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.     

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.     

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.     

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.     

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.     

A study of particle arrangement in magnetic tapes

Particle dispersion and particle orientation in magnetic tapes are experimentally interpreted by using a large-scale model which consists of commercial carbon steel wires and a vinyl chloride binder. Results suggest that the particle orientation in the plane through the thickness of the coating was broader than that in the plane of coating. Even though the micromagnetic equations are not properly scaled, comparison of the large-scale model and γ-Fe2O3assemblies shows general agreement with respect to packing fraction. The coercive force of an isolated single γ-Fe2O3particle is found to be 420-440 Oe, compared to 380 Oe found for packing fractions used in commercial magnetic tapes.     

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 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.     

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.     

High coercivity sputter-deposited maghemite thin-film disk

The fabrication and read-write characteristics of a high-coercivity sputter-deposited maghemite (γ-Fe2O3) thin-film disk medium is discussed. By employing a low sputtering gas pressure, severe internal strain is introduced into the films. This internal strain markedly increases coercivity. Furthermore, the films are composed of fine crystallites (300-400 Å in diameter), which result in extremely low media noise. A high coercivity (1060 Oe) γ- (Fe0.954Co0.02Ti0.02Cu0.015)2O3thin-film disk medium with a thickness of 0.095 μm exhibited superior read-write characteristics (e.g., a 2493 FRPM recording density D50and a 42 dB signal-to-noise ratio). These results show that sputter-deposited maghemite films have a promising potential for use as high-density disk media.     

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.     

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.     

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).     

Recording characteristics of coated disks having high coercivity powder

The recording characteristics of coated disks having high coercivity powder, in particular metal powder, are given. We manufactured various disks containing γ-Fe2O3(coercivity: 350 Oe), iron-cobalt metal powder (400 - 800 Oe) and iron metal powder (650 Oe), respectively. The recording characteristics of these disks were measured by using a Mn-Zn ferrite head whose saturation magnetization was 5,000 gauss. It was found that disks with higher coercivity showed a capacity for increased bit density, but at the same time, poor overwrite characteristics. The optimum coercivity proved to be about 650 Oe. The typical values for metal powder coated disks (650 Oe) were: critical bit density: 19,000 BPI reproduced output: 1 mV at 19,000 BPI overwrite: -30 dB.     

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.     

High gradient magnetic separation: Theory versus experiment

The experimental performance of a high gradient magnetic separator has been previously reported by other workers in some detail for a CuO/Al2O3slurry. Less detailed results were also reported for slurries of Mn2O3, Al, and α-Fe2O3particles with Al2O3representing a 20:1 range in particle sizes and a 200:1 range in magnetic susceptibility. Examination of these results indicates that many layers of particles build up on each filter fiber. Accordingly, in this paper we extend the original particle trajectory model for the calculation of filter performance, to include the build-up of multiple layers of particles on the fibers. Good agreement is obtained between the calculated recoveries and purities for all of the particles and the experimentally reported values using a filter packing efficiency and a mechanical trapping term, derived from the CuO data, as adjustable parameters.     

Polymer-coated metal and alloy particles for magnetic recording

Fine particles of cobalt and an iron-cobalt alloy were prepared and encapsulated in a plastic film to protect them from oxidation. The magnetic properties of the coated powders were determined and compared with the properties of coated iron powders studied previously. The thermal degradation of magnetic properties was also investigated. Polymer-coated iron and cobalt powders were incorporated in a Hypalon rubber matrix to obtain additional magnetic property data and to study recording characteristics. The experimental materials exhibited better high-frequency response in magnetic recording than a standard γ-Fe2O3material, and one material had a higher signal output as well.     

An investigation of magnetic oxides' defects and conflict of anisotropies for diluted powders

Magnetostatic measurements and rotational hysteresis were carried out on extremely diluted samples of pure, surface modified, bulk doped iron oxides and CrO2. It is found that in all these materials but pure γ-Fe2O3a conflict of anisotropy is active. As a result, the values of remanence ratio jrcan be either lower or higher than the theoretical value of ½ for strictly uniaxial particles depending on the angle between the two anisotropy axis and the relative value of the anisotropy constants. It is therefore found that whilej_{r} < 1/2for acicular magnetite and CrO2, the introduction of cobalt always produces an increase of jr: the extreme effect of such conflict can be found in high remanence isotropic particles. Such a conflict is minimized in Surface Modified Materials. From rotational hysteresis, Hcvs. angle with applied field and, over all, from CF and IFF parameters, it is suggested that the postreatment of the iron oxides with the Co-containing solutions, beside the increase of Hc, has a beneficial effect on the morphology of the particles.     

CEMs and Faraday rotation study of γ-Fe2O3- Fe3O4films obtained by a new pyrolisis technique

Thin film iron oxides prepared by a new pyrolisis technique are studied by means of CEM spectroscopy and Faraday rotation measurements. It is shown that Fe3O4spinel oxides are obtained when the deposition is performed under Ar atmosphere. These spinel-ferrite films present an important magnetization component perpendicular to the film plane. It is also shown that the Fe3O4films are converted to γ-Fe2O3by oxidation in air while retaining a uniaxial magnetic anisotropy. We interpret this induced magnetic anisotropy as arising from a magnetoelastic coupling with the substrate. Faraday rotation hysteresis loops confirm the existence of a strong induced uniaxial magnetic anisotropy in these films.     

Remanence loss in γ-Fe2O3tapes as related to reptation, viscosity, and print-through

The reptation and magnetic viscosity in γ-Fe2O3tape as a function of an applied field is found by using a rotating sample magnetometer (RSM) which measures the decrease in remanent magnetization MR, as the sample spins in a field at different frequencies. A modified RSM is used to find the decrease in MRwhen a reverse field is repeatedly applied along a stationary axis. Reptation is found only when the sample rotates in a field. Magnetic viscosity effects peak at fields near the coercive field and scale with print-through in tapes.