Soft Magnetic Particles Research Articles

Carbon nanotube-coated silicated soft magnetic carbonyl iron microspheres and their magnetorheology

Core-shell structured magnetic microspheres were fabricated by coating the surface of silicated soft magnetic carbonyl iron particles with multi-walled carbon nanotubes using a facile layer-by-layer method involving an alternative sequential adsorption of positive and negative polyelectrolytes. The morphology of the fabricated particles was observed by scanning electron microscopy. The magnetorheological (MR) behavior affected by the non-magnetic coatings was examined using a rotational rheometer under a controlled magnetic field. The flow curves of typical MR characteristics with improved shear stresses were observed under a range of magnetic field strengths.

Direct observation of particle rearrangement during cyclic stress hardening of magnetorheological gels

Swollen physical magneto-responsive gels were obtained by self-assembly of triblock copolymers containing dispersed soft magnetic particles. When subject to simple shear cyclic loading–unloading cycles in the presence of a magnetic field, magnetorheological (MR) gels show a stress hardening behavior. The stress-hardening was characterized by the increase of the maximum stress against the number of cycles. Most of the stress increases are achieved in the first and second cycles and only minor effects take place in the subsequent ones. The cyclic behaviour of MR gels with dilute particle volume fraction is also studied with the help of a homemade shear cell mounted on an optical microscope. Despite the difference of particle volume fraction, we are able to relate cyclic stress hardening to the change in particle–particle distribution observed directly at the microscopic scale. While anisotropic particle strings created by a pre-structuring process (original sample alignment) still has many defects, the history of simple shear in the presence of the magnetic field gradually removes the defects of the particle strings and helps them to relax towards a zipped cluster. We show here the first directly observed example of the shear-annealing effect in MR gels. The shear induced rearrangement of the particle network serves as the microscopic origins of the above mentioned cyclic stress hardening behavior of MR gels and seems to be fully reversible after the removal of the magnetic field. Results show that a slow quiescent process also takes place and it is suggested to be related to solvent flow outwards of the clusters, while they get more compact. Interestingly, all directly observed rearrangements including string breaking seem to be fully reversible after removal of the magnetic field.

Anisotropy induced large exchange bias behavior in ball milled Ni–Co–Mn–Sb alloys

We report the effect of decrease in the grain size on the structural, magnetic and exchange bias (EB) behavior in ball milled Ni50−xCoxMn38Sb12 (x=0 and 5) Heusler alloys. The existence of a wide range of grain sizes in the ball milled samples results in dramatic changes in the structural and magnetic properties. For x=0, a large EB field of 3.2 kOe is observed in the ball milled sample, compared to a value of 245 Oe of the bulk sample. This increase is attributed to the enhanced exchange coupling between the soft and hard magnetic particles.

COUPLED MAGNETIC FIELD AND VISCOELASTICITY OF FERROGEL

Composed of a soft polymer matrix and magnetic filler particles, ferrogel is a smart material responsive to magnetic fields. Due to the viscoelasticity of the matrix, the behaviors of ferrogel are usually rate-dependent. Very few models with coupled magnetic field and viscoelasticity exist in the literature, and even fewer are capable of reliable predictions. Based on the principles of non-equilibrium thermodynamics, a field theory is developed to describe the magneto-viscoelastic property of ferrogel. The theory provides a guideline for experimental characterizations and structural designs of ferrogel-based devices. A specific material model is then selected and the theory is implemented in a finite element code. Through numerical examples, the responses of a ferrogel in uniform and non-uniform magnetic fields are analyzed. The dynamic response of a ferrogel to cyclic magnetic fields is also studied, and the prediction agrees with our experimental results. In the reversible limit, our theory recovers existing models for elastic ferrogel, and is capable of capturing some instability phenomena.

The Experimental Analysis of Modifier on Fluid Magnetic Abrasivetool

Fluid magnetic abrasivetool is a kind of suspension, which is composed of soft magnetic particles, surfactants, base liquid and so on so forth, so it can have strong magnetorheological characteristics and grinding characteristics. The stability of magnetorheological fluid is one of the most important indicator to evaluate the quality of magnetorheological fluid. Sodium hexametaphosphate and PAA have perfect function to maintain dispersionstability and retrogradation, adding nano-silica can improve the retrogradation of magnetic abrasivetool, which can modify redispersibility of magnetic abrasivetool.

ChemInform Abstract: Magnetorheology: Materials and Application

AbstractReview: 129 refs.

Interaction forces between soft magnetic particles in uniform and non-uniform magnetic fields

The influence of the magnetization of a soft magnetic sphere on the surrounding magnetic field is measured and characterized. The interaction force between two soft magnetic particles is directly measured using an ultra precision load sensor in uniform and non-uniform magnetic fields. The interaction force largely follows an inverse fourth power law as a function of separation distance between particle centers. At small distances, the effect of magnetization of one particle on the magnetization of its adjacent particle causes the attractive (repulsive) force to be larger (smaller) than that predicted by the inverse fourth power law. The theoretical prediction based on a modified dipole model, that takes into account the coupling effect of the magnetization among soft magnetic particles, gives excellent agreement with the measured force in a uniform magnetic field. The interaction force under a non-uniform applied magnetic field can be reasonably predicted using the dipole–dipole interaction model when the local magnetic field is used to determine the magnetization.

Nano-sized Fe soft-magnetic particle and its magnetorheology

As a new magnetoresponsive magnetorheological (MR) material under an applied magnetic field, magnetic Fe nanoparticles were synthesized from a simple process of thermal decomposition of pentacarbonyl iron using oleyl amine and kerosene at 150 °C. Morphology of the fabricated Fe nanoparticle was examined using both scanning electron microscopy and transmission electron microscopy. MR characteristics of the nano-sized magnetic particle-based MR fluid dispersed in non-magnetic carrier fluid was investigated using a rotational rheometer under different external magnetic field strengths, focusing on their flow behaviors at a steady shear mode and yield stress. Flow curve was also found to be fitted well with the Casson equation.

Bat head contains soft magnetic particles: Evidence from magnetism

Recent behavioral observations have indicated that bats can sense the Earth's magnetic field. To unravel the magnetoreception mechanism, the present study has utilized magnetic measurements on three migratory species (Miniopterus fuliginosus, Chaerephon plicata, and Nyctalus plancyi) and three non-migratory species (Hipposideros armiger, Myotis ricketti, and Rhinolophus ferrumequinum). Room temperature isothermal remanent magnetization acquisition and alternating-field demagnetization showed that the bats' heads contain soft magnetic particles. Statistical analyses indicated that the saturation isothermal remanent magnetization of brains (SIRM(1T_brain)) of migratory species is higher than those of non-migratory species. Furthermore, the SIRM(1T_brain) of migratory bats is greater than their SIRM(1T_skull). Low-temperature magnetic measurements suggested that the magnetic particles are likely magnetite (Fe3O4). This new evidence supports the assumption that some bats use magnetite particles for sensing and orientation in the Earth's magnetic field.

Rotating field driven structure of particulate magnetic filler in a drying polymer layer

This paper presents computational studies of the dissipative self-organization of a system of soft magnetic particles in a drying thin layer of polymer solution set under a rotating magnetic field. The structure transformations during the layer drying is modeled for different situations including constant/increasing viscosity, regard/disregard of the hydrodynamic interactions, slow or fast drying with respect to the structure formation. All these factors are found influential on the final pattern of the particles distribution in the settled polymer film.

Effect of addition of soft magnetic alloy particles on the flux trapping in Gd123 bulk superconductors

Pinning stability and the introduction of magnetic flux pinning is an essential problem in applications of high-Tc superconductors. Study on the role of addition of a variety of metal oxides into GdBa2Cu3O7−δ (Gd123) bulk superconductors was carried out. We found that the addition of 0.05 wt. % of soft magnetic alloy particles Fe-Cu-Nb-Si-Cr-B (Fe-B) into the Gd123 contributes to the enhancement of the critical current density (Jc) under a wide range of applied magnetic fields up to 3 T. The Fe-B particles refined less than 10 μm by ball milling indicate no remarkable contribution on the Jc under the magnetic field. The reduction of the Ba content resulted in the appearance of a peak of Jc which has been observed in the Gd/Ba solid solution with rich Ba content. These results let us discriminate the effect of the magnetic particles from other conventional flux pinning mechanism. The peak of Jc under magnetic field was not only observed in the part along the c-axis under the seed of the sample but also in the growth sector around the periphery of the Gd123 bulk with Fe-B addition. It indicates that the magnetic particles inclusions play an important role on the homogeneous enhancement of Jc and the high flux pinning performance.

Spatial variation of superconducting properties of Gd123 Bulk superconductors with magnetic particles addition

Magnetic flux trapping and the homogeneity of the flux pinning are essential problems in the practical application of high-temperature superconductors. We have conducted study on the role of addition of soft magnetic Fe-B alloy particles contribute to the enhancement of the critical current density (Jc) under wide-range of magnetic field. Magnetic flux trapping was enhanced in Gd123 bulk superconductor with suitable amount of magnetic particles addition. In addition, it can be effective as pinning center enhance the Jc of the bulk in both the a–b growth sector and the c-growth sector under magnetic field. However, the Tc of the Gd123 bulk was decreased obviously by addition of magnetic particles. The study on the spatial variation of superconducting properties indicates that the performance of the upper part of the bulk is better than the bottom. By comparing the superconducting properties of the Gd123 bulk with magnetic particles addition and without magnetic particles addition, we concluded that there is a trace of the formation of homogeneous pinning properties in the magnetic particles addition Gd123 bulk.

Soft magnetic composite particles of reduced iron coated with poly(p-xylylene) via chemical vapor deposition polymerization

Soft magnetic composite particles of reduced iron coated with ultrathin films of poly(p-xylylene) (PPX) were prepared via chemical vapor deposition (CVD) polymerization. The structure, morphology, magnetic properties, and chemical stability of PPX encapsulated reduced iron particles were studied. It can be confirmed from FTIR spectrum that the polymer covered outside the reduced iron particles is PPX. The thickness of PPX film is averagely 100 nm according to the results of transmission electron microscopy (TEM). The mean diameter of the composite particles is about 6.0 μm. The resistance to thermooxidation of the composite particles is much higher than that of the reduced iron particles due to the protection of PPX ultrathin film outside the iron powder. After being encapsulated with PPX ultrathin film, the resistance to acidic corrosion of the reduced iron particles is obviously improved. Although the saturation magnetization (Bs) of the composite particles is reduced from 1.42 T to 1.33 T after the coating process, their coercive force (Hc) decreases from 10.6 kA/m to 6.02 kA/m, which is advantageous for the application of soft magnetic particles in MR fluid.

Magnetic Properties of CoFeSiB Powders, Suspensions, and in Rigid Matrix

The aim of this study is to investigate the magnetic properties of micrometric soft magnetic CoFeSiB particles as powders, in suspensions and embedded in a rigid polymer matrix. The CoFeSiB particles were obtained by milling in a planetary ball mill amorphous magnetic microwires prepared by an in-rotating-water quenching technique. The milled powder was sieved by means of a sieve shaker using a series of micrometric mesh sieves. A series of powders with particles in the following dimensional ranges were obtained: 40-63, 63-80, and 80-100 micrometers. The particles were imaged by mean of SEM that revealed that the particles had irregular shapes. Also, magnetic measurements were performed for all the powders. The powders were further used to prepare suspensions of magnetic particles in a viscous liquid carrier which are known as magnetorheological (MR) fluids. Magnetic measurements made on magnetorheological suspensions reveal that particles mobility and structuring result in increased susceptibility. Finally, we prepared samples of magnetic CoFeSiB particles embedded in a polymer rigid matrix. The particles were in the same concentration as in the case of the MR fluids. The magnetic measurements on the material thus prepared indicated that the magnetic saturation is closed to the magnetic saturation of the MR fluid but no changes in the susceptibility compared to powders were observed.

Magnetorheology: materials and application

As one of the most important field-responsive intelligent and smart soft matter materials, magnetorheological (MR) fluids, consisting of magneto-responsive magnetizable particles suspended in non-magnetic fluids, have drawn a lot of attentions in both academia and industry as their physical and rheological characteristics can be controlled with external magnetic field strength. In this highlight, preparation methods and MR properties of various magnetic composites with soft magnetic particles and polymers are reviewed. In addition, some industrial applications, such as a MR dampers and a MR polishing, are briefly summarized.

Synthesis and magnetic softness of sub-micron amorphous particles

Magnetically soft amorphous particles were successfully synthesized in aqueous solutions. The advantages of the particles are their high saturation magnetization and submicron size. In addition to the original synthesizing process, particle precipitation in a magnetic field is studied. In the process of precipitation in the magnetic field the particles are bonded into long chains leading to an appreciable reduction of the demagnetizing field that has been an obstacle to the high-permeability performance of soft magnetic particles. The microstructures, magnetic softness of the chains, and significant enhancement of the permeability in the frequency range 0.05∼10 GHz are demonstrated.

Theoretical and experimental approach to characteristic magnetic measurement data of polymer bonded soft magnetic composites

Soft magnetic composites consisting of an organic binder and FeCoV particles have been produced and examined afterward regarding the dependence of magnetic field H at polarization J on filler content C. In general, the applied magnetic field to achieve a certain value of polarization is proportional to the inverse filler content, following H∼1∕C, and the polarization is directly proportional to the filler content, following J∼C. With use of the evaluated material data and the inner demagnetization factor Ni a model for calculating the permeability of soft magnetic composites has been derived. As input values the permeability of the soft magnetic particles μ, the filler content C, and the geometric demagnetization factor of the particles Np are used. The elaborated approach shows an excellent agreement with experimental permeability data of the FeCoV composites.

Magnetic properties of hybrid-soft magnetic composites

Soft magnetic composites containing an organic binder and mixtures of two different soft magnetic particles – iron and nano-crystalline Fe 73.5Si 15.5B 7Cu 1Nb 3 – in varying mixing ratios have been prepared at a total filler fraction x mag = 0.5 and examined regarding the magnetic properties. Results show that the coercivity is decreased by replacing the magnetically harder material with the magnetically softer material in the composite until a certain threshold value of the mixing ratio is reached. Above this threshold the coercivity stays nearly constant. Total losses P / f , saturation polarization J s and inner demagnetization factor N i follow a linear mixing rule. The composite’s permeability μ is a result of filler fraction, mixing ratio, coercivity, polarization and inner demagnetization factor and due to that complex dependence hardly predictable.

Analysis and fabrication of patterned magnetorheological elastomers

This paper presents analysis, fabrication and characterization of patternedmagnetorheological (MR) elastomers. By taking into account the local magnetic field inMREs and particle interaction magnetic energy, the magnetic-field-dependentmechanical properties of MREs with lattice and BCC structures were theoreticallyanalyzed and numerically simulated. Soft magnetic particles were assembled in apolydimethylsiloxane (PDMS) matrix to fabricate new MR elastomers with uniform latticeand BCC structures, which were observed by a microscope. The field-dependentmoduli of the new MR elastomers were characterized by using a parallel-plateMR rheometer. The experimental results agreed well with numerical simulations.

Microwave permeability spectra of flake-shaped FeCuNbSiB particle composites

The effective permeability of flake-shaped FeCuNbSiB particles/nonmagnetic matrix composition in high frequency was measured and calculated. In contrast to the relatively larger size and irregular shape particles, the flake particles have higher permeability. The results are attributed to the different magnetic loss mechanisms. According to the skin-effect criterion, we find that the magnetic loss in flake particles is mainly caused by the natural resonance, compared with the eddy current effect in the larger size and irregular shape particles. Using the shape anisotropy, the flake soft magnetic particles overcome the difficulty of the relatively small intrinsic anisotropies and increase the natural resonant frequencies to the gigahertz range, leading to the higher real part and imaginary part of the permeability. The resonance peak of flake particles was simulated using the combination of the Landau–Lifshitz–Gilbert equation and Bruggeman’s effective medium theory considering a random spatial distribution of magnetic easy axes. Our theory simulation agrees well with the experimental data.