SUPERPARAMAGNETISM IN GRANULAR COMPOUNDS AS SEEN BY MÖSSBAUER SPECTROSCOPY

Résumé
Les films granulaires /1/ sont des matériaux composites constitués de particules de métal (15 à 200 Å) dispersés dans une matrice amorphe et isolante. Les composés étudiés Fe-SiO₂ /2/, Fe-Al₂O₃ et Fe-ZrO₂ ont été préparés par copulvérisation de Fe et de SiO₂ (ou Al₂O₃ ou ZrO₂). La microstructure et le diamètre des grains de fer dépendent de la proportion relative de fer et de matrice. L'effet Mössbauer sur le ⁵⁷Fe permet d'étudier la transition ferro superparamagnétisme et de préciser des propriétés de l'interface métal-isolant en corrélation avec la microscopie électronique. Différentes compositions ont été étudiées de 4,2K 1 300K. Pour les matériaux peu riches en fer, les grains sont de petit diamètre et les composés sont presque amorphes. Pour les composés contenant de 50 à 60 % de fer, les diamètres des grains varient entre 50 et 100 Å et plusieurs sites de fer sont mis en évidence : deux sites, d'intensités faibles, correspondent d'une part à du fer paramagnétique dilué dans la matrice, d'autre part à des composés non définis Fe-O matrice (H = 200 à 250 kOe à 4,2K) ; un site principal correspond au fer massif métallique ; deux sites simulant une répartition de champs hyperfins (370 ≤ H ≤ 420 kOe) correspondent à des atomes de fer à l'interface /3/. Les intensités de ces sites varient beaucoup avec le diamètre des grains et la nature de la matrice.

Abstract
Granular films are composite materials made of small metals grains (15-200 Å) in an amorphous insulating matrix /1/. The granular compounds studied here were Fe-SiO₂ /2/, Fe Al₂O₃ and Fe-ZrO₂ with different ratio Fe/insulating matrix. These samples were prepared by cosputtering of Fe and SiO₂ (or Al₂O₃ or ZrO₂). The microstructure was studied by electron microscopy. Mössbauer effect was used to study the transition superparamagnetism ↔ ferromagnetism as a function of T and to get information about the interface Fe-matrix /2/. In 0.35 Fe, 0.65 ZrO₂, the spectrum shows a large distribution of H_hpf around the value of metallic iron, this sample is amorphous and relaxation effects are present at 4.2K. In 0.53 Fe, 0.45 Al₂O₃ the distribution of H_hpf is narrow, this sample is almost amorphous. In other samples with higher concentration of iron, a magnetically ordered well defined spectra appear at 4.2K. The following features are shown : a paramagnetic site (low intensity) was found which corresponds to very tiny Fe clusters. A magnetic site (low intensity) with H_hpf = 200 to 250 kOe corresponds to some iron-oxygen-matrix compound. The magnetic site with H_hpf ≈ 340 kOe corresponds to bulk metallic iron. Other sites with H_hpf ≈ 370 kOe and 410 kOe correspond to iron atom at the surface of the grains /3/. Spectra at different temperature on samples 0.6 Fe, 0.4 Al₂O3 and 0.5 Fe, 0.5 ZrO₂ are typical of relaxation behaviour with transition temperature around 300 and 500K respectively. The nature of the matrix is of significant importance on the behaviour of the magnetic grains. For a given grain diameter the number of sites with H_hpf > 340 kOe depends strongly of the nature of the matrix. Amorphous material appears when the concentration of iron is very low. The size diameter measured from electron diffraction photograph is consistent with the size deduced from Mössbauer experiments. Fe-Al₂O₃ gives the narrowest linewidth, this means that the granular compound is well defined : interface effect are the lowest, concentrations of paramagnetic Fe and Fe compound are very low.Furthermore in Fe-Al₂O₃ there is a narrow range of grains diameter. Fe-Al₂O₃ appears to be the best defined granular compound, i.e. : magnetic metalgrains in an insulating matrix with a sharp distribution of diameters.