Thickness dependence of the magnetic percolation threshold in as-deposited and annealed Fe-SiO2 granular thin films

The magnetic properties and microstructure of as-deposited and annealed Fe-SiO2 granular thin films were studied. As-deposited films have a maximum in coercivity at an Fe volume fraction (Fe vol %) similar to 62% independent of film thickness. Iron grains in as-deposited films are well defined, nearly equiaxial and similar to 5 nm in diameter. From 66 to 90 Fe vol %, some as-deposited films showed an unusual well defined in-plane uniaxial anisotropy. The magnetic percolation threshold, x(p), as indicated by the maximum in the H-c vs Fe vol % curve, changed after the films were annealed. The percolation threshold (x(p)) of films annealed at 420 degrees C for 30 min shifted to similar to 47 Fe vol % except for the 5 nm films, whose x(p) remained unchanged. After annealing at 510 degrees C for 3 h, a strong thickness dependence of the percolation threshold was revealed in films thinner than 40 nm, with values ranging from 78 Fe vol % to less than 44 Fe vol %. The shifts of x(p) in both 420 and 510 degrees C annealed films can be explained by the effects of reduced dimensions in very thin films. (C) 1998 American Institute of Physics. [S0021-8979(98)01322-X].