M.Th. Rekveldt

A domain size effect in the magnetic hysteresis of NiZn‐ferrites

The domain size and the ac‐hysteresis of NiZn‐ferrites have been investigated as a function of grain size. It is found that the ac‐hysteresis or energy dissipation at MHz frequencies is substantially reduced in fine‐grained, monodomain NiZn‐ferrites, which is attributed to the absence of intragranular domain walls.

The initial permeability of polycrystalline MnZn ferrites : the influence of domain and microstructure

The validity of nonmagnetic grain‐boundary (NMGB) models for the initial permeability of polycrystalline ferrites is examined. The domain size in a series of wet‐chemically prepared polycrystalline MnZn ferrites, in the demagnetized state, has been determined by neutron depolarization. A transition in the intragranular domain structure from mono‐ to two domain is observed at grain size D≊4 μm. An expression for this transition has been derived for a dense magnetic material. The grain size dependence of the initial permeability of the ferrites studied, particularly for monodomain grains, is consistent with the NMGB model. The grain‐boundary width and composition have been determined with a nanoprobe (1.5 nm resolution). Composition variations extend ≊10 nm into the grain. The findings suggest that, although the NMGB model considers ‘‘nonmagnetic’’ grain boundaries, in practice the grain boundary may be hard magnetic.

Ferrite/pearlite band formation in hot rolled medium carbon steel

Abstract The influence of the microsegregation of Mn, Si, and Cr on the austenite decomposition during isothermal transformations in hot rolled medium carbon steel has been studied by neutron depolarisation, electron probe microanalysis (EPMA), and optical microscopy. Eight specimens of the same alloy were held at 1173 K for 30 min and were rapidly cooled to different isothermal transformation temperatures. Two-dimensional EPMA maps of the specimen annealed at 1013 K showed that microsegregation of alloying elements in hot rolled steel is strongly related to the ferrite/pearlite band formation. The local variations in alloying element concentration lead to variations in local transition temperatures, which were calculated with the thermodynamic database MTDATA. Similar EPMA maps for the specimen transformed at 953 K demonstrate the presence of microchemical bands, while optical microscopy reveals the absence of microstructural bands. It is shown that the formation of microchemical bands is a prerequisite for band formation, but that the kinetics of the phase transformation determines the actual formation of microstructural bands. A quantitative model has been developed, which describes the observations in terms of the relative difference between ferrite nucleation rates in regions with a high and low local undercooling and the subsequent growth of the ferrite. The isothermal transformation experiments have led to generalised nucleation and growth criteria for the formation of microstructural bands.

Neutron depolarization in particulate media: A review of theory and experimental results

Abstract The polarization change of a polarized neutron beam due to transmission through a magnetic medium is analyzed in a neutron depolarization (ND) experiment. This change yields useful information about the micro-magnetic state of the medium, such as the correlation length and the mean local orientation of fluctuations in the local magnetization. These fluctuations are related to the intrinsic particle properties and magnetic interparticle correlations. A review of the ND theory and in particular of its application to particulate media is given. Furthermore, the results of ND experiments on several types of particulate media are discussed.

In-situ study of pearlite nucleation and growth during isothermal austenite decomposition in nearly eutectoid steel

The evolution of the microstructure during the isothermal austenite/pearlite transformation in a nearly eutectoid steel was studied by the three-dimensional neutron depolarization technique, which simultaneously provides information about the pearlite fraction, the average pearlite colony size, and the spatial distribution of the pearlite colonies during the transformation. The in-situ measurements show that the pearlite nucleation rate increases linearly with time with a temperature-dependent slope. The in-situ measured average pearlite growth rate is accurately described by the Zener-Hillert theory, which assumes that volume diffusion of carbon is the rate-controlling mechanism. The measured overall transformation rate deviates from the predictions of the theory developed by Kolmogorov, Johnson, Mehl, and Avrami.

Correlations in ferromagnetic domain structures studied by means of the neutron depolarization technique

Abstract The polarization change of a polarized neutron beam after transmission through a partly magnetized ferromagnetic material can be described by a (3 × 3) depoalarization matrix. An earlier interpretation of this matrix in terms of domain quantities such as the reduced mean magnetization m , the mean domain size δ and the mean square direction cosines γ x , γ y and γ z of the inner magnetization within the domain is extended by taking into account different correlations occuring in real domain structures, such as the correlation between neighbouring domain orientations expressed by the mean direction cosine K between these orientations and the correlation between the domain dimensions and the domain magnetization orientation of an arbitrary domain. The latter correlation can be expressed by means of the quantities Δ and I`, where Δ is a measure for the average shape anisotropy of the domains with respect to the direction of domain magnetization, and Γ is a measure for the movement of domain walls during the magnetization process of the sample. The possibilities and limitations of the resulting theory on neutron depolarization experiments are discussed.

Relation between grain size and domain size in MnZn ferrite studied by neutron depolarisation

Abstract The domain size has been determined by means of neutron depolarisation in a series of polycrystalline MnZn ferrites of varying grain size but unaltered composition. For ferrite samples with grain sizes between 0.3 and 3 μm, it is found that the domain size is identical to the grain size i.e. no magnetic domain walls are present within the grains.

Static and dynamic neutron depolarization studies of ferromagnetic domain structures

A polarized neutron beam passes through a ferromagnetic sample. In front of the sample a polarization turner turns the polarization towards the x, y or z direction. A second polarization turner behind the sample enables one to analyse any component of the polarization vector. In this way a (3×3) depolarization matrix can be measured from which various domain quantities are determined in the bulk, such as the mean magnetization in three dimensions (Mi, i=x,y,z), the mean domain size and the mean local direction of magnetization within the domains. Applying a periodic magnetic field on the sample and using a multi‐channel analyser the time dependent depolarization matrix determines the mean periodic time dependence of the parameters mentioned. Among the possible applications of the method static and dynamic experiments are discussed. Static experiments have been carried out on an oblate Ni ellipsoid with a magnetic field in the y‐direction in the [311] crystal direction in the plane of the ellipsoid. From t...

The effect of intragranular domain walls in MgMnZn-ferrite

The effect of grain size, D, on the intragranular domain state and initial permeability, μi, of polycrystalline MgMnZn ferrites has been investigated. The intragranular domain state is found to change from mono- to two domain at D=2.9±0.2 μm. Despite this change, μi follows a continuous dependence on D, which can be described by the nonmagnetic grain boundary model. The results show that in spinel ferrites with an anisotropy of 72 J/m3, μi is still due to rotation of magnetization.

The ferrite and austenite lattice parameters of Fe–Co and Fe–Cu binary alloys as a function of temperature

The lattice parameters of Fe-15 Cu, Fe-2% Cu, Fe-1% Co, and Fe-2% Co binary alloys were determined by means of neutron diffraction at temperatures around the austenite-ferrite phase transformation (860--1350 K). While the thermal expansion coefficients prove to be similar to those of Fe for all alloys, Cu and Co have an opposite effect on the lattice parameter of Fe. Addition of Cu increases the lattice parameter in both ferrite ({alpha}) and austenite ({gamma}), while Co decreases the lattice parameter. For all alloys, the {alpha} {leftrightarrow} {gamma} phase transformation introduces a volume change of 1.0%. Evidence is found that both ferrite and austenite are slightly strained ({epsilon} < 8 {times} 10{sup {minus}4}) when both phases are present simultaneously.