Analysis of the mobility of migrating austenite–ferrite interfaces

Dilatometric studies assisted by high-temperature laser scanning confocal microscopy provide a comprehensive experimental picture with regard to cyclic austenite-to-ferrite transformations in Fe–C alloys. The validity range for the sharp interface and effective mobility approach is identified by comparing modelling results with calculations based on experiments. The interface velocity for the austenite-to-ferrite transformation in pure iron is exclusively controlled by the intrinsic interface mobility conforming to the upper boundary of mobilities. The austenite-to-ferrite transformation in Fe–C alloys under conventional cooling and heating conditions is primarily controlled by carbon diffusion in austenite. The lower boundary of the temperature-dependent interface mobility has been established for an Fe–C alloy over a wide range of temperatures during cycling transformation. Austenite-to-ferrite transformations in Fe–C–X alloys are characterized by still lower effective mobilities depending on both temperature and composition, because substitutional elements X give rise to a solute drag effect. An estimate for the effective mobility valid for the austenite-to-ferrite transformation in lean Fe–C–Mn alloys is provided.     

Mössbauer spectroscopy studies of the magnetic properties of ferrite nanoparticles

Studies of the ferrite nanoparticles prepared by the chemical decomposition of iron chlorides with a various ratio ξ = Fe³⁺/Fe²⁺ are herein presented. The microstructure and the magnetic properties have been studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Mössbauer spectroscopy (MS). The TEM studies show that the nanoparticles have almost a spherical shape with the diameter of (12 ± 2) nm for all samples. The measured XRD pattern was mainly composed of lines which were indexed with a cubic spinel structure. The analysis of the Mössbauer data shows that the microstructure of the nanoparticles consists of the core formed by nonstoichiometric magnetite and maghemite shell. A small amount of hematite, probably on the surface of the nanoparticles with ξ = 1.75, 2.0, was detected. At temperatures T ≤ 150 K the spin canting of surface maghemite with ξ = 2.25 was observed while for the samples with ξ = 1.75, 2.0 such effect was suppressed by the presence of hematite on the surface of the nanoparticles. Infield Mössbauer spectra with ξ = 1.75, 2.0 show that magnetic moments of the magnetite/maghemite core are parallel while magnetic moments of the surface hematite are perpendicular to the direction of the external magnetic field.     

On the nucleation of cementite on bainitic ferrite–austenite interphase boundaries

Among all possible variants of the Isaichev orientation relationship between cementite and ferrite, a single major cementite variant has been observed to appear in bainite. Interphase boundary nucleation of cementite on ferrite–austenite semi-coherent interfaces is considered a plausible reason for this observation. With the aid of known crystallographic relations and habit planes of the ferrite–cementite, ferrite–austenite and austenite–cementite phases, a model for cementite nucleation has been proposed. The interphase-boundary nucleus is assumed to form on a semi-coherent ferrite–austenite interface and to possess ferrite–cementite and austenite–cementite habits as two main facets of the nucleus. It is shown that interphase cementite nucleation will be viable if the energies of all facets of the nucleus are in the semi-coherent range.     

Effect of nanoparticles on the magnetic properties of Mn–Zn soft ferrite

In this paper, the effect of nanostructures on the magnetic properties like the specific saturation magnetization (σ_S) and the coercivity (H_C) for Mn_0.4Zn_0.6Fe₂O₄ ferrite prepared by the co-precipitation method has been presented. We have shown by means of X-ray diffraction that the resulting ferrite is made up of nanoparticles, and that the average size of these nanoparticles calculated with the Scherrer formula depends upon the sintering temperature. When the sintering temperature is increased from 500 to 900 °C, the average nanoparticle diameter varies from 19.3 to 36.4 nm. The nanoparticle phase is further confirmed by scanning electron microscopy (SEM). Both results are found to be in good agreement. The magnetic properties are explained on the basis of the single-domain and multi-domain theory.     

Substitutional effect of Cr ions on the properties of Mg–Zn ferrite nanoparticles

The effect of Cr³⁺ substitution in Mg–Zn ferrite, with a chemical formula Mg_0.5Zn_0.5Cr_xFe_2−xO₄ (x=0.0–1.0), synthesized by a sol–gel auto-combustion reaction is presented in this paper. The resultant powders were investigated by various techniques, including X-ray diffractometry (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), vibrating sample magnetometry (VSM), and DC resistivity. The XRD pattern revealed that the cubic spinel structure is maintained for the all the compositions. The particle sizes measured from XRD and TEM are in good agreement with each other. The cation distribution suggests that Mg²⁺, Cr³⁺ and Fe³⁺ have strong preference towards octahedral B-site. The theoretical lattice constant and experimental lattice constant match each other very well. The IR analysis supports the presently accepted cation distribution. The saturation magnetization decreases linearly with increasing Cr³⁺ content. Curie temperatures are obtained by the Laoria and AC susceptibility techniques. The dc resistivity has been investigated as a function of temperature and composition.     

Structural studies on the yttrium-doped cobalt ferrite powders synthesized by sol–gel combustion method

Y_0.2CoFe_1.8O₄ nanopowders were prepared using a sol–gel combustion method. Metal nitrates, such as yttrium nitrate, cobalt nitrate and ferric nitrate, were used as the source materials. Citric acid and polyvinyl alcohol were used as the burning agent and agglomeration reducing agent, respectively. The pH of the precursor was maintained at 7. The mean crystallite size of the prepared ferrite was in the range of ∼20–70 nm. The inverse spinel structure, cubic morphology, and the identification of functional groups of the yttrium-doped cobalt ferrite were analyzed systematically using several analytical tools.     

Magnetic losses of the soft magnetic composites consisting of iron and Ni–Zn ferrite

Ferromagnetic powders which are surrounded by an electrically insulating film (soft magnetic composites (SMCs)) exhibit unique magnetic properties, such as relatively low magnetic losses and 3D isotropic magnetic behavior. In some electromagnetic applications, including microwave frequency range applications, it is necessary to increase electrical resistivity without any noticeable reduction in magnetic properties. To achieve this purpose, electrically resistant materials, for example, ferrites with acceptable magnetic properties, are suitable candidates. This paper focuses on the effects of the synthesized Ni–Zn ferrite addition on the magnetic properties of the SMCs containing Ni–Zn ferrite within iron particles. The structure was studied by means of X-ray diffraction (XRD). The microstructure and the powder morphology were examined by the use of scanning electron microscopy (SEM). The magnetic measurements on powders and samples were carried out using a vibrating sample magnetometer (VSM) and an LCR meter, respectively. The results indicate that the lowest magnetic loss and the highest magnetic permeability are related to the composites with 20 wt% ferrite and 2 wt% ferrite, respectively. Also, the composites with 10 wt% ferrite show a good combination of magnetic loss and magnetic permeability in the range 0–500 kHz.     

Response comments to the article “some comments on magnetization reversal in uniaxial ferrite magnets”

The previous paper contains no new information but is rather based on some wrong conclusions drawn from a paper entitled “Magnetization reversal in ferrite magnets” by Hadjipanayis et al. published in J. Magn. Magn. Mat. 81 (1989) 318. The authors of the previous paper (Paoluzi and Turilli) missed altogether the objectives of the article by Hadjipanayis et al. which were to explain the differences observed in the temperature dependence of coercivity of bulk and small particle BaFe₁₂O₁₉ magnets and relate them to the magnetization reversal processes. The conclusions were that the bulk sintered magnets behave as “nucleation”-type, the ultrafine particles as “single domain”-type and the aerosol coarser particles show a behavior which is a mixture of the above two. In this paper we dispute in detail the main points made by the authors.     

Magnetic ordering in Ni–Cd ferrite

A series of Ni_1−xCd_xFe₂O₄ (0.0≤x≤0.8) were prepared by conventional double sintering ceramic method and sintered at 1200 °C for 6 h. X-ray diffraction results confirmed the single-phase spinel structures of all the samples. The Curie temperature decreases linearly with increasing Cd content, which is explained due to the weakening of the A–B exchange interaction. The sample with x=0.7 shows re-entrant type of spin glass phase transitions. The magnetic moment and saturation magnetization at 20 K are found to increase with Cd content up to x=0.5 and then tends to decrease for x>0.5. The increase in magnetic moment with cadmium is attributed to Neel's two sublattice (A- and B-sublattice) collinear models according to which the magnetic moment is the vector sum of the lattice magnetic moment. The decrease in magnetization for x>0.5 obeys the Yafet–Kittel (Y–K) model. The increase in Y–K angles for x>0.3 indicates the increased tendency for triangular spin arrangements on B-sites. This suggests the existence of a canted spin structure in the ferrite system with higher content of Cd.     

Characterization of Ni–Cu mixed spinel ferrite

Crystal structure, X-ray density, porosity, compressive strength of Ni_1−xCu_xFe₂O₄ have been investigated along with scanning electron microscopy (SEM) to study the effect of composition and microstructure on the magnetic and electrical properties. The formation of single-phase ferrite is confirmed by the X-ray diffraction. Tetragonal deformation is observed for the sample of composition x=1, i.e. for pure CuFe₂O₄ Crystal structure for samples of other compositions are face centered cubic (FCC). SEM micrographs exhibit increase in grain size with the increase of copper content. Compressive strength decreases with the increase of Cu. Initial magnetic permeability and saturation magnetization is maximum for the composition of x=0.2, i.e. for Ni_0.8Cu_0.2Fe₂O₄, which can be attributed to the maximum sintered density obtained for this composition. Resistivity decreases with the increase of Cu content.     

Neutron diffraction studies of the diluted spinel ferrite ZnxMg0.75−xCu0.25Fe2O4

Neutron diffraction studies have been performed in the temperature range 1050 K⩾T⩾10 K on the spinel system Zn_xMg_0.75−xCu_0.25Fe₂O₄ (x=0.0, 0.25, 0.5 and 0.75) prepared in the ceramic sintering method. Cation distributions in all the four compositions have been determined from the analysis of neutron data. A perturbed ferrimagnetic order has been observed in the compositions with x values lying in the low to intermediate range, where apparently, randomly canted spins and small fluctuating clusters are superimposed on the ferrimagnetic long range order. A complete breakdown of the ferrimagnetic stability occurs at x=0.75 as indicated by the absence of magnetic long range order. A huge diffuse scattering signal appears at the low-Q region broadening the (1 1 1) diffraction line in the low temperature neutron patterns of this composition indicating the formation of large magnetic clusters. A cluster spin glass state is suggestive for this composition.     

Magnetic enhancement in nano-sized Ni–Zn ferrite

Nanocrystalline Ni_0.35Zn_0.65Fe₂O₄ synthesized by mechanical alloying method and subsequent annealing at 300°C has been characterized by XRD, TEM and Mössbauer spectroscopic techniques. Mössbauer spectroscopic study divulges the enhancement of magnetic order, ordering temperature and magnetization in nano-crystalline sample compared to its bulk counterpart. This magnetic enhancement has mostly been prompted by cation redistribution in the nanosized sample. Zinc having strong A site affinity determines the nature and intensity of site exchange of cations, which has a strong influence in the genesis of enhancement/reduction in magnetic property of nano-crystalline Ni–Zn ferrite samples.     

Thermal properties of gamma-ray irradiated Co–Zn ferrite

Co-Zn ferrite samples of the system Co_1-xZn_xFe₂O₄ (x = 0, 0.2, 0.3, 0.5, 0.6 and 0.8) were prepared using the usual ceramic double sintering technique. Thermal conductivity and specific heat were measured at different temperatures for different compositions. The effect of Co-60 γ-irradiation dose (10⁶ rad) on the thermal conductivity and specific heat were studied.     

Experimental investigation of the wave profile excited by a narrow transducer in a dielectric–ferrite–dielectric structure

The results from an experimental study of the diffraction profiles of magnetostatic backward volume waves (MSBVW) excited by a finite linear transducer placed on the surface of tangentially magnetized ferrite film in a dielectric–ferrite–dielectric structure are presented. Theoretical calculations and experimental data are compared.     

Mössbauer studies of the H2 reduction effects on magnetic properties of Ba–Sr hexagonal ferrite

Hyperfine Interactions - The concept of composite magnets with hard materials and soft materials have been applied for increasing specific saturation magnetization (σs) of M-type hexagonal...     

Speed of a ferrite—ferroelectric microwave planar resonator

The speed of a ferrite—ferroelectric microwave planar resonator consisting of a YIG film and a barium strontium titanate plate is investigated. It is shown that the speed of the resonator is governed by the rate of resonance frequency electrical tuning and the tuning is accompanied by a change in the electrical bias. The rate of frequency tuning in the resonator is 1.3–3.3MHz/fus.     

Mössbauer study of nanocrystalline Ni–Zn ferrite

Ni_1−xZn_xFe₂O₄ (0.0⩽x⩽1.0) nanoparticles have been prepared by the polyvinyl alcohol (PVA) sol–gel method. The lattice parameter of Ni–Zn nanoparticle is larger than that of the bulk material. The Mössbauer spectra of the samples showed the presence of ultrafine particles exhibiting superparamagnetic relaxation at room temperature and an ordered magnetic structure at 77 K.