Magnetic and Structural Properties of Granular Iron-Silicon Dioxide Thin Films
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MAGNETIC AND STRUCTURAL PROPERTIES OF GRANULAR IRON-SILICON DIOXIDE THIN FILMS M.J. Carey, F.T. Parker, and A.E. Berkowitz, University of California, San Diego, Department of Physics (B-019) and Center for Magnetic Recording Research (R-001), La Jolla, CA 92093
Abstract Fe-SiO2 granular films were produced by co-sputtering from separate Fe and SiO2 targets. Films deposited on silicon nitride membranes were thin enough to observe directly by transmission electron microscopy (TEM). The coercive force (Hc) was strongly dependent on temperature and volume fraction of Fe as observed in previous investigations, but with lower Hc. Two features of the films with low Fe concentrations were remarkable. The hysteresis loops show high remanence, and the room temperature TEM images show stable domain structure and ripple patterns. This behavior is inconsistent with a model of non-percolating, non-interacting superparamagnetic Fe grains.
Introduction Granular solids are composites of fine grains of metals in an insulating matrix. Their properties are strongly dependent on the volume fraction of the grains. At a volume fraction near 60%, the metal grains physically touch, creating a continuous network through the solid [1], i.e. percolation is established. Thin granular films below percolation which consist of separated, noninteracting magnetic grains with high Hc might be of use as high density recording media. The lack of exchange interaction between grains could allow for sharp magnetization reversals to be written without high noise. Xiao and Chien have shown that granular thin films of Fe in SiO 2 exhibit anomalously high Hc just below percolation [2]. In their work, samples consisting of nearly equiaxed grains of a-Fe embedded in an amorphous SiO 2 matrix were produced by high rate sputter deposition from composite targets. As the Fe concentration increased from about 30 to 60 vol%, the low temperature Hc increased steadily from 500 Oe to 2500 Oe. The remanence ratio (Mr/Ms) increased from 0.5 to 0.6 over the same range, indicating increasing interactions among grains. In this study, Fe-SiO2 thin granular films with Fe volume fractions from 19% to 62% were produced and studied by transmission electron microscopy (TEM), electron diffraction, Missbauer spectroscopy, vibrating sample magnetometry (VSM), and Lorentz electron microscopy to correlate the structure and magnetic properties. Experi~ment Granular films were sputtered from separate 2 inch diameter Fe and SiO 2 targets. The background pressure was 5x10- 7 torr. Substrates were mounted on a platform rotating about 1 revolution per second alternately over the Fe and the SiO 2 targets. The SiO 2 was sputtered at 400 watts, giving a deposition rate of 0.30 A/second. The Fe power was varied from 50 to 350 watts resulting in rates from 0.08 to 0.45 A/second. The target to substrate distance was 6 inches and the argon pressure was 4 mtorr. 60A of SiO 2 was sputtered as an initial layer; this enhanced the heterogeneity of the films. An additional 60A of SiO 2 was deposited o
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