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The transformation of α-FeOOH into γ-Fe2 O3 via Fe3 O4 is 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. 相似文献
3.
Bottoni G. Candolfo D. Cecchetti A. Corradi A. Masoli F. Ceresa E. Salvioli L. 《IEEE transactions on magnetics》1980,16(1):68-70
Magnetic characteristics (coercivity, remanent coercivity, remanence/saturation ratio, coercivity factor) and rheological properties (tap density, apparent density and wettability) for a number of γ-Fe2 O3 samples are analyzed. The samples are the following: I. γ-Fe2 O3 produced from densified α-FeOOH at various densification times; II. Densified γ-Fe2 O3 withdrawn from the densifier at different densification times; III.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. 相似文献
4.
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. 相似文献
5.
Oxidation kinetics and x-ray diffraction of the intermediate products have been investigated for several kinds of Fe3 O4 powders during the oxidation to γFe2 O3 . From these results, a two-components-hypothesis as an intermediate state of the Fe3 O4 - γFe2 O3 system 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. 相似文献
6.
Cobalt modification of a number of commercial samples of γ-Fe2 O3 had 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. 相似文献
7.
The effect of an organic coating on the magnetic properties of γ-Fe2 O3 particles has been studied using Mössbauer absorption, infrared absorption, and magnetization measurements; particular emphasis has been placed on the magnetic structure at the γ-Fe2 O3 - organic interfaces. Experiments were performed on fine γ-Fe2 O3 particles (≈ 300 Å) and also on conventional micron-size acicular γ-Fe2 O3 particles. 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. 相似文献
8.
Fine γ-Fe2 O3 particles produced by a process which involves hydrothermal conversion of iron hydroxides to α-Fe2 O3 have 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-Fe2 O3 . 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-Fe2 O3 issim 13deg , it is concluded that the improved particle morphology does not lead to an improvement in the high-field alignment. 相似文献
9.
Accidental printing effect of magnetic recording tapes using ultrafine particles of acicular γ-Fe2 O3
Five kinds of acicular γ-Fe2 O3 powder 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. 相似文献
10.
The signal-to-noise ratio SNRw of γ-Fe2 O3 thin 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 SNRw relationship 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 SNRw of 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. 相似文献
11.
Pressure-induced signal loss in Fe3 O4 and γ-Fe2 O3 thin 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-Fe2 O3 thin films prepared from α-Fe2 O3 show better resistance to pressure than do Fe3 O4 and other γ-Fe2 O3 films. 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-Fe2 O3 thin film disks. 相似文献
12.
Measurements of the remanent coercive force Hr have been made upon large numbers of individual 'tape' particles of γFe2 O3 . The greatest range of values of Hr sound 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 Hr were undoubtedly single, but that many of lower Hr were multiple. Particles of similar morphologies have been observed in recording tape. A value for Hr of 1100 0e is shown to be consistent with the magnetization reversing by the buckling process. 相似文献
13.
Acicular γ-Fe2 O3 particles 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-Fe2 O3 particles, 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-Fe2 O3 particles, and the migration of cobalt ions is extremely difficult. 相似文献
14.
Some aspects of the alternating and rotational hysteresis losses of very diluted samples of γ-Fe2 O3 particles are examined. The packing fractionp of 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 uponp forp < 0.04 ; the rotational losses increase considerably asp approaches 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 decreasingp forp < 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. 相似文献
15.
Cusido J. Obradors X. Fontcuberta J. Isalgue A. Canal F. Cortadellas J. Langlet M. Joubert J. 《IEEE transactions on magnetics》1987,23(1):74-76
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 Fe3 O4 spinel 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 Fe3 O4 films are converted to γ-Fe2 O3 by 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. 相似文献
16.
The reptation and magnetic viscosity in γ-Fe2 O3 tape 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 MR when 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. 相似文献
17.
Mollard P. Collomb A. Devenyi J. Rousset A. Paris J. 《IEEE transactions on magnetics》1975,11(3):894-900
The authors give the chemical, morphological, crystallographic, and magnetic properties of solid solutions between cubic iron sesquioxide γFe2 O3 and cobalt ferrite CoFe2 O4 obtained 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. 相似文献
18.
The influence of R1 {=Fe2 O3 /(Y2 O3 +Sm2 O3 +Lu2 O3 )} in the melt composition on film properties, and growth characteristics, has been investigated for (YSmLuCa)3 (FeGe)5 O12 . The garnet phase is the primary phase when R1 is 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, KY, KSm, KLuand KGeincrease, and KFeand KCadecrease, with increasing R1 . 相似文献
19.
The degree of preferred orientation in magnetic recording tape was investigated by an X-ray pole figure technique and by measuring magnetic properties. Acicular γFe2 O3 particles 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. 相似文献
20.
Mössbauer measurements were carried out in compound BaMg2 Fe16 O27 (Mg2 W) by utilizing both polycrystalline and single-crystal samples. The resonant γ-absorption spectra have been measured with the absorbers at temperatures of 85 to 800°K and in some cases in the presence of a 15-kOe external magnetic field. The values of the hyperfine magnetic fields Hhf at the Fe57nuclei in the different sublattices as functions of temperature have been measured. By fitting the curve of the saturation magnetization σ versus temperature, the cation distribution over the available lattice sites has been deduced. From the value of σ extrapolated at 0°K, it turns out that the number of Bohr magnetons(n_{B})W per elementary cell is higher than the value obtained by adding the corresponding values for theS andM structures. The Curie temperature of the compounds has also been measured and is equal to (440 ± 5)°C. 相似文献