P.T. Por

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.

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.

Construction and testing of a multichannel polariser for thermal neutrons

A curved polarising multichannel neutron guide with total beam cross section 26 × 56 mm2, containing 20 channels was produced at PNPI. The concave sides have a supermirror coating consisting of the alloys Co64Fe34V2 and Ti60Zr40, giving θcλ = 34(±0.8) mrad/nm. The convex side is a natural Ni layer with θcλ = 17.5 mrad/nm. Both sides have an antireflecting underlayer based on an alloy of TiZr and Gd. All coatings were produced by sputtering. The characteristic bending angle for which the system is designed is 5.79 mrad giving a characteristic wavelength equal to 0.17 (±0.05) nm. The transmission and polarisation as functions of λ were measured at IRI by means of “Lannor precession spectroscopy” and the “3 polariser 2 shim method”, respectively. The transmission is equal to the theoretical value within 10%; the polarisation is 0.98 at λ = 0.16 nm decreasing to 0.75 at λ = 1.0 nm.

Adiabatic rotators for 3-D neutron polarisation analysis

Abstract A set of two polarisation rotators designed according to the principle of adiabatic rotation of the polarisation vector of a polychromatic thermal neutron beam is described. The adjustment of the polarisation vector along the axes of the laboratory coordinate system is examined by 3-dimensional polarisation analysis over the λ range of 0.15–0.55 nm. The spectra needed for this analysis are obtained by means of the method of Larmor precession followed by Fourier transformation.

Separating the polarising power from depolarisation in a set-up with 3 neutron polarisers

Abstract A method of separating the polarising power of neutron polarisers from the depolarisation in a set-up composed of three neutron polarisers, the so-called “three polariser two shim method”, is described. It appears that, using this method, the net polarising power of the second polariser, i.e. without any depolarisation effects between the polarisers, can be determined. Also the application to magnetic Bragg scattering experiments is discussed.

Magnetic permeability and intra-granular domain structure in polycrystalline ferrites

Abstract A transition from mono- to multiple-domain grains has been identified in a series of MnZn ferrites with grain size ranging from 0.3–16 μm using the neutron depolarisation technique. Despite this transition to mono-domain grains, the initial permeability displays a grain size dependence consistent with the a recently proposed model, where non-magnetic grain boundaries are considered to modify the effective permeability of the polycrystalline ferrites. In contrast, the absence of domain walls within the mono-domain grains excludes the use of the domain wall permeability model of Globus in this situation.

Theory of neutron depolarisation in particulate dispersions

Abstract Neutron depolarisation has been simulated to study the microstructure of a particulate dispersion predicted by a Monte Carlo simulation. The zero-field configuration predicts a magnetic correlation length approximately equal to the particle diameter, in agreement with experiment. Moreover, the ND results indicate a random structure of particles.

An experimental study of the magnetic behaviour of particulate dispersions I: Chromium dioxide

Abstract A detailed study of the magnetic behaviour of CrO 2 dispersions is described. The study includes static and dynamic measurements and also some preliminary results on the structure of the dispersion obtained by Neutron Depolarisation which was carried out in an attempt to determine the long-range order in the dispersion. The measurements are interpreted in terms of a simple model of the evolution of dispersion quality during the milling process. Magnetic measurements are shown to be useful in characterising dispersion quality although a number of techniques are required in order to probe different aspects of dispersion behaviour.

Neutron depolarisation and noise studies of Ba-ferrite tapes as a function of milling time

Ba-ferrite particles have been dispersed and coated onto polyethylene terephthalate (PET) film and magnetic structure investigated using a range of magnetic measurements including noise studies and neutron depolarisation (ND). Noise measurements were made on lengths of tape for samples cycled through Isothermal Remanent Magnetisation (IRM) and DC Demagnetisation (DCD) measurement processes and interpreted in terms of the total noise power and spectral response. ND measurements were analysed in terms of magnetic correlations within the films. Total noise power exhibited a minimum around the remanent coercivity (H/sub r/), as observed for most particulate media. ND values of correlation function, /spl zeta/ displayed a peak rather than a dip around H/sub r/. The similarity in the respective peak/dip positions indicates that both measurement techniques were sensitive to the same micro-structural details.

The magnetic structure of metal-evaporated tape studied by neutron depolarisation

Abstract In this paper the results of a neutron depolarisation study of the magnetic structure of a two-layer metal-evaporated tape are presented. It appears that the tape consists of small magnetic grains with a diameter of 10 nm. These small grains form due to the exchange coupling magnetic domains with a domain period of 210 nm. The magnetisation orientation of the domains is for both layers 45°. The two layers are magnetically not identical.