High coercivity in heterogeneous Co-rich CoAg very thin films

A study of the microstructural and magnetic properties of phase separated Co-rich CoAg very thin films is presented. In the as-deposited state these films form in a very fine grained metastable alloy and are magnetically soft (H_c<20 Oe) in the thickness (5-50 nm) and Co composition range investigated (Co volume percent >65%). After annealing at 420°C for 30 minutes phase separation and grain growth occur and the coercivity is now thickness dependent with a maximum around 15 nm. The maximum coercivity as a function of composition is expected to occur around a volume concentration of 50% for bulk or thick film granular materials. However, for films thinner than 20 nm we have found a maximum H_c>800 Oe at a Co concentration of 70 vol%. This shift in the percolation threshold may originate in the reduced dimensionality of very thin films. Magnetic force microscopy images show a transition in the magnetic domain structure as a function of film thickness that is consistent with this description and the results obtained by transmission electron microscopy