Remanent Coercivity Research Articles

The preparation and characterization of polymer-derived Fe/Si/C magnetoceramics

In this work, the preparation of polymer-derived Fe/Si/C ceramics was achieved by synthesis and pyrolysis of the preceramic hyperbranched polyferrocenylsilane. The Fe/Si/C ceramics obtained under various experimental conditions such as different pyrolytic temperatures, atmospheres and heating rates, were investigated by TG–MS, XRD, and XPS. Moreover, the magnetic properties of the ceramics were investigated by the vibrating sample magnetometer (VSM). Experimental results indicated that the pyrolysis of the preceramic polymer started above 250°C and completed among 800–1000°C. The crystallization of the pyrolytic product started at 500°C. Besides the heat treatment temperature, the atmosphere and heating rate also played important roles in the composition and magnetic properties of the corresponding Fe/Si/C ceramics. Fe existed as α-Fe in ceramics sintered at 900°C under N2 and NH3 atmospheres. However, the main components in ceramic product sintered under air were γ-Fe2O3 and carbon. Although all of the three ceramic products sintered under different atmosphere were ferromagnetic with low remanent magnetization and coercivity, the ceramics sintered under NH3 had the largest saturation magnetization. The saturation magnetization of the Fe/Si/C ceramics further ascended when fabricated under NH3 with a slower heating rate, in which Fe existed as α-Fe, Fe3O4 and γ′-Fe4N.

Large batch recycling of waste Nd–Fe–B magnets to manufacture sintered magnets with improved magnetic properties

The waste Nd–Fe–B sintered magnets up to 500 kg per batch were recycled to manufacture anisotropic sintered magnets by combination of hydrogen decrepitation (HD) and alloying technique. Magnetic properties and thermal stability of both the waste magnets and recycled magnets were investigated. The recycled magnet exhibits magnetic properties with remanence (Br) of 12.38 kGs, coercivity (Hci) of 24.89 kOe, and maximum energy product [(BH)max] of 36.51 MGOe, respectively, which restores 99.20% of Br, 105.65% of Hci, and 98.65% of (BH)max of the waste magnets, respectively. The volume fraction of Nd-rich phase in the recycled magnets is about 10.1 vol.%, which is bigger than that of the waste magnets due to the additive of Nd3PrFe14B alloy containing more rare earth. The remanence temperature coefficient (α) and coercivity temperature coefficient (β) of the recycled magnets are −0.1155%/K and −0.5099%/K in the range of 288–423 K, respectively, which are comparative to those of the waste magnets.

Experimental mixtures of superparamagnetic and single‐domain magnetite with respect to Day‐Dunlop plots

AbstractDay‐Dunlop plots are widely used in paleomagnetic and environmental studies as a tool to determine the magnetic domain state of magnetite, i.e., superparamagnetic (SP), stable single‐domain (SD), pseudosingle‐domain (PSD), multidomain (MD), and their mixtures. The few experimental studies that have examined hysteresis properties of SD‐SP mixtures of magnetite found that the ratios of saturation remanent magnetization to saturation magnetization and the coercivity of remanence to coercivity are low, when compared to expected theoretical mixing trends based on Langevin theory. This study reexamines Day‐Dunlop plots using experimentally controlled mixtures of SD and SP magnetite grains. End‐members include magnetotactic bacteria (MSR‐1) as the SD source, and a commercial ferrofluid or magnetotactic bacteria (ΔA12) as the SP source. Each SP‐component was added incrementally to a SD sample. Experimental results from these mixing series show that the magnetization and coercivity ratios are lower than the theoretical prediction for bulk SP magnetic size. Although steric repulsion was present between the particles, we cannot rule out interaction in the ferrofluid for higher concentrations. The SP bacteria are noninteracting as the magnetite was enclosed by an organic bilipid membrane. Our results demonstrate that the magnetization and coercivity ratios of SD‐SP mixtures can lie in the PSD range, and that an unambiguous interpretation of particle size can only be made with information about the magnetic properties of the end‐members.

Magnetic, in situ, mineral characterization of Chelyabinsk meteorite thin section

AbstractMagnetic images of Chelyabinsk meteorite's (fragment F1 removed from Chebarkul lake) thin section have been unraveled by a magnetic scanning system from Youngwood Science and Engineering (YSE) capable of resolving magnetic anomalies down to 10−3 mT range from about 0.3 mm distance between the probe and meteorite surface (resolution about 0.15 mm). Anomalies were produced repeatedly, each time after application of magnetic field pulse of varying amplitude and constant, normal or reversed, direction. This process resulted in both magnetizing and demagnetizing of the meteorite thin section, while keeping the magnetization vector in the plane of the thin section. Analysis of the magnetic data allows determination of coercivity of remanence (Bcr) for the magnetic sources in situ. Value of Bcr is critical for calculating magnetic forces applicable during missions to asteroids where gravity is compromised. Bcr was estimated by two methods. First method measured varying dipole magnetic field strength produced by each anomaly in the direction of magnetic pulses. Second method measured deflections of the dipole direction from the direction of magnetic pulses. Bcr of magnetic sources in Chelyabinsk meteorite ranges between 4 and 7 mT. These magnetic sources enter their saturation states when applying 40 mT external magnetic field pulse.

Pressure effect on magnetic hysteresis parameters of single-domain magnetite contained in natural plagioclase crystal

This study investigates pressure effects on the magnetic properties of non-interacting single-domain (SD) magnetite. Using a high-pressure cell specially designed for a Magnetic Property Measurement System, magnetic hysteresis measurements were conducted under high pressures of up to 1 GPa on natural plagioclase crystals containing much acicular SD magnetite. Coercivity and saturation magnetization were nearly constant with pressure, while saturation remanent magnetization and coercivity of remanence decreased with pressure at moderate rates of −8 per cent GPa–1 and −18 per cent GPa–1, respectively. These results suggest that temperature effects govern the magnetic behaviour of acicular SD magnetite grains in the middle and lower crusts.

Synthesis and microwave-absorbing properties of Co3Fe7@C core–shell nanostructure

Co3Fe7@C core–shell nanoparticles with high performance of microwave-absorbing properties were prepared by hydrothermal method and heat treatment. The transformation of structural, morphological and magnetic properties among the carbon-encapsulated composites, which were annealed at three different temperatures, were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). XRD analysis indicated the phase composition of Co3Fe7/CoFe2O4, Fe3C/Co3Fe7 and pure Co3Fe7 at different annealing temperatures. TEM confirmed the Co3Fe7@graphite core–shell nanostructure with an average particle size of 180 nm. The saturation magnetization (M s) increased monotonically with the increase in temperature, which was attributed to the crystal growth and purity of metallic core. Co3Fe7@graphite nanoparticles exhibited the hysteretic loops of soft ferromagnetic behavior with high M s of 222.85 emu g−1, weak remanent magnetization (M r) and coercivity (H c). For Co3Fe7@graphite nanomaterial, a reflection loss exceeding −20 dB was obtained between 2.8 and 10.2 GHz, which almost covering from S-band to X-band. The maximum reflection loss is −26.8 dB at 9 GHz with 1.8 mm thickness. The excellent microwave absorption properties result from the proper electromagnetic match in core–shell nanostructure and the strong natural ferromagnetic resonance.

Synthesis of uniform Co NPs with high saturation magnetization and investigation on removal of methyl orange from aqueous solution by Co/MWCNTs composite

Uniform sphere-like cobalt (Co) nanosized particles (NPs) were synthesized by a solvothermal method, where ethylene glycol (EG) was used as solvent and ethylenediamine (EDA) as an additive. The sphere-like Co NPs show high saturation magnetization, small remanent magnetizations and coercivities, which the saturation magnetization (237emu/g) is higher than those reported in references. A composite (the Co NPs decorated on multi-walled carbon nanotubes (MWCNTs)) was facilely fabricated via one-pot solvothermal method. The composite can be conveniently separated from aqueous solution by an external magnetic field. The pore size, surface area, and pore volume of the composite were determined to be about 2.37nm, 15.33m2/g, and 0.0731cm3/g, respectively. The composite can be used to remove methyl orange (MO) from the aqueous solution, and the maximum adsorption capacity is about 170mg/g, which is higher than those reported in the References.

Sub-micron period lattice structures of magnetic microtraps for ultracold atoms on an atom chip

We report on the design, fabrication and characterization of magnetic nanostructures to create a lattice of magnetic traps with sub-micron period for trapping ultracold atoms. These magnetic nanostructures were fabricated by patterning a Co/Pd multilayered magnetic film grown on a silicon substrate using high precision e-beam lithography and reactive ion etching. The Co/Pd film was chosen for its small grain size and high remanent magnetization and coercivity. The fabricated structures are designed to magnetically trap 87Rb atoms above the surface of the magnetic film with one-dimensional and two-dimensional (triangular and square) lattice geometries and sub-micron period. Such magnetic lattices can be used for quantum tunneling and quantum simulation experiments, including using geometries and periods that may be inaccessible with optical lattices.

Synthesis, structural and magnetic behavior studies of Zn–Al substituted cobalt ferrite nanoparticles

A series of nano-sized Zn–Al substituted cobalt ferrite Co(1−x)Zn(x)Fe2−xAlxO4 with 0.0⩽x⩽1.0 have been synthesized by chemical co-precipitation technique. The XRD spectra revealed the single phase spinel structure of Co(1−x)Zn(x)Fe2−xAlxO4 with average size of nanoparticles are estimated to be 17–30nm. These are small enough to achieve the suitable signal to noise ratio, which is important in the high-density recording media. The FTIR spectra show the characteristic of two strong absorption bands at 560–600cm−1 corresponds to the intrinsic stretching vibrations of the metal at the tetrahedral site and lowest band is observed at 370–410cm−1 corresponds to octahedral site. The crystalline structures of nanoparticles composite were characterized by Field Emission Scanning Electron Microscopy (FE-SEM). The magnetic properties such as saturation magnetization, remanence magnetization, and coercivity were calculated from the hysteresis loops. Saturation magnetization were found to increase up to x=0.4 while remanence magnetization and coercivity continuously decrease with increasing Zn–Al concentration. The stability in coercivity while increase in saturation magnetization confirms that the Co0.6Zn0.4Fe1.6Al0.4O4 ferrite sample is suitable for applications in high-density recording media.

Evolution of magnetic properties and microstructure of Hf2Co11B alloys

Amorphous Hf2Co11B alloys produced by melt-spinning have been crystallized by annealing at 500–800 °C, and the products have been investigated using magnetization measurements, x-ray diffraction, and scanning electron microscopy. The results reveal the evolution of the phase fractions, microstructure, and magnetic properties with both annealing temperature and time. Crystallization of the phase denoted HfCo7, which is associated with the development of coercivity, occurs slowly at 500 °C. Annealing at intermediate temperatures produces mixed phase samples containing some of the HfCo7 phase with the highest values of remanent magnetization and coercivity. The equilibrium structure at 800 °C contains HfCo3B2, Hf6Co23, and Co, and displays soft ferromagnetism. Maximum values for the remanent magnetization, intrinsic coercivity, and magnetic energy product among the samples are approximately 5.2 kG, 2.0 kOe, and 3.1 MGOe, respectively, which indicates that the significantly higher values observed in crystalline, melt-spun Hf2Co11B ribbons are a consequence of the non-equilibrium solidification during the melt-spinning process. Application of high magnetic fields during annealing is observed to strongly affect the microstructural evolution, which may provide access to higher performance materials in Zr/Hf-Co hard ferromagnets. The crystal structure of HfCo7 and the related Zr analogues is unknown, and without knowledge of atomic positions powder diffraction cannot distinguish among proposed unit cells and symmetries found in the literature.

Ferromagnetic Property of Co and Ni Doped TiO2 Nanoparticles

The Co and Ni doped diluted magnetic semiconductor nanoparticle TiO2 is prepared by sol‐gel method. Ti0.97Co0.03O2, Ti0.97Ni0.03O2, Ti0.97Co0.06O2, and Ti0.97Ni0.06O2 samples were characterized by X‐ray scattering techniques and high resolution transmission electron microscope. The results show that there are no other phases existing in TiO2. As to the sample of high‐concentration dopant, the X‐ray scattering techniques have explored the existing of CoTiO3 and NiTiO3. The ferromagnetic measurement shows that the magnetization of the sample of high‐concentration dopant increases in the same external magnetic field. However, the relatively higher dopant Co and Ni may form more interstitial ions and paramagnet matters, reducing the oxygen vacancy concentration and finally leading to the decrease of remanent magnetization and coercivity of the materials.

Synthesis and Characterization of CoxZn1−xAlFeO4 Nanoparticles

Nanocrystalline powders of cobalt and aluminum co-substituted zinc ferrites with general formula CoxZn1−xAlFeO4 (x = 0.0–1.0) have been synthesized for the first time. Using the citrate-microwave technique and the citric acid as combustion–complexion agent (fuel), materials with spinel mono-phase cubic spinel structure were successfully prepared. The characterization of products was done by XRD, SEM and VSM. The crystallite size estimated by Scherrer formula has been found in the range of 7.7–9.6 nm. The magnetic properties were studied by room temperature (RT) VSM magnetization measurements. The small remanent magnetization (Mr) and coercivity (Hc) values reveal the superparamagnetic nature of nanoparticles (NPs) at RT. The extrapolated saturation magnetization (Ms) is maximum for Co0.8Zn0.2AlFeO4 (17.15 emu/g) and minimum for ZnAlFeO4 particles (4.22 emu/g). This case is attributed to high or low amount of cation distribution change from normal to mixed spinel structure. The average magnetic diameters (D mag) were calculated from magnetic fit studies of M–H spectra. D mag values are between 8.17 and 8.46 nm and this range is in great accordance with the obtained diameters from XRD measurements. The small Mr/Ms ratios (maximum, 0.219) specify the uniaxial anisotropy according to Stoner–Wohlfarth model for CoxZn1−xAlFeO4 NPs. RT effective anisotropy constants (K eff ) were calculated by using Ms and Hc values. K eff constants increased with increasing Co content in the spinel NPs.

Magnetic composites of CoFe2O4 nanoparticles in a poly(aniline) matrix: Enhancement of remanence ratio and coercivity

Magnetic composites of cobalt ferrite nanoparticles and poly(aniline) have been synthesized in acid media and characterized by X-ray diffraction studies, scanning and transmission electron microscopy observation, thermogravimetric analysis, conductivity and infrared spectroscopy measurements. The magnetic behavior was studied through DC magnetization measurements; hysteresis loops were observed, showing ferromagnetic behavior for particles and composites. The remanence ratio increases as the polymer contents increases, the coercivity decreases first and then increases, and the saturation magnetization does not follow the ferrite mass fraction in the composite. It is proposed that an indirect exchange mechanism (RKKY-like) between the nanoparticles and PANI localized spins mediated by the conduction electrons is responsible for the observed magnetic behavior. The results suggest the presence of particle–matrix interactions related to the amount of polymer present, thus, in turn suggesting that the material magnetic properties could be controlled through the nanoparticles to polymer ratio.

Cubic superparamagnetic nanoparticles of NiFe2O4 via fast microwave heating

This study demonstrated the possibility of using microwave heating as a fast and cheap method for synthesizing superparamagnetic nanoparticles. In this sense, NiFe2O4 samples were subjected to microwave heating at various temperatures to determine the lowest temperature at which the crystalline phase of the nanoparticles occurs. X-Ray powder diffraction, 57Fe Mossbauer spectroscopy, and transmission electron microscopy of the samples were performed to confirm the formed nanoparticles. It was observed a cubic structure of inverse spinel type with good crystallinity. The magnetic properties of the samples were studied using a vibrating sample magnetometer and was found to zero values to remanent magnetization and coercivity field. This behavior suggests superparamagnetic features for all samples. The crystallite size (9, 10, and 12 nm) and saturation magnetization (31–45 emu/g) were used as a function of the increase of the temperature treatment time. Blocking temperature was found by tracing remanent magnetization versus temperature.

Effect of Sm content on energy product of rapidly quenched and oriented SmCo5 ribbons

The Sm-content dependence of phase composition, anisotropy, and other magnetic properties of Sm1+δ Co5 (δ ≤ 0.12) ribbons melt spun at 10 m/s has been studied. The samples consist of hexagonal SmCo5 grains whose c axes are preferentially aligned along the long direction of the ribbon. The lattice parameter a and the cell volume (V) increase with increasing Sm content δ, whereas c decreases. Sm addition appears to improve the degree of the preferred orientation of the c-axis and to increase the mean grain size, which weakens the effective intergranular exchange coupling. Therefore, the remanence ratio, coercivity, and squareness of the hysteresis loops are significantly enhanced. The remanence ratio of 0.91 and the maximum energy product of 21.2 MGOe, which is the highest value reported so far for Sm–Co ribbons, are achieved for δ = 0.06. High performance in combination with simple processing may facilitate high-temperature applications for anisotropic Sm1+δ Co5 ribbons.

Hydrothermally-induced changes in mineralogy and magnetic properties of oxidized A-type granites

Abstract The changes in magnetic mineralogy due to the hydrothermal alteration of A-type granitic rocks have been thoroughly investigated in samples from the granite of Tana (Corsica, France), and compared with other A-type granites: Meruoca (NE Brazil), Bushveld (South Africa), Mount Scott (Wichita Mountains, Oklahoma, USA) and the stratoid hypersolvus granites of Madagascar. The altered red-colored samples and their non-altered equivalents were magnetically characterized by means of magnetic susceptibility measurements, hysteresis loops, remanent coercivity spectra, and Lowrie test. It is shown that hydrothermalization in magnetite-bearing granites is related to the formation of fine-grained magnetite and hematite, and to coeval depletion in the content of primary low-coercive coarse-grained magnetite. These mineralogical changes give typical rock magnetic signatures, namely lower susceptibility magnitudes and anisotropy degrees, prolate AMS (anisotropy of magnetic susceptibility) fabrics and increased coercivities. Optical microscopy and SEM (scanning electronic microscopy) images suggest that the orientation of the secondary magnetic minerals is related to fluid-pathways and micro-fractures formed during the hydrothermal event and therefore may be unrelated to magma emplacement and crystallization fabrics. Changes in magnetic mineralogy and grain-size distribution have also to be considered for any paleomagnetic and iron isotope studies in granites.

Micromagnetic Simulation of a Perpendicular Exchange-Coupled Magnet

The magnetization switching process of a perpendicular exchange spring system was investigated using micromagnetic simulations. An exchange spring hysteresis loop was observed in this work. Details like coercive field and remanent magnetization are discussed .

Magnetic properties prediction of NdFeB magnets by using support vector regression

A novel model using support vector regression (SVR) combined with particle swarm optimization (PSO) was employed to construct mathematical model for prediction of the magnetic properties of the NdFeB magnets. The leave-one-out cross-validation (LOOCV) test results strongly supports that the generalization ability of SVR is high enough. Predicted results show that the mean absolute percentage error for magnetic remanence Br, coercivity Hcj and maximum magnetic energy product (BH) max are 0.53%, 3.90%, 1.73%, and the correlation coefficient (R2) is as high as 0.839, 0.967 and 0.940, respectively. This investigation suggests that the PSO-SVR is not only an effective and practical method to simulate the properties of NdFeB , but also a powerful tool to optimatize designing or controlling the experimental process.

In-plane Uniaxial Anisotropy and Magnetization Reversal Mechanism of FeCo Films by Strip Pattern

Strip-like FeCo films were patterned by a traditional lithograph process from intrinsically isotropic continuous FeCo films. The strip-patterned FeCo film shows a strong in-plane uniaxial magnetic anisotropy with easy axis along the length direction of the strip. The angular dependences of remanence ratio, switching field, and coercivity indicate that the magnetization reversal mechanism of the strip-patterned FeCo film is coherent rotation and domain wall depinning when the applied field is near the hard axis and easy axis, respectively. The consistency of the experimental hysteresis loops of the strip-patterned FeCo film and calculated hysteresis loops with a simple in-plane uniaxial anisotropy model indicates that the strip-patterned FeCo film behaves as a single domain. The absence of the domain wall and the strong in-plane anisotropy field make the strip-patterned FeCo films have much potential for high-frequency application.

In situ growth of SrFe12O19

SrFe12O19 is a highly anisotropic ferrimagnetic compound with relatively high remanence and high coercivity, which is used in permanent magnets. Permanent magnets are everywhere in our daily life and they are responsible for the interconversion between motion and electricity in electrical components ranging from headphones to wind turbines. Three key parameters, important for making permanent magnets, are an anisotropic structure, size of the nanocrystallites and the microstructure. In situ X-ray powder diffraction has been used to follow the growth kinetics of SrFe12O19 under hydrothermal conditions. Synthesis of SrFe12O19 (Sr-Hexaferrite) nanocrystals by hydrothermal methods have the advantage of allowing exhaustive control of the reaction parameters. We have studied the growth and kinetics of SrFe12O19 by carring out time resolved synchrotron experiments at MAX-lab, Sweden. The experiments were carried out at elevated pressure (250 bar) and in temperatures ranging from 250 to 400 oC. The diffraction data allow us to follow the evolution of the crystallite size as function of temperature, time and composition. By controlling the composition of the precursor we can tailor the size of the nanocrystallites. The obtained data have shown that the synthesis takes place through a conversion of tiny hexagonal shaped FeOOH nanocrystallites into the SrFe12O19. Several ex situ studies under comparable conditions have been carried out to compare the magnetic properties and the obtained nanocrystallites have been investigated using high resolution laboratory powder diffraction data.