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.     

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.     

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.     

Preparation of M- and W-type hexaferrite particles by the glass crystalization method on the basis of the pseudo-ternary system Fe2O3- BaO - B2O3

The preparation of M- and W-type Ba-hexaferrite particles by the glass crystallization method was investigated. For this purpose that part of the pseudoternary phase diagram Fe2O3-BaO-B2O3was elaborated in which primary crystallization of BaFe12O19occurs. This three-dimensional region was found to be located within the triangle Fe2O3-BaB2O4-BaFeO2,5. The lowest temperature of primary crystallization of BaFe12O19is close to 800°C near the composition 55 mol% BaO, 20 mol% B2O3and 25 mol% Fe2O3. Using the knowledge of that system it was also possible to synthesize W-type Ba-hexaferrite particles (BaZn2Fe16O27) by the glass crystallization method. Magnetic data of M- and W-type hexaferrite pigments prepared by that method are given. Particle size from a tenth of a micron to more than ten microns was obtained.     

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.     

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.     

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.     

Magnetostriction in some magnetic oxide compacted powders

Magnetostriction measurements have been made on a series of compacted powder samples of oxide pigments used in magnetic recording, to provide information for the evaluation of the role of magnetostriction in the recording process. Two composition series were measured at 294 K: one from γFe2O3through a range of partial reductions to Fe3O4, and another from 0 to 4 at. % Co in γFe2O3; also samples of CrO2. In addition, a 2 at. % Co-doped Fe3O4powder was measured as a function of field and temperature from 77 to 300 K. Magnetostriction was measured using a semiconductor strain gage on a rotating sample, in fields up to 9 kOe.     

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.     

Magnetic properties of iron-nitride particles prepared with gas evaporation method

Magnetic properties of iron nitride particles prepared with reactive gas evaporation method were investigated. Major products for higher NH3gas pressure region is γ'Fe4N and αFe. For lower pressure region αFe was produced, which was oxidized to Fe3O4after exposure to air. It is found that particles consist of γ'Fe4N and γFe are more persistent to oxidation comparing to those of αFe.     

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.     

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.     

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.     

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.     

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.     

Preparation of thin films in the system γFe2O3- Fe3O4for recording media by spray pyrolysis of organometallic solutions using an ultrasonic pump

Thin films of magnetic spinel iron oxides were prepared by pyrolysis of an ultrasonically generated aerosol of organometallic compounds. The bases of the method and experimental procedure are described. Polycrystalline films of good adherence and homogeneity were obtained. Lattice parameters were observed to be modified by film substrate strain interaction and to depend strongly on the hydrogen content. The oxidation state of iron was discussed in regard to the experimental growing conditions. Corresponding magnetic properties were investigated. Pure magnetite Fe3O4films withH_{c} = 400Oe andM_{s} = 400EMU/cm³and gamma ferric oxides films withH_{c} = 400Oe andM_{s} = 250EMU/cm³have been obtained without post deposition heat treatment. Annealing magnetite films in air resulted in a considerable increase of their coercivity (up to 800 Oe).     

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.     

Fabrication and properties of Fe/Fe oxide and Co/Co oxide films

Evaporation of Fe and Co at a residual O2pressure of 10^-5to 10^-4Torr yielded magnetic films of high coercivities up to 500 Oe and magnetizations of about 1000 EMU/cm³. While the Co films were continuously deposited in the O2atmosphere, a sandwich structure of Fe and Fe2O3was formed by intermittently altering the evaporation conditions. Both kinds of films were produced in the same geometrical arrangement. The Fe/Fe2O3films showed shape anisotropy caused by the oblique incidence deposition, while no uniaxial anisotropy was observed on the Co films. For both, Co films and Fe/Fe2O3sandwich films, it was possible to adjust the magnetic properties within a certain range. In both cases the coercivity increased considerably by annealing at 200°C.     

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.     

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.