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.     

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.     

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.     

Correlation of textural and surface properties with cobalt modification efficiencies of γ-Fe2O3

Cobalt modification of a number of commercial samples of γ-Fe2O3had been carried out by implementing the so-called "Epitaxial Process". The initial powder coercivities and specific surface areas of these samples ranged from 280-400 Oe and 12-30 m²/g, respectively. The cobalt modification efficiencies were found to range from 0-50, Oe/% Co. Textural properties (surface area and porosity) and surface chemistry of these samples were found to be responsible for the observed variation and an interesting correlation between cobalt efficiency and the degree of microporosity of these samples was obtained. For a given external surface area, the cobalt efficiency increased linearly with the degree of microporosity. The correlation obtained is explained in terms of induced conflict of anisotropies and improvement in particle morphology in view of the recent findings of Corradi, et. al.     

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.     

Magnetic characteristics of γ-Fe2O3dispersions

A technique to evaluate the magnetic properties of disc and tape oxide dispersions utilizing a vibrating sample magnetometer has been developed. The method permits an assessment of dispersion quality prior to disc or tape coating and can be utilized to optimize oxide formulations as well as for quality control.     

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.     

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.     

Accidental printing effect of magnetic recording tapes using ultrafine particles of acicular γ-Fe2O3

Five kinds of acicular γ-Fe2O3powder of which axial length varies from 0.17 to 0.80 μm with almost the same axial ratio are manufactured. The variance of coercive force and printing effect as the average axial length of the particles varies is shown. Coercive force of which particle axial length is about 0.35 μm shows maximum value. The printing effect increases according to the decrease of axial length. The printing effect increases with contacting time. The temperature dependence of coercive force and remanent magnetization are measured from 25 to 200°C. These two properties decrease as the temperature increases, but the decreasing inclination becomes steeper as the axial length is smaller. The relationship between the printing effect and the decreasing inclination of coercive force is nearly linear. From the temperature dependence of the printing effect, the activation energy of the printing effect can be calculated. The activation energy of the smallest particle is about 0.05 eV and the others are about 0.15 eV. This measured activation energy is discussed in connection with the relaxation equation.     

Signal-to-noise ratio studies on γ-Fe2O3thin film recording disks

The signal-to-noise ratio SNRwof γ-Fe2O3thin film disks has been investigated in a linear bit density range of 77-1500 flux reversals per millimeter (FRPM), for several media whose average crystallite sizes were changed by Ti doping. The noise voltage was found to increase as the bit-cell length became shorter, and as the average crystallite size became larger. A SNRwrelationship to the average crystallite size, the resolution, and the bit-cell length was empirically determined as a simple formula. For a 10-μm core width and for a 1.0-μm bit-cell length, an SNRwof more than 35 dB was estimated to be attainable from the medium which had a 0.90 resolution and had crystallites averaging less than 0.06 μm in size.     

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 measurements on single acicular particles of γFe2O3

Measurements of the remanent coercive force Hrhave been made upon large numbers of individual 'tape' particles of γFe2O3. The greatest range of values of Hrsound for any one sample was 300r<1100 0e, and other samples of different origins gave rather similar results. Using the data for two sets of samples, together with other known factors, the remanent loops of the corresponding tapes were synthesized. The synthesized loops resembled the experimental loops, but the remanent coercive forces were larger by 20 to 60 0e: very good agreement was obtained for one sample. The effect of a.c. demagnetization was investigated. This showed that particles of the highest Hrwere undoubtedly single, but that many of lower Hrwere multiple. Particles of similar morphologies have been observed in recording tape. A value for Hrof 1100 0e is shown to be consistent with the magnetization reversing by the buckling process.     

A contribution to the investigation of fine-particle solid solutions between cubic iron sesquioxide γFe2O3and cobalt ferrite CoFe2O4

The authors give the chemical, morphological, crystallographic, and magnetic properties of solid solutions between cubic iron sesquioxide γFe2O3and cobalt ferrite CoFe2O4obtained as fine particles by a new preparation method. This method gives the whole range of compositions and allows shifting of the magnetic and morphological features in a direction suitable for magnetic recording applications.     

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.     

Alternating and rotational hysteresis losses of γ-Fe2O3powders in a nonmagnetic matrix

Some aspects of the alternating and rotational hysteresis losses of very diluted samples of γ-Fe2O3particles are examined. The packing fractionpof the particles ranges from 0.001 to 0.120. The coercive force is found to be independent ofp. The alternating and rotational hysteresis losses appear dependent uponpforp < 0.04; the rotational losses increase considerably aspapproaches zero. The values of the rotational hysteresis integral are close to the theoretical ones corresponding to the model of magnetization reversal of chain of spheres with fanning, but they increase slightly with decreasingpforp < 0.04. Generally, the ratio of rotational to alternating losses is about twice that for bulk materials. All these effects are ascribed to a weakening of the magnetic interactions among particles.     

Influence of R1parameter {=Fe2O3/(Y2O3+ Sm2O3+ Lu2O3)} on magnetic bubble properties of (YSmLuCa)3(FeGe)5O12

The influence of R1{=Fe2O3/(Y2O3+Sm2O3+Lu2O3)} in the melt composition on film properties, and growth characteristics, has been investigated for (YSmLuCa)3(FeGe)5O12. The garnet phase is the primary phase when R1is kept between 10 and 60. The temperature coefficient for the bubble collapse filed changes from -0.29 to -0.19 %/°C, and the growth rate with 10°C supercooling changes from 0.60 to 0.15 μm/min, by increasing R1=10 to 60. Distribution coefficients, K^Y, K^Sm, K^Luand K^Geincrease, and K^Feand K^Cadecrease, with increasing R1.     

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.     

On the texture of γFe2O3particles in magnetically and mechanically oriented magnetic tape

The degree of preferred orientation in magnetic recording tape was investigated by an X-ray pole figure technique and by measuring magnetic properties. Acicular γFe2O3particles dispersed in PVA film were oriented by mechanical stretching and magnetic orientation. The pole figures of these samples indicated the texture with [110] to the orientating direction. The degree of orientation in stretched film was much higher than in magnetically oriented film. Further, it was observed that the particles tend to align uniaxially in mechanical orientation, and biaxially in magnetic orientation.     

Simultaneous substitution of Co2+and Zn2+ions in submicronic acicular γ - Fe2O3particles

In iron sesquioxide of acicular shaded γ-Fe2O3simultaneous substitution of Co²⁺and Zn²⁺ions leads to the formation of mixed-defect ferrites and modulation of magnetic properties is of interest for their application to high density magnetic recording. It is shown that the coercive force, remanent magnetization and saturation-magnetization are controlled by a judicious choice of the contents of Co²⁺and Zn²⁺, while it is indispensable to optimize the morphological features like the average size of the crystallites, the shape, the size and texture of the particles. A high value of coercive force (650-700 Oe) and of the remanent magnetization (35-45 emu/g) had been obtained with a minimal content of cobalt ions (Co²⁺= 2.5 to 3% by wt.) permitting limits to the magnetocrystalline anisotropy of these compounds and their thermal variation near the ambient temperature. The influence of the zinc content had been systematically studied notably in relation to its effect on the structural, morphological and magnetic properties of the ferrites.