Synthesis and Magnetic Characteristics of Granular Co Dispersions in a Polymer Matrix
- PDF / 436,369 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 66 Downloads / 150 Views
SYNTHESIS AND MAGNETIC CHARACTERISTICS OF GRANULAR Co DISPERSIONS IN A POLYMER MATRIX Eric Kay, C. Laurent, S.S.1P. Parkin and D. Mauri IBM Research Division, Almaden Research Center, San .Jose, CA 95120-6099
ABSTRACT We have produced thin film composite matrices containing granular transition metals in the zero valent state and have found onset of percolation at a much lower volume fraction than was the case for Au, albeit for significantly smaller transition metal particles. At volume fractions below percolation these granular composites are superparamagnetic down to 6°K. Remanent magnetization and hysteretic ferromagnetic behavior was not observed until we were slightly above the onset of electrical percolation. We interpret this result as indicative of ferromagnetic ordering among the transition metal grains. No significant reduction in magnetic moment due to size effects or to chemical interaction with the matrix was observed. A very small size dispersion of the metal clusters was indicated both from magnetic a well as electron microscopy measurements. INTRODUCTION Numerous papers have been published on matrix isolation of granular metal particles in a variety of inorganic, insulating matrices such as A120, and SiO2 . We have developed a one step vapor deposition process which results in uniform dispersions of granular metal particles in a polymeric thin film matrix. In particular, we report on the incorporation of cobalt in plasma polymerized propane. At low metal volume fraction the cobalt forms very small, roughly spherical, clusters with an approximate diameter of 10A, and with little size dispersion as revealed by transmission electron microscopy. We determined the magnetic moment of these clusters by measuring their superparamagnetic behavior. We reach the surprising conclusion that the moment is consistent with these clusters having a bulklike magnetization at low temperature ( < 50 K). Furthermore, the Curie temperature was found to lie above room temperature. This result is significant if one considers the extremely small size of the clusters which have roughly half of their atoms on the surface, i.e., in contact with the polymer. This implies first that there is no significant interaction of Co with the matrix; second, that the small cluster dimensions encountered here do not greatly affect the magnetic moment. T'his is in accordance with earlier experiments where Co clusters of -15 A diameter were produced in a metallic (Cu) rather than an insulating matrix. I,2 We investigated the whole range of volume fractions from 0% to 100% Co and observe an electrical percolation threshold at -22 vol % Co. The onset of percolation is also reflected in the magnetic properties by the appearance of hysteresis in the magnetization curves at room temperature. FILM SYNTHESIS Since the polymer film formring process involves a somewhat unique plasma polymerization approach a brief review of this process in the context of producing uniform dispersions of granular metals in such polymeric matrices seems in order. Injec
Data Loading...
