Interparticle Interaction Effects in Nonmiscible CoAg Thin Films With High Co Concentration

Abstract

AbstractA series of Co-rich CoAg heterogeneous thin films, with Co concentrations greater than 65 % volume fraction and film thickness in the range 5 nm - 50 nm, was prepared by dc magnetron sputtering. Annealing in high vacuum (P < 3x10-7 Torr) above 400°C was necessary to promote phase separation between the two elements. Maximum room temperature coercivities, Hc ∼ 900 Oe, were obtained at a thickness of 20 nm for CoAg 70:30 vol % films. These coercivities are almost two orders of magnitude larger than the values observed in as-deposited films. The concentration of maximum coercivity, that is usually associated to the threshold for magnetic percolation, is much higher than the 40-50 vol % commonly observed in bulk granular materials. The shift in the magnetic percolation limit was related to the reduced dimensionality of the very thin films. Coercivity was found to vary both with film composition and with thickness. Delta M curves and time decay (magnetic viscosity) measurements were done to determine the interparticle interactions and the activation volumes. A maximum in the remanent coercivity (coincident with the maximum in Hc) was found for a film thickness φ ∼ 15-20 nm in all the alloys. At this same thickness a negative minimum in the interparticle interaction parameter α (indicating that the particle interactions favor the demagnetized state) was observed. The absolute value of a increases for larger Co concentrations. The irreversible susceptibility is an increasing function of film thickness and the magnetic viscosity has a minimum for φ ∼ 15-20 nm. The activation volume also increases with thickness, changing from (15 nm)3 to (40 nm)3 for the more diluted alloys, values similar to the average particle size determined by transmission electron microscopy.