Polarisations of magnetic fluids

Colteu, A.

doi:10.1016/0304-8853(83)90406-7

Cited by 15 publications

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“…The magnitude of the effect can be assessed by substituting the value of a e which can be determined from the work of Colteu [36] as (25) This gives a texture dependent contribution to the relative permittivity as The magnitude of the effect can be assessed by substituting the value of a e which can be determined from the work of Colteu [36] as (25) This gives a texture dependent contribution to the relative permittivity as…”

Section: R(a)mentioning

confidence: 99%

The Magnetic Properties of Fine Particles

Chantrell

O’Grady

1994

Applied Magnetism

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Magnetic fine particles find a large number of applications, perhaps most notably as particulate recording media. In addition, the ideas of fine particle magnetism are applicable in a number of areas as diverse as geomagnetism and biomagnetism. The purpose of this chapter is to introduce the fundamental aspects of fine particle magnets, including magnetisation reversal phenomena, interaction effects and time dependence, and to describe applications to geomagnetism and recording media. UNITS Note that in this chapter cgs units are used throughout, with the relationship between flux density, field strength and magnetization being B = H + M with M = 47r1. 113 115shown. It can be seen that the alignment increases both the value of He and the remanent magnetisation Mr' For this reason particulate recording media are magnetically aligned to some degree as will be seen later. INTRINSIC PARTICLE PROPERTIESThe magnetic behaviour of a fine particle system is to a large extent governed by the intrinsic properties of the particles. The most important property is the magnetic anisotropy, which, as will be seen later, provides an energy barrier to magnetisation rotation which, importantly, can be large enough to result in irreversible magnetic behaviour. The anisotropy arises from three main effects: * * * Crystalline Anisotropy. This has its ongm in the non-spherical shape of electron orbitals which give rise to preferred orientations in the lattice. The second necessary factor is a strong spin-orbit coupling which links the magnetic moment to the spatial anisotropy. The result is an energy term having the symmetry of the crystal lattice. The two most familiar forms are uniaxial anisotropy Ea = K 1 sin 2 8 + K 2 sin 4 8 and cubic anisotropy:where (a,,8,1) are the direction cosines. For the case of uniaxial anisotropy it is often found that Kl»K2 and only the first term is retained.Shape Anisotropy. In any magnetised body there exist demagnetising fields due to free poles on the surface. For a non-spherical body the demagnetising effects are anisotropic. This leads (for an ellipsoid of revolution) to an anisotropy energy which is uniaxial in form with an anisotropy constant K = 1/2(NB -NA)I;b where NB and N A are the demagnetising factors along the principal axes of the ellipsoid.Strain Anisotropy. This is essentially a magnetostrictive effect and is often described by a uniaxial anisotropy with an effective anisotropy constant K" = ~).s 0' where >-s is the saturation magnetostriction.The particle anisotropy represents one of the dominant factors in the behaviour of a fine particle, the second major factor being its size which, as will be seen later, is largely responsible for determining the mode of magnetisation reversal. It should also be noted that the particle surface plays an important role in relation to the magnetic behaviour. For example, the loss of exchange coupling in the surface can lead to a reduction in the magnetisation of the particle. In addition, there exists the possibility of various types of spin-pinning...

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