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.     

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.     

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.     

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.     

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.     

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.     

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 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.     

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.     

Electronic diffusion and directional order in the magnetic semiconductor HgCr2Se4

The existence of a directional order has been shown in a HgCr2Se4single crystal reheated with Hg atmosphere, by measuring the magnetic aftereffect and the anisotropy. The directional order is attributed to an electron migration between Cr²⁺and Cr³⁺on B sites; the n-type carriers created by the thermal treatment with Hg are shown to be mostly due to the Cr²⁺ions formed during the treatment.     

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.     

Peculiarities of the Anisimkin Jr.' plate modes in LiNbO3 and Te single crystals

Quasi-longitudinal (QL) Anisimkin Jr.' modes discovered recently in quartz plates are found now in 2 other piezoelectric crystals, belonging to trigonal symmetry. In 128°Y,X+90°-LiNbO3, 2 different QL modes may propagate simultaneously for small plate thickness h/λ = 0 to 0.06 (h is thickness, λ is wavelength). The velocity of the 1st mode, v1, is close to the longitudinal bulk wave velocity vL. It is varied with h/λ and piezoelectrically stiffened (maximum Kn² = 39% at h/λ = 0.08). The velocity of the 2nd QL mode, v2, is close to the shear-horizontal bulk wave velocity vQSH, not varied with h/λ and not stiffened (Kn² = 0). On the contrary, 210°Y,X-Te crystal supports only one QL-mode, but it is unusually wideranging and low-dispersive: the mode exists for all h/λ from 0 to 2.5 with velocity vn almost permanent and equal to vL in the whole range. This mode is piezoelectrically stiffened (maximum Kn² = 2% at h/λ = 0.13). The variety of the Anisimkin Jr.' modes in different crystals makes them attractive for liquid sensors, where the amount of suitable waves is very restricted.     

Trivalent chromium in the solid solution system CrxMo1-xO2 (x ≤ 0.5)

the solid solution system Cr_xMo_1-xO₂ for x ≤ 0.5 was investigated. The following three phases were identified from x-ray powder diffraction analyses: monoclinic distorted rutile (CrMoO₄) for 0.36 ≤ x ≤ 0.5, tetragonal rutile for 0.12 ≤ x < 0.36 and monoclinic (MoO₂) for 0 ≤ x < 0.12. From magnetic data it was shown that throughout the whole range the magnetic ion was trivalent chromium (d³) and the pentavalent molybdenum (d¹) due to its metallic Mo-Mo interactions does not contribute to the magnetic moment.     

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.     

Properties of the W-type hexaferritePbZn2Fe16O27

W-type Pb-hexaferrites were prepared by standard ceramic methods. The lattice constants found by refinement were a=0.59140±0.00006 nm and c=3.29209±0.00041 nm. The X-ray density of a typical composition PbZn_1.9Fe_15.3O_25.8 was ρ=5.32 g/cm³ and the Vickers microhardness value h _v=6 kN/mm². A plot of the saturation magnetization versus temperature is given. The extrapolated value of the saturation magnetization (H→∞, T→0) was σ_s=108 emu×g^-1, and the Curie temperature was T_c=600±20 K     

Remarks on extinction effects in MgF2 single crystals

Extinction effects are very often severe in ionic single crystals. Corrections for these effects depend mainly on theoretical models which describe primary extinction effects only poorly. In order to reduce the primary extinction, different thermal treatments were applied to MgF₂ samples. γ-ray rocking curves were recorded to visualize the effects of these thermal treatments. The aptitude of a sample to give the best results in neutron and X-ray intensity measurements was decided on the basis of the shape of the γ-ray rocking curve profiles. The isotropic mosaic spread extinction parameter values obtained from γ-ray, X-ray and neutron techniques were of the same order of magnitude.     

The piezoresistive effect in the p-TlInSe2 single crystals

The experimental investigations of the directed deformation influence on the electroconductance of p-T1InSe₂ single crystals have been carried out. The presence of a strong piezoresistive effect has been found at p-T1InSe₂ crystal tension or compression along the crystallographic axis [001]. It is shown that the observed effect has a qualitative explanation, based on a 4-ellipsoidal model with the more probable arrangements of extrema being at Г, T, and N points of the Brillouin band.