High coercivity in heterogeneous Co-rich CoAg very thin films

A study of the microstructural and magnetic properties of phase separated Go-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 degrees C for 30 minutes phase separation and grain growth occur and the coercivity is now thickness dependent with a maximum around 15 mn. 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 thin ner 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.