Granular Magnetism
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Granular Magnetism C.L. Chien
Department of Physics & Astronomy, The Johns Hopkins University Baltimore Maryland 21218 Abstract Recent studies of granular magnetic solids involving Fe and its alloys embedded in either an insulating medium (e.g. SiO 2 A120 3) or a metallic medium are described. Below the percolation threshold, isolated magnetic granular exhibit single-domain properties and superparamagnetic relaxation. Giant coercivity has been uncovered in granular Fe-SiO 2, with I- as high as 3 kOe at 5 K and 1.1 kOe at 300 K. Some recent results on granular Fe in a metallic medium, and other granular magnetic systems will also be discussed.
A. Introduction Granular metal solids, pioneered by Abeles et al. 1.2 in the 1970's, cpnsist of metal granules dispersed in an immiscible medium. The granules, as small as 10 A, can be made to be isolated or connected, as dictated by the metal volume fraction of the sample. This size range is much smaller than that achievable by usual powder metallurgy and chemical methods, and is the size range where finite-size effects may be observed. Because of their unique nanostructures, granular solids display a rich variety of conducting, superconducting, optical, magnetic and other physical properties. They provide a fertile ground for the exploration of nanostructure-induced properties. The two relevant extra degrees of freedom, unique to granular metal solids, are the granule size, which be can controlled by process conditions, and the metal volume fraction (xv), which can be experimentally varied between 0 and 1. For samples with small values of xv, the metal granules are isolated from each other and protected by the matrix. Electrically, granular solids with low x, and in an insulating matrix are themselves insulating, apart from tunneling among metallic grains, which is of course interesting in its own right. For ferromagnetic granules, all the granules are single domain, since the granules are smaller than the critical size for a single domain. On the other hand, for samples with large values of x, the conducting paths and magnetic closure structure are readily facilitated. Thus, the metal granules form an infinite network which exhibits metallic conductivity and bulk magnetic properties. The percolation threshold (xd)is the unique volume fraction at which an infinite network of connecting granules first forms. Experimentally, for a wide variety of granular metal solids, values of xp have been found to lie in the range 0.5 -0.6. Many granular solids consist of a metal and an insulator, the latter includes SiC 2, A12 0 3, MgO, etc. Granular solids are therefore also nanocomposite materials, which can reap benefits from both components. Since these insulators are usually mechanically harder, and more wear and corrosion resistant than the metal companion, granular metals exhibit much superior auxiliary properties while retaining their metallic characteristics. In this paper, we will primarily discuss the magnetic properties of a variety of granular magnetic solids. The granules m
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