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 %) ∼62% independent of film thickness. Iron grains in as-deposited films are well defined, nearly equiaxial and ∼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, xp, as indicated by the maximum in the Hc vs Fe vol % curve, changed after the films were annealed. The percolation threshold (xp) of films annealed at 420 °C for 30 min shifted to ∼47 Fe vol % except for the 5 nm films, whose xp remained unchanged. After annealing at 510 °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 xp in both 420 and 510 °C annealed films can be explained by the effects of reduced dimensions in very thin films.