Structural and Magnetic Properties of Fe/Al 2 O 3 , Granular Systems
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STRUCTURAL AND MAGNETIC PROPERTIES OF Fe/Al20, GRANULAR SYSTEMS J.L. DORMANN+ AND D. FIORANI++ CNRS, 92195 Meudon-Cedex, +Laboratoire de Magn~tisme, รท+ITSE, CNR, CPlO. 00016 Monterotondo Stazione, Italia.
France.
ABSTRACT Several samples of granular material consisting of Fe small particles an alumina matrix were studied by means of various techniques. dispersed in Structural results show that the particle surface is partially oxidized with occurrence of bonds with the matrix. Depending on the particle size, one observes b.c.c. a-Fe or amorphous structure. Magnetic properties are relevant of superparamagnetism phenomena, but the strong magnetic interactions between particles (due to the close compact arrangement) lead to a phase transition is also briefly discussed the magnetic volume It at zero temperature. determination and the influence on the properties of the magnetic disorder arising at the particle surface.
INTRODUCTION Several studies have been devoted to the magnetism of granular films which are composite materials constituted of small particles finely dispersed in an insulating matrix. After the pioneer works on Ni particles in alumina the Fe particles in silica [2-5] and in alumina [1], and silica matrices have been extensively studied by means of various techniques. In [6-17] small grains of FelooexCox[18], FeO 3 [19] and amorphous FeoBo addition, alumina matrix have also been investigated (21] in [20] and Fe 6 4 Cr 6 DB 20 essentially by means of Mossbauer spectroscopy. are relevant of particles of such magnetic properties The review papers [22-24]) as the particle phenomenum (see superparamagnetism diameter is smaller than around 120 A. Nevertheless some complexities arise, the close compact arrangement of the considering due to the fact that, the dipolar magnetic interactions between particles are large and particles, In this case the Neel's theory, valid for isolated modify the properties. particles, is not directly applicable. On the other hand, simplified formulae expressed via the Langevin relating the magnetisation vs applied field, are not always valid since in granular samples, the 23], function [14, particles cannot move and reorientations of the easy magnetisation axis do such as in ferrofluids. Another interesting question arising in not occur, concerns the effects of spin disorder at the surface. We such a material point out that around 40 % Uf the atoms lie near or on the surface for a particle with a diameter of 40 A. For small particles, Mossbauer experiments [24] have shown that disordered canted magnetic structure appears at the but there are no definitive results for particles embedded in an surface, amorphous matrix, with possible bonds at the interface [5, 15]. On the other hand, the influence of the magnetic interactions on the magnetic structure of not been analysed. the particle surface has still we will summarize the results obtained on several in this paper, Fe/A1 2 0 3 granular samples from the structural as well as magnetic points of view. We will describe also briefly th
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