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.     

Correlation between magnetic and electroacoustic parameters of audio magnetic tapes

Coercivity, squareness, remanent coercivity, coercivity factor, rotational hysteresis loss and its integral were measured on two sets of tapes (γ-Fe2O3and crO2) and related to their electroacoustic performances. The static magnetic measurements explain some of the properties of the particulate media: it was found that while CrO2rotates either coherently or incoherently depending on the direction of the applied field, γ-Fe2O3rotates incoherently, and the departure from the model of "fanning chain of spheres" can be related to the imperfections of the particles which are sources of demagnetizing fields. Good agreement with audio performance is found not only for parameters like coercivity and squareness, whose role is well known, but also with the other static parameters which were previously disregarded as ruling analogue recording.     

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).     

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.     

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.     

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.     

Characteristics of barium ferrite tape for magnetic contactduplication

The magnetic properties and electromagnetic characteristics of Ba-ferrite tape for R-DAT magnetic contact duplication (MCD) were investigated. The tapes employed Ba-ferrite particles averaging 0.06 μm in diameter with an aspect ratio between 4 and 5. Ba-ferrite tapes were prepared with perpendicular, longitudinal, and no orientation, and their MCD and ring-head recording (RHR) characteristics were compared. Several distinctive characteristics were observed in MCD, and the desirability of perpendicular orientation at short wavelengths in MCD applications is clear. Tilted orientation was also investigated, and its output level was similar to that of perpendicular orientation in MCD, while in RHR it proved superior to perpendicular orientation in the short wavelength region. The magnetization mode of the slave tape was studied by waveform analysis using the FFT method and in conjunction with the tapes' magnetic properties, such as uniaxial anisotropy energy, remanence coercivity, and squareness ratio     

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.     

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.     

Processing of BSCCO 2212 and 2223 Superconductor Compounds

High-T_c BSCCO superconductor tapes were prepared by the conventional powder-in-tube method. Some tapes involved partial melting, while the others involved solid-state processing only. Bulk samples were prepared by powder metallurgy. X-ray diffraction (XRD) showed the presence of both BSCCO 2223 (T_c = 105 K) and BSCCO 2212 (T_c = 80 K) phases in all the materials. For the mostly BSCCO 2212 phase samples, x-ray studies indicated that tape #1 which involved partial melting at 850^°C for 0.3 h had a higher degree of basal orientation than either a tape #2 sample which involved partial melting at 855^°C for 0.5 h, or a bulk (#2) sample. For the mostly BSCCO 2223 phase samples, however, a comparison of bulk (#1.), tape #3 (solid state processing at 840^°C) and tape #4 (partial melting at 865^°C for 0.5 h) samples, showed that the solid-state processed tape (#3) had the highest degree of basal orientation. Direct current magnetic susceptibility measurements were used to follow the transition at T_c. Critical current density, J_c, values were estimated from DC magnetic hysteresis loops for all bulk and tape samples.     

Mössbauer spectroscopy in ferromagnetic metal surfaces

Mössbauer experiments were made utilizing Co⁵⁷or Fe⁵⁷as a microprobe for studying the magnetic properties of ferromagnetic metal surfaces. For the emission spectroscopy, carrier-free Co⁵⁷atoms were electrodeposited on a surface of bulk iron. The absorption spectroscopy was applied to a sample whose surface was selectively enriched with Fe⁵⁷. In both cases, the spectra at 4.2 K showed that all iron atoms in the surface are ferromagnetic, but the reduced hyperfine field suggests a partial decrease of the local magnetic moment.     

The effect of austenitizing time on martensite morphologies and magnetic properties of martensite in Fe–24.5%Ni–4.5%Si alloy

The effect of austenitizing time on the formation of martensite in Fe–24.5%Ni–4.5%Si alloy has been studied by means of transmission electronmicroscope (TEM), scanning electronmicroscope (SEM) and Mössbauer spectroscopy technique. TEM and SEM observations revealed that the martensite morphology was found to be closely dependent on the austenitizing time. The orientation relationship between austenite and thermally induced martensite was found as the Kurdjumov-Sachs type. The volume fraction changes of martensite and austenite phases, the hyperfine magnetic field of martensite phase and isomery shift values have been determined by Mössbauer spectroscopy. The Mössbauer study also revealed that the martensite volume fractions increased with increasing austenite grain size.     

Synthesis of ultrafine amorphous Fe–B alloy nanoparticles using anodic aluminum oxide templates

Ultrafine amorphous Fe–B alloy nanoparticles are self-assembled within anodic aluminum oxide templates by combining a preparation process of Fe–B nanoparticles with a template method. Scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry, X-ray diffraction spectrometry, Mössbauer spectrometry, and vibrating sample magnetometry are employed to study the morphology, chemical composition, structure, and magnetic properties of the nanoparticle assemblies, respectively. The results show that the alloy particles are amorphous with a boron content of 24 at. % and can be in shape of sphere and rod by controlling the duration of preparation. There is a narrow distribution of the sizes of spherical nanoparticles with an average diameter below 35 nm in relatively short preparation time, while rods are found in longer time. The measurements of magnetic properties indicate that the nanoparticles are mostly in superparamagnetic state and the self-assembly of the nanoparticles has a weak magnetic anisotropy with an easy direction perpendicular to the template plane.     

Detecting and describing the inhomogeneity of critical current in practical long HTS tapes using contact-free method

The nondestructive and contact-free apparatus for measuring local critical current of long high temperature superconducting (HTS) tapes is presented. The local critical current of tape is acquired by using Hall probe array sensor to measure the remanent field after exposed to dc external magnetic field since the critical current is proportional to remanent field based on Bean critical state model. A detailed experiment on multifilamentary Bi2223/Ag tape is made to validate reproducibility, reliability, resolution, nondestructiveness and usefulness for manufacturer and user of tapes. The parameter COV (coefficient of critical current variation) is suggested for quantitatively describing the inhomogeneity and quality of practical long HTS tape based on Gaussian statistical analysis. The developed apparatus can detect HTS tape at velocity of 100 m/h with resolution smaller than 3 mm in liquid nitrogen.     

Magnetic properties of nanocomposites of mixed oxides of iron and chromium synthesized under different oxidative environments

Synthesis of nanocomposites of mixed oxides of Fe and Cr in a copolymer matrix of aniline and formaldehyde at room temperature along with IR, XRD and⁵⁷Fe Mössbauer studies on as synthesized as well as the samples on heating at different temperatures is described. The XRD and⁵⁷Fe Mössbauer studies show the formation of nanosized iron oxide particles. These studies further show the formation of nanosized particles of metal oxides and formation of solid solution of iron and chromium oxide on heating the samples at 700°C. Further, the IR studies show that the polymeric backbone is strongly influenced by different reaction conditions and lead to variable magnetic character in the heated samples as shown by their Mössbauer studies.     

Magnetization and Mossbauer spectra of non-crystalline Y3Fe5O12

Non-crystalline Y₃Fe₅O₁₂ is studied by X-ray, electron microscope, magnetic susceptibility, and Mössbauer effect techniques. The material appears to consist of X-ray amorphous platelets of several microns, which are agglomerations of particles of about 200 Å. Mössbauer spectra at room temperature show Fe in tetrahedral surrounding, while at liquid helium temperature hyperfine splitting for both octahedral and tetrahedral sites is observed. Magnetization measurements show that individual particles order magnetically at ～850 K, much higher than well-crystallized YIG (560 K). These differences are consistent with a high degree of crystallographic disorder and a small particle size. Crystallographic ordering and particle growth are induced by annealing. At 680°C crystalline YIG is formed.     

Synthesis of Superparamagnetic Nanoparticle Ni0.50Zn0.50Fe2O4 Using Wet Chemical Method

The nanoparticles of Ni_0.50Zn_0.50Fe₂O₄ were prepared using cotton base wet chemical method. The X-ray diffraction (XRD) spectrum shows single phase cubic spinel structure, and crystallize size calculated using the Debye–Scherrer formula for 311 planes was found to be 41 nm ± 5 %. The nano range sizes of particle were confirmed using transmission electron microscope (TEM) and atomic force microscopy (AFM) images. The porosity value of the sample is promising towards sensor applications. Infrared spectroscopic results reveal two main absorption bands, indicating sample is having single phase spinel structure with two sublattices. The high DC resistivity of the sample is attributed to nano range particle sizes. The superparamagnetic (SPM) nature was confirmed from a variation of magnetic moment with applied magnetic field using VSM. The Mössbauer spectrum recorded at room temperature shows two major sextets corresponding to A- and B-site Fe ions. The relaxed sextet is associated with the atoms near the surface of the nanoparticles experiencing disordered spins. The central doublet in Mössbauer spectra of the sample indicates SPM particles. The isomer shift results show that all the Fe ions are in high state.     

Recovery of highly strained Fe–28 at.-%Al alloy

Abstract

Recovery of a highly strained Fe–28 at.-%Al alloy has been studied by means of electrical resistivity, Mössbauer spectroscopy, magnetic polarisation, and coercivity measurements. The amount and state of defect rich volumes (interfacial phase) were investigated. Three recovery steps were found in the temperature dependences of resistivity and magnetic polarisation. The first two steps are characterised by almost the same activation energies (?₁=120±5 and ?₁=127±1 kJ mol^?1) derived from resistivity measurements, and can be ascribed to the annihilation of defects and relaxation of stresses accompanied by cell wall sharpening and the onset of subboundary formation. The appearance of backward recovery stresses was manifested as changes of spin orientation. The third step is ascribed to the process of cell wall sharpening. The observation of the third and further steps was complicated by overlapping caused by magnetic transformations and ordering effects.

MST/1663     

Orientation process of acicular oxides in the magnetic tape layer

The orientation of acicular particles in the active layer of a magnetic tape is presently a usual technological process applied in magnetic tape production. The theory of this process is dealt with. A mathematical model is deduced, describing the behavior of an isolated agglomerate of acicular particles in an external magnetic field and the change of its microstructure, i.e., the orientation of acicular particles in the agglomerate. The term for the density of probability ω(α) of the angle distribution α of the longer particle axes in the orientated agglomerate is also deduced. Also the approximate relation between the orientation degree, defined by the mean quadratic deviationsof the angles α of the particular particles and by the bevel values γ of the deformed agglomerate, is shown. It appears that, besides the orientation by means of the external magnetic field, it would also be possible to orientate the magnetic suspension by other methods.     

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.