar coupling coefficient, KP ~ 51.2%, piezoelectric coefficient, d33 ~ 512 pC/N and bipolar strain (~ 0.16%) were recorded for BZT-0.50BCT ceramic composition.

Rietveld analysis, enhanced magnetic, dielectric and ferroelectric properties of Gd3+ and Ti4+ co-doped LaFeO3 multiferroic

Undoped lanthanum iron oxide (LaFeO3 i.e. LFO) and Gd3+-Ti4+ co-doped La0.85Gd0.15Fe0.90Ti0.10O3 (LGFTO) samples are prepared by solid state reaction method. The X-ray diffractograms (XRD) analyses by Rietveld refinement confirm that both the prepared samples show distorted perovskite structure having space group Pbnm. X-ray photoelectron spectroscopy of both LFO and LGFTO suggests absent of impurity in the prepared samples. Isothermal magnetic hysteresis loops (M−H) show that the magnetic moment of LGFTO is enhanced compared with LFO not only at low but also at room temperature. Thermal variation of magnetization shows that the ferromagnetic phase is dominated over the antiferromagnetic phase below ~56 K. The frequency dependent dielectric constant of LGFTO follows Cole-Cole relaxation in the high frequency region. The polarization vs. electric field (P-E) loops of LGFTO shows a high value of polarization and low value of coercive field compared to LFO, which indicates improved ferroelectric properties of LGFTO.

Impedance, dielectric, and magnetic properties study of La2CrMnO6 ceramics

Double perovskite La2CrMnO6 (LCrMO) ceramics was prepared using solid state reaction technique sintered at 1433 K in ambient air. Wide range impedance spectroscopy study conducted in 20 Hz – 1 MHz over 85–440 K range allowed for separation of grain bulk and grain boundary effects. Electrical features corresponded to grain bulk and grain boundary properties were investigated. The features of imaginary part of impedance indicated electrical relaxation phenomenon occurrence in LCrMO. The temperature dependence of most probable relaxation times, τZ, obtained from impedance spectra collected below 120 K were fitted using Arrhenius relation and the activation energy equal to 0.14 eV was estimated. The Kramers-Kronig transform of the dielectric permittivity enabled to distinguish relaxation process over 85–170 K temperature range. The variable range hopping of small polaron model was fitted with better accuracy to the dielectric relaxation times, τdiel, temperature dependence, which indicated occurrence of Fermi glass related to local disorder. AC and DC conductivity relaxation processes, which corresponded to grain bulk and grain boundary contributions, respectively, were discriminated. Magnetic susceptibility of LCrMO ceramics was measured over 5–300 K range. Bifurcation between zero field cooled and field cooled dependence occurred below ~120 K. Magnetization M(H) hysteresis loops were not dependent on temperature and exhibited low values of coercive field, ~108 Oe and remnant magnetization, ~6 emu g−1, over 5–200 K range. The magnetic spin glass features were deduced. We note that coupling between electric and magnetic properties was detected: anomaly around 87 K in magnetization, related to magnetic transition, is accompanied by deviation of dielectric relaxation times discerned from the VRH temperature dependence.

Solution combustion synthesis of Ca hexaferrite using glycine fuel

Abstract Ca 1-X La X Fe 12 O 19 (x = 0 and 0.4) polycrystalline powders were synthesized by the solution combustion route using glycine as a fuel and nitrates as oxidizers. After the combustion reaction, the prepared powders were calcined at 1100 °C and 1200 °C in a static air atmosphere for 1 h. The thermo gravimetric (TG) study was carried out to understand the ignition temperature. The structural and morphological studies of the prepared hexaferrite powders were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. Magnetic properties of these samples were also measured by vibrating sample magnetometer (VSM). The XRD results of the samples which consisted of La as dopant and calcined at 1200 °C for 1 h, indicated the formation of calcium hexaferrite and also α-Fe 2 O 3 . However, the XRD results showed that CaFe 4 O 7 and α-Fe 2 O 3 phases were formed in the sample with the same condition but without using any dopant. The micrograph of SEM showed that the calcium hexaferrite particles were regular hexagonal platelets with the size range of 1–2 μm. Maximum magnetization (M Max ), remanent magnetization (M r ), and coercivity (H c ) were measured from the hysteresis loops. Low values of coercive field (23.52 kA m −1 ) and maximum magnetization (58.75 Am 2 kg −1 ) were originated from calcium hexaferrite particle which calcined at the temperature of 1200 °C for 1 h.

Processing, structure and magnetic properties correlation in co-precipitated Ca-ferrite

La-substituted hexagonal calcium ferrite, Ca1−XLaXFe12O19 (x varies from 0 to 0.6 with the step of 0.2), was synthesized by applying co-precipitation method, in which the molar ratio of Fe3+/(Ca2++La2+) was 11. The ferrite precursors were prepared from aqueous solution of calcium nitrate, ferric nitrate and lanthanum nitrate by co-precipitation of calcium, iron and lanthanum ions by using an aqueous base of sodium hydroxide (1.5M) at the pH of 14 and at room temperature. These precursors were calcinated with different amount of La at different temperature of 700, 1100 and 1200°C for constant calcination time of 1h in a static air atmosphere. Some tests such as simultaneous thermal analysis (STA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM) were carried out to investigate the thermal behavior, crystallographic properties, morphology and magnetic properties of the precursor powders which were calcinated at different temperatures. The powder XRD patterns of samples which consisted of La as dopant and were calcinated at 1200°C for 1h, indicates the formation of calcium hexaferrite and also α-Fe2O3 besides Magnetoplumbite-phase (M-phase). However, the results showed that CaFe4O7 and α-Fe2O3 phases were formed in the sample with the same condition but without using any dopant. The results of SEM showed that the calcium hexaferrite particle were regular hexagonal platelets with the size range of 1–2µm. The magnetic properties such as maximum magnetization (MMax), remanent magnetization (Mr) and coercivity (Hc) were measured from the hysteresis loops. Low values of coercive field (16.3kAm−1) and maximum magnetization (50.6Am2kg−1) were obtained from calcium hexaferrite particle in optimum amount of La (X=0.4) which calcinated at the temperature of 1200°C.

High Tc ferroelectricity in Ba-doped ZnO nanoparticles

Effects of Ba doping on structural, optical and ferroelectric properties of ZnO nanoparticles prepared by a low cost thermal decomposition method are presented. The substitution of Ba on Zn sites of the wurtzite structure of ZnO was observed by X-ray diffraction. Some structural transformation in the morphology of nanostructure with Ba doping was observed. Redshift in band gap is observed in the UV–visible spectra after Ba doping, supported by photoluminescence spectra and showing enhanced defect states with Ba doping. In dielectric studies, high value of dielectric constant and transition temperature at (~330°C) were observed. High value of remnant polarization (1.01µCcm−2) and low value of coercive field (2.02kVcm−1) were also observed in ferroelectric studies which can be useful for potential applications.

Phase States in Multicomponent PZT-Based Materials Depending on Poling Conditions

Phase states of the PZT-based multicomponent solid solutions belonging to the rhombohedral area of the phase х-Т diagram with low coercive field values are considered. The solid-solution samples in the initial state are proved to be heterophase (R3c and P4mm mixtures) and reveal relaxor properties. The anomalies of dielectric characteristics obtained in poled samples are due to the R3c→R3m phase transition. The manifestations of relaxor properties above the temperature of the dielectric permittivity maximum are caused by the R3m phase in the P4mm and Pm3m phase mixture.

Fatigue effect in ferroelectric crystals: Growth of the frozen domains

The model of the fatigue effect during cyclic switching caused by growth of the frozen domain area with charged domain walls has been proposed. It was claimed on the basis of the previous experimental results that for switching in increasing field the frozen domain area started to grow at the given sub-threshold field value and stopped at the threshold field. The influence of the shape and frequency of the field pulses used for cyclic switching has been considered. The uniaxial ferroelectric stoichiometric lithium tantalate single crystals produced by vapor transport equilibration with record low value of coercive field have been chosen as a model material for experimental verification of the model. The formation of the charged domain walls as a result of cyclic switching has been revealed by analysis of the domain images obtained by optical and Raman confocal microscopy. It has been shown that the fatigue degree is equal to the fraction of the frozen domain area. The experimental dependence of the switched charge on the cycle number has been successfully fitted by modified Kolmogorov-Avrami formula. The experimentally observed frequency independence of fatigue profile for rectangular pulses and frequency dependence for triangular pulses has been explained by proposed model.

Polarization reversal and jump-like domain wall motion in stoichiometric LiTaO3 produced by vapor transport equilibration

The polarization reversal and domain structure evolution has been studied in stoichiometric lithium tantalate prepared by vapor transport equilibration process. The first in situ visualization of domain kinetics has demonstrated the jump-like motion of few strictly oriented plane domain walls, which leads to short isolated current pulses in the switching current data. The proposed model of jump-like domain wall motion caused by interaction with pinning centers representing the areas with increased value of the threshold field is based on the effect of retardation of bulk screening. The derived formulas were applied successfully for analysis of the field dependence of the total switching time. The durations of wall jumps and wall stays (rest times) extracted from the switching current data are analyzed separately. The deceleration of the wall motion velocity during jump is controlled by the trail of residual depolarization field produced by bound charges and screening charges in the area behind the wall. The duration of the rest time is governed by the bulk screening of residual depolarization field. The value of Hurst exponent 0.75 obtained by fractal analysis of the switching current data has confirmed the essential influence of prehistory on the domain wall motion. The measurements of the coercive field by switching in bipolar triangular pulses in wide range of the field ramp rate have allowed us to extract the record low value of coercive field 60 V/mm for quasi-static polarization reversal.

Sol–gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis

Nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4) have been prepared by a sol–gel method where the ultra-sonication technique has been adopted to reduce the agglomeration effect among the nanoparticles. The samples were heat-treated at three different temperatures and the formation of the nanocrystalline phase was confirmed by X-ray diffractograms (XRD). The average particle size of each sample has been estimated from the (311) peak of the XRD pattern using the Debye–Scherrer formula and the average sizes are in the range of 10–21nm. The average particle size, crystallographic phase, etc. of some selected samples obtained from the high-resolution transmission electron microscopy are in agreement with those estimated from the XRD patterns. Static magnetic measurements viz., hysteresis loops, field cooled and zero field cooled magnetization versus temperature curves of some samples carried out by SQUID in the temperature range of 300 to 5K clearly indicate the presence of superparamagnetic (SPM) relaxation of the nanoparticles in the samples. The maximum magnetization of the SPM sample annealed at 500°C is quite high (68Am2/Kg) and the hysteresis loops are almost square shaped with very low value of coercive field at room temperature (827.8A/m). The particle size, magneto-crystalline anisotropy, etc. have been estimated from the detailed theoretical analysis of the static magnetic data. The dynamic magnetic behavior of the samples was also investigated by observing the ac hysteresis loops and magnetization versus field curves with different time windows at room temperatures. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses, etc. were extracted from dynamic measurements. Dynamic measurements confirmed that the samples are in their mixed state of SPM and ordered ferrimagnetic particles, which is in good agreement with the results of static magnetic measurements. Mössbauer spectra of the samples recorded at room temperature (300K) and at different temperatures down to 20K confirmed the presence of the SPM relaxation of the nanoparticles of the samples.

Ni–Zn nanoferrite for radar-absorbing material

Nanoparticles of nickel–zinc ferrite have been prepared by using the citrate precursor method. According to scanning electron microscopy (SEM), the particle size is nanometric for the powder calcined at 350 °C/3.5 h. The phase formation has been studied by applying different calcining atmospheres, such as air and argon. Pure Ni–Zn ferrite has been observed when calcined in argon at the temperature of 350 °C. Hysteresis analyses have been done with magnetization of 53.01 emu/g at 350 °C and obtaining 84.62 emu/g at 1100 °C due to an optimization of domains formation at high temperature. Measures of reflectivity of Ni–Zn ferrite/epoxy composite have been obtained below 21% at 350 °C and above 96% at 1100 °C with a coercive field of 26.61 Oe. Low value of coercive field increased the mobilization of domains wall and increased the radiation absorption.

Nanocrystalline Cu-free HITPERM alloys with improved soft magnetic properties

The crystallization behaviour in Fe44.5Co44.5Zr7B4 alloy is investigated by DSC-calorimetry, transmission electron microscopy and magnetization measurements. Two distinct crystallization steps, which correspond to the formation of α′-FeCo phase (Tx1 = 774 K) and the crystallization of the remaining amorphous matrix (Tx2 = 957 K) are observed. Typical size of the nanograins in the samples that show advanced degree of crystallization is about 25 nm. The measurements of the hysteresis loops are performed in the temperature range 300 K–700 K. The specimens in the optimum heat treated stage exhibit fairly low values of coercive field (Hc < 35 Am–1) in whole temperature range investigated. These values of coercive field are lower than those reported for the original HITPERM alloys.

Structure and magnetism of electrodeposited ZnSe–Co granular films

Abstract ZnSe–Co granular films have been electrodeposited under potentiostatic condition on polished stainless steel substrates. X-ray diffraction and transmission electron microscopy measurements indicate the formation of polycrystalline granular films with Co particles dispersed in an ZnSe matrix without evidence of intermediate compound formation. Magnetic measurements reveal low values of coercive field and remanence, indicating that the overall magnetic response of the deposited films is determined by multidomain nanosized particles.

La0.95Mg0.05MnO3: an ideal ferromagneticsystem?

Detailed measurements of the field- and temperature-dependent ac susceptibilityand magnetization of LaMnO3 substituted with 5 at.% of the divalent cationMg are presented. This system is a semiconducting ferromagnet at lowtemperature; in particular, analysis of these data yields Tc = 147.2±0.2 K,with critical exponents δ = 4.75±0.15 (from the field dependence of thesusceptibility along the `crossover' line, and confirmed by measurements alongthe critical isotherm), γ + β = 1.75±0.05 (from the temperaturedependence of the crossover line) and γ = 1.39±0.05 (from thetemperature dependence of the susceptibility along the same line). Withinexperimental uncertainty these exponent values agree with those predicted by theisotropic, three-dimensional Heisenberg model. Nevertheless this system exhibitssome unusual characteristics, specifically in the temperature dependence of the(low-) field-cooled and zero-field-cooled magnetization. The latter have beenmodelled by a Preisach-based approach, which helps to resolve a ubiquitousdilemma in the doped manganites, viz. the appearance of a technicalhardness irreconcilable with low coercive field values. A possible origin forthe large, reversible component in the response below Tc is presented.

Epitaxial τMnAl/NiAl magnetic multilayers on AlAs/GaAs

We have successfully grown a novel type of epitaxial metallic multilayers consisting of ultrathin ferromagnetic tetragonal τMnAl and nonmagnetic (CsCl-type) NiAl on (001) GaAs substrates using molecular beam epitaxy. Reflection high energy electron diffraction and cross-sectional transmission electron microscopy (TEM) show that monocrystalline MnAl/NiAl multilayers are formed with the expected epitaxial orientations and that the c axis of the tetragonal structure of the ultrathin MnAl film is aligned perpendicular to the substrate. Composition modulation is clearly evidenced by TEM even when the thickness of each MnAl and NiAl layer is as thin as 3 monolayers. Perpendicular magnetization of the MnAl/NiAl multilayers is shown by both magnetization and magnetotransport measurements at room temperature, exhibiting remarkably square hysteresis loops, high values (Mr≳300 emu/cm3) of remanent magnetization, and relatively low values of coercive field (0.2&amp;lt;Hc&amp;lt;1 kOe).