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.     

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.     

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.     

Magnetic properties of amorphous (fe, ni)80B20, (fe, ni)80B19si1, and (fe, ni)80P14B6alloys

Magnetic measurements were performed on amorphous (Fe, Ni)80B20, (Fe, Ni)80B19Si1, and (Fe, Ni)80P14B6alloy ribbons to yield the values of the magnetic moment per transition metal atom at 0 K and the Curie temperature. A close fit to the moment data, obtained by allowing not only the moments on Ni and Fe atoms to vary with Fe concentrationx, as revealed by neutron diffraction in crystalline alloys, but also the number of electrons per atom npand nbthat P and B atoms, respectively, donate to the transition metaldbands as described by the relationsn_{p} = 1.74 + 0.75 (1 - x)andn_{b} = 0.58 + 1.20 (1 - x), suggests a moment of 0 andsim0.13 mu_{B}per Ni atom in amorphous Ni80P14B6and Ni80B20alloys, respectively. The functional dependence of the Curie temperature onx, when followed closely for low Fe concentrations, gives the critical concentration xcabove which ferromagnetism appears in amorphous FexNi80-xB20and FexNi80-xP14B6alloy series asx_{c} cong 5and 8 at % Fe, respectively. The results of a theory based on coherent-potential approximation have been fitted to the measured values of Curie temperature in order to arrive at the Fe concentration dependence of the exchange interaction (JNiNi) between Ni-Ni pairs. The physical significance of the variation of JNiNiwithxobtained thereby has been discussed.     

Sublattice magnetization and cation distribution in BaMg2Fe16O27(Mg2W) ferrite

Mössbauer measurements were carried out in compound BaMg2Fe16O27(Mg2W) 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 Hhfat 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})Wper elementary cell is higher than the value obtained by adding the corresponding values for theSandMstructures. The Curie temperature of the compounds has also been measured and is equal to (440 ± 5)°C.     

Magnetic moments and curie temperatures of (Fe, Ni)80(P, B)20amorphous alloys

The magnetic moment per transition metal atom at 0°K and the Curie temperature were obtained for a series of (Fe, Ni)80(P, B)20amorphous quenched alloy ribbons. Fe/Ni and P/B compositions were varied separately. The moment data can be fitted well by assigning 2.1 Bohr magnetons per Fe atom and 0.6 per Ni atom, with the moment being lowered by 0.3 per B atom and 1.0 per P atom. Alternatively, moments varying with composition, as shown by neutron diffraction in crystalline alloys, combined with a lowering of 1.2 per B atom and 2.1 per P atom, also fit well. For a given P/B composition, Tcshows a broad maximum at Fe:Ni of about 3:1. For a given transition metal composition, Tcincreases with increasing B content.     

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 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.     

The role of Ar+, CH+4, O+2and backscattered Pb+ ions during Nb/Oxide/PbAuIn edge junction fabrication

A study has been made of the correlation between energetic positive plasma ions and the resulting Josephson tunnel barriers fabricated in an RF plasma. Typical edge junction current densities of 4000 Amp/cm²and Vm's of 25 mV were achieved. An in-situ mass spectrometer system was designed to extract, energy analyze, and identify the positive ion species incident on the RF cathode/sample surface. The argon positive ion measurements are correlated to Nb sputter rates and back scattered Pb and Nb rates - required for a clean Nb interface. The CH4positive ions are correlated to the growth of niobium carbide which reduces sub-oxide formation while optimizing junction quality. Finally the niobium oxide growth is correlated to the O2positive ion measurements. A simple model for oxide growth consisting of a time dependent O+2ion induced term and a time independent or very fast initial state term accounts for the known junction tunnel current dependencies on the RF plasma parameters of pressure, voltage, frequency, time, and ion to substrate angle.     

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.     

Dielectric, piezoelectric, and ferroelectric properties of MnCO3-added 74(Bi1/2Na}1/2) TiO3-20.8(Bi1/2K1/2)TiO3-5.2BaTiO3 lead-free piezoelectric ceramics

74(Bi1/2Na1/2)TiO{i3}-20.8(Bi1/2K1/2)TiO3-5.2BaTiO3-x MnCO3 lead-free piezoelectric ceramics were synthesized by conventional solid oxide routine. The tetragonal 74(Bi1/2Na1/2)TiO3-20.8(Bi1/2K1/2)TiO3-5.2 BaTiO3 (BNKB) exhibits high depolarization temperature Td of 195°C; however, its properties are far from satisfactory for practical application and need to be improved. The experiments show that the addition of MnCO3 reduces the tetragonality c/a and increases the cell volume. In addition, it revealed that the suitable addition of MnCO3 promotes the sintering and increases the densities of BNKB ceramics. The addition of MnCO3 also enhances the relaxor behavior of BNKB ceramics due to the reconstruct of the disorder arrays. Due to the effect of the crystal lattice, grain growth, and relaxor behavior, the optimal electric properties were realized at MnCO3 addition x of 0.16: the dielectric permittivity εr = 1047, dielectric dissipation tanδ = 0.022, piezoelectric strain d33 = 140 pC/N, mechanical coupling kp = 0.18, mechanical quality Qm = 89 while the depolarization temperature Td stays relatively high at 175°C. The effect and mechanism of Mn doping on the electrical properties were discussed in detail.     

Ferro-antiferromagnetic transition in the alloy system Fe65(Ni1-xCrx)35at low temperatures

The magnetic and electric properties of iron-nickel "invar" alloys and stainless steels are measured at low temperatures. It is shown that "invar" alloys such as stainless steels exhibit one-directional anisotropy, and their electric and galvanomagnetic properties are rather specific. It is shown also that the concentration dependence of Neel temperatures and magnetoresistance for the systems Fe-Ni and Fe65(Ni1-xCrx)35are similar.     

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.     

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.     

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.     

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.     

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.     

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.     

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).