Magnetic properties of Fe4N NAD Fe pigments

The Fe4N pigments were prepared by nitridation of the Fe pigments in an ahnosphere of ammonia-hydrogen. The Fe pigments, the precursors, were prepared by reduction in hydrogen of the iron oxyhydrates having various particle sizes. The realtionship between the coercivity and the specific surface area of Fe4N pigment was clarified. The relationship between the magnetic properties of the Fe pigments, the precursors, and those of the Fe4N pigments was also clarified. The chemical stabilities of the Fe4N pigment were also investigated. From the results of electron diffraction patterns, it is clear that the Fe4N pigment particle maintains the original shape of the starting material aud that it consists of fine unit particles which link together to form a stereo-network structure, and that the unit particle is observed to be a single crystal, as in the case of the Fe pigment.     

Performances of metal pigment tapes with 600-700 Oe coercivity

Audio or video recording performances of various magnetic tapes including Co-modified γ-Fe2O3, metal and Fe4N tapes having their coercivity range of 600-700 Oe were investigated. The tapes were prepared by the conventional method using the above magnetic pigments which were prepared to give their coercivity range of 600-800 Oe. The electromagnetic characteristics of the tapes were discussed in comparison with the commercial reference tape. The metal tapes were superior to the other two tapes on Sensitivity (S), Frequency Properties (F), Maximum Output Level (MOL) and Dynamic Range (D.R.) The Fe4N tape has an excellent property on Distortion (THD).     

On the microstructure and magnetization process of γ-Fe2O3

Experimental evidence is reported for γ-Fe2O3single particles being made up of several microcrystals chained together and their boundaries being sources of demagnetizing fields. This leads to a very defined picture of the magnetization reversal in partially alligned γ-Fe2O3particles: rotation is incoherent but the magnetization vectors are scattered in such a way that Hcivs. angle Ψ (between the orientation direction and the applied field) does not follow the known magnetization reversal modes. The importance of the reduction temperature in the process: α-FeOOHunderrightarrow{red.}Fe3O4underrightarrow{ox.}γ-Fe2O3becomes extremely important if considered as the parameter which rules the pore closure within the particles, i.e. the size and number of microcrystals chained in a single particle.     

Pressure-induced signal loss in Fe3O4and γ-Fe2O3thin film disks

Pressure-induced signal loss in Fe3O4and γ-Fe2O3thin film disks has been investigated. Through gauzes, pressure of 20 to 500 g/cm²were applied to the surface of a disk rotating 300 revolutions per minute (rpm). The result was extremely large signal losses of 40 to 90 percent. Several types of losses characteristic of ferrite surfaces were observed: initial loss, subsequent loss, and steady loss. These phenomena were consistently explained by a model that shows that the removal of a small amount of unstable surface crystallites by pressure reduces the leakage flux density. Wiping the rotating disk surface with gauze at a pressure of about 1000 g/cm²was effective in reducing the loss. It was confirmed that Ti doped γ-Fe2O3thin films prepared from α-Fe2O3show better resistance to pressure than do Fe3O4and other γ-Fe2O3films. Surface lubrication of the medium was also confirmed to be effective in reducing the loss. Contact-start-stop (CSS) induced signal loss and the possibility of pressure-induced missing signal errors due to thin film pinholes were also examined for typical γ-Fe2O3thin film disks.     

On the behaviour of the rheological and magnetic parameters of densified γ-Fe2O3

Magnetic characteristics (coercivity, remanent coercivity, remanence/saturation ratio, coercivity factor) and rheological properties (tap density, apparent density and wettability) for a number of γ-Fe2O3samples are analyzed. The samples are the following: I. γ-Fe2O3produced from densified α-FeOOH at various densification times; II. Densified γ-Fe2O3withdrawn from the densifier at different densification times; III. Densified and non-densified materials available on the market. The magnetic data as a function of the densification degree (i.e. vs. apparent density) confirm the increase of interactions and the re-assembling of the loose particles in a distribution of single particles, sheaf and ring shaped clusters. The wettability of the I samples is largely different from that of II samples; an hypothesis is made: densification produces a rough cleaning of the particles surface thus bringing out the reactive sites of the surface that can therefore be easily wetted out. Samples III confirm this hypothesis.     

High-field magnetism in non-polar γ-Fe2O3recording particles

Fine γ-Fe2O3particles produced by a process which involves hydrothermal conversion of iron hydroxides to α-Fe2O3have been investigated. Such particles appear to lack pores or dentrites which cause internal magnetic poles, and exhibit superior properties for magnetic recording. Mössbauer spectroscopy, x-ray diffraction, and transmission electron-microscopy were used. The particles were of length 250-500 nm and width 35-50 nm; some samples had cobalt adsorbed onto the surface. Mössbauer spectra were collected at temperatures from 4.2 to 300 K, and in zero and 5 T applied magnetic fields. In general the patterns are typical of bulk γ-Fe2O3. The 300 K spectra of Co-doped samples show sub-patterns of weak intensity associated with at least two distinct additional components with reduced splitting. One of these has Bhf= 45.2(2) T; its origin is suggested to be iron-atoms lying in or close to the cobalt-modified surface. In a 5 T field, the iron-atom moments deviate from the applied field direction by average angles in the range 13-15°. Because the angle for conventional similarly sized γ-Fe2O3issim 13deg, it is concluded that the improved particle morphology does not lead to an improvement in the high-field alignment.     

A magnetic recording tape based on iron particles

Tapes based on acicular iron particles give an outstanding recording performance compared to γ-Fe2O3and CrO2tapes. Especially in the short wavelength region the signal to noise ratio is, respectively, 12 and 7 dB better, whereas the bias noise level is somewhat lower. The iron tapes have a high coercivity, between 80 and90 times 10^{3}A/m, and hence the required bias is about 9 dB higher than for γ-Fe2O3. The print through properties are excellent.     

Formation and magnetic properties of γ-Fe2O3particles surface modified with crystallized cobalt-ferrite

Acicular γ-Fe2O3particles were heated at 90°C in alkali solution containing Co²⁺and Fe²⁺with Co²⁺/Fe²⁺ratio of 0.5. The coercivity of resultant particles increased linearly with increasing the Co²⁺content, and the coercivity of 900 Oe was obtained for the particles with Co²⁺content of 7 wt%. The shape of the particles is acicular, and an appreciable variation of morphology by the treatment in alkali solution was not observed. Cobalt-ferrite was expected to crystallize epitaxially on the surface of γ-Fe2O3particles, and the increase of coercivity was attributed to the magnetic anisotropy of the cobalt-ferrite. A variation of coercivity by annealing at 60°C and print-through were small compared with those of the particles in which iron were homogeneously substituted by cobalt ions. Such stability was explained by considering that a very high concentration of cobalt ions exist only on the surface of γ-Fe2O3particles, and the migration of cobalt ions is extremely difficult.     

Mössbauer study of orientation of particles in magnetic recording tapes

Mössbauer spectrum method was applied on tapes prepared using γ-Fe2O3, cobalt-epitaxial iron oxides and Fe particles to estimate the direction of orientation of the particles in the tapes. The coercivity and the squareness of the tapes was in the range of about 340 to 1140 Oe and 0.82 to 0.86, respectively. The γ-ray was applied to the tapes from the direction of the tape travel and the direction perpendicular to the tape plane. The angle of the orientation of the particles in the tapes was 29 -34 degree from the direction of the tape travel. The direction of magnetic moment in remanent state determined by Mössbauer method nearly agreed with that determined by hysteresis measurement. In the remanent state, the degree of discrepancy between the direction of orientation of particles and the direction of magnetic moment in particles increased with decreasing coercivity. It was considered that the magnetic moment in particles with lower coercivity tended to be inclined to the direction of tape plane.     

Influence of reduction temperature on coercivities, coercivity factors and rheological properties of γ-Fe2O3and silica coated γ-Fe2O3

The transformation of α-FeOOH into γ-Fe2O3via Fe3O4is studied in order to determine the influence of the reduction temperature on the magnetic and rheological properties of the final product. The study was carried out both on pure and on silica coated α-FeOOH. It was found that the reduction temperature at which a maximum of coercivity is obtained, varies with particle size, and it is much higher for silica coated samples which also show higher coercivity at the optimum reduction temperature. Coercivity factors (CF percent) and Mr/Ms values are also dependent on particle size. It is shown that coating with silica enables the reduction to be carried out at higher temperatures without significant collapse of the acicular shape.     

Investigations of high-coercivity alnico alloys

For Alnico alloys containing 32-42 percent Co, 0-9 percent Ti, 6-8 percent Al, 14 percent Ni, 3 percent Cu, 0-0.5 percent S, 0-2 percent Nb, remainder Fe, the liquidus and solidus temperatures and the upper limit of the (α + γ) phase field were determined. The greatest influence is exerted by titanium in that all temperatures decrease as the Ti content increases. The influence of cobalt on temperature is considerably smaller, while sulfur has no influence at all. The addition of 2 percent Nb resulted only in the upper limit of the (α + γ) phase field being shifted to temperatures 20 to 30°C lower. Alnico alloys containing about 39 percent Co and 7-8 percent Ti were brought to columnar crystallization by controlled cooling, using a seed crystal. The seed contained 6 percent Ti and the lowering speed was between 0.50 and 3.75 cm/h. At a lowering speed of 0.50 cm/h and a temperature gradient of 5 to 10°C/mm, it was possible to obtain columnar crystallization without using a seed. The following magnetic values were obtained:(BH)_{max} = 11.0-11.7MG.Oe,BH_{c} = 1780-2100Oe,B_{r} = 8920-10 050gauss.     

The influence of modified intermetallics and Si particles on fatigue crack paths in a cast A356 Al alloy

Mechanical fatigue tests were conducted on uniaxial specimens machined from a cast A356-T6 aluminium alloy plate at total strain amplitudes ranging from 0.1 to 0.8% ( R = − 1). The cast alloy contains strontium-modified silicon particles (vol. fract. ~6%) within an Al–Si eutectic, dispersed α intermetallic particles, Al₁₅ (Fe,Mn)₃ Si₂ (vol. fract. ~1%), and an extremely low overall volume fraction of porosity (0.01%). During the initial stages of the fatigue process, we observed that a small semicircular fatigue crack propagated almost exclusively through the Al–1% Si dendrite cells. The small crack avoided the modified silicon particles in the Al–Si eutectic and only propagated along the α intermetallics if they were directly in line with the crack plane. These growth characteristics were observed up to a maximum stress intensity factor of ~ K trmax = 7.0 MPa m^1/2 (maximum plastic zone size of 96 μm). When the fatigue crack propagated with a maximum crack tip driving force above 7.0 MPa m^1/2 the larger fatigue crack tip process zone fractured an increased number of silicon particles and α intermetallics ahead of the crack tip, and the crack subsequently propagated preferentially through the damaged regions. As the crack tip driving force further increased, the area fraction of damaged α intermetallics and silicon particles on the fatigue fracture surfaces also increased. The final stage of failure (fast fracture) was observed to occur almost exclusively through the Al–Si eutectic regions and the α intermetallics.     

Grain-size dependence of anhysteresis in iron oxide micropowders

The low-field anhysteresis of small equidimensional multidomain particles of Fe3O4is found to be surprisingly large. For particle sizes below about 0.075 μm the anhysteretic susceptibility equals or surpasses that of elongated or doped equidimensional single-domain γFe2O3particles used in conventional recording tapes.     

Theory of domain wall motion induced by microwave magnetic fields

A general theory is developed that applies to arbitrary polarization and takes account of damping and of the dipolar interaction between domains. The effect of the microwave field on the domain structure can be characterized by a pressure on the domain walls and by an alignment energy, both of which are proportional to the square of the rf magnetic field and become large in the vicinity of a resonance. For circular polarization the pressure tends to decrease the Larmor-domains (domains in which the imposed sense of polarization coincides with the sense of the natural spin precession) for frequencies outside the resonance region. Inside the resonance region, however, the pressure tends to increase the Larmor-domains. A linearly polarized field also exerts a pressure on the domain walls, with the polarity dependent upon the orientation of the field to the wall normal. In a linearly polarized magnetic field the domain walls tend to become aligned parallel to the rf field at frequencies ω near the low-frequency resonance (ω =γHa, γ = gyromagnetic ratio, Ha= anisotropy field) and perpendicular to the rf field at frequencies near the high-frequency resonance (ω = γ[Ha(Ha+ 4πM0)]^1/2, M0= saturation magnetization).     

Magnetization of a two-component Stoner-Wohlfarth particle

The free energy of an arbitrary two-component Stoner-Wohlfarth particle is derived and general criteria for stable states of magnetization are established. Hysteretic properties of coaxial prolate spheroids composed of γ-Fe2O3coated iron particles are calculated numerically and the specific extension to epitaxial cobalt ferrite γ-Fe2O3particles amongst other two-component systems is indicated.     

Characterization of ultrafine γ-Fe(C), α-Fe(C) and Fe3C particles synthesized by arc-discharge in methane

Ultrafine -Fe(C), -Fe(C) and Fe3C particles were prepared by arc-discharge synthesis in a methane atmosphere. The phases, morphology, structure and surface layer of the particles were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) techniques and X-ray photoelectron spectroscopy (XPS). It was found that the mean particle size ranged from 9.8 to 12.8 nm. The surface of particles mostly consisted of a carbon layer and a little oxide. Phase transformation from -Fe(C) to -Fe(C) was studied by annealing in vacuum and by differential thermal analysis and thermogravimetry (DTA–TGA) measurement. The abundance of -Fe(C) was determined by a magnetization measurement to be approximately 30%. Phase transformation occurred between 300 and 500 °C in a flowing argon atmosphere. The Fe3C particles oxidized to -Fe2O3 and carbon dioxide at 610 °C or so. © 1998 Chapman & Hall     

Magnetic structure at γ-Fe2O3-organic interfaces

The effect of an organic coating on the magnetic properties of γ-Fe2O3particles has been studied using Mössbauer absorption, infrared absorption, and magnetization measurements; particular emphasis has been placed on the magnetic structure at the γ-Fe2O3- organic interfaces. Experiments were performed on fine γ-Fe2O3particles (≈ 300 Å) and also on conventional micron-size acicular γ-Fe2O3particles. The presence of a coating was confirmed by infrared-absorption spectra and by somewhat reduced saturation magnetizations. Although some increase in coercivity is observed for micron-size samples, the origin of this increase is suggested to lie in the change in packing factor when the nonmagentic organic materials are introduced. No substantial increase in coercivity is observed for coated fine particles. Mössbauer analyses have revealed that the surface magnetic structure of the pigment is unchanged by the organic coatings; neither a magnetically "dead" layer nor a pinned structure is formed. It appears that some effects reported for nonmagnetic coatings earlier may be secondary ones.     

Orientation relations between carbon nanotubes grown by chemical vapour deposition and residual iron-containing catalysts

Orientation relationships between the growth direction of carbon nanotubes and encapsulated residual iron-containing particles have been determined using transmission electron microscopy. The nanotubes that are prepared by Fe-catalysed chemical vapour deposition on sol–gel Fe(NO₃)₃-tetraethyl orthosilicate substrates are the helical multiwall type. Nanoscale particles of both the low-temperature α-Fe (ferrite) and high-temperature γ-Fe (austenite) were found in the cavity of the carbon nanotubes with , and parallel to the tube growth direction, respectively. Cementite Fe₃C, the most abundant Fe-containing phase in present samples was also found to be entrapped in nanotubes with or parallel to the tube axis. The metastable retention of γ-Fe particles at room temperature is ascribed to the strain energy induced at the particle-nanotube interface due to volume expansion upon the γ- → α-Fe phase transformation. The decomposition of initially high aspect-ratio, rod-shape particles into a string of ovulation, while encapsulated in carbon nanotubes is accounted for by the Rayleigh instability. Ovulation leading to reduced particle size has also contributed to increase the surface energy term that counterbalances the total free energy change of phase transformation from γ- to α-Fe and further aids to the metastable retention of γ-Fe.     

自组装法制备磺化聚苯乙烯@Fe3O4磁性复合颗粒及其磁性能

实现结构可控、均匀包覆是制备核-壳复合材料的关键。采用离子交换法完成了磺化聚苯乙烯(PSS)表面Na+与溶液中Fe2+和Fe3+的交换,于碱性条件下制备了PSS表面负载Fe₃O₄(PSS@Fe₃O₄)的磁性复合颗粒。通过称重法计算了Fe₃O₄最大包覆率;通过振动样品磁强计(VSM)测试了不同负载含量下PSS@Fe₃O₄复合颗粒的磁性能;通过XRD、衰减全反射-FTIR (ATR-FTIR)、SEM-EDS分析了PSS@Fe₃O₄磁性复合颗粒的化学组成和微观结构。结果表明,随着Fe2+/Fe3+浓度增加,PSS@Fe₃O₄磁性颗粒的饱和磁化强度也随之增大,最大饱和磁化强度为7.51 emu/g,并具有明显的磁响应性;Fe₃O₄均匀包覆在PSS表面,最大包覆率为8.3 wt%。PSS@Fe₃O₄磁性复合颗粒有望用于磁流变、医学及水处理领域。      

Origin of coercivity changes during the oxidation of Fe3O4to γ-Fe2O3

Oxidation kinetics and x-ray diffraction of the intermediate products have been investigated for several kinds of Fe3O4powders during the oxidation to γFe2O3. From these results, a two-components-hypothesis as an intermediate state of the Fe3O4- γFe2O3system is proposed, instead of a homogenious solid solution which has been widely recognized. The effect of the changes in particle size caused by the two phases oxidation products on the variations of magnetic coercivity was emphasised.