Shift in the magnetic percolation threshold of phase separated Co-rich CoAg very thin films due to reduced dimensionality

An investigation of the structural and magnetic properties of phase separated Co-rich CoAg very thin films (5 nm-50 nm) as a function of film thickness and Co concentration is presented. In the as-deposited state the films are very fine grained and the coercive field is relatively low (H-c <20 Oe). However, after annealing at 420 degrees C for half an hour in high vacuum, grain growth is promoted and a drastic change in the magnetic properties is observed. The coercivity now has a strong thickness dependence with a maximum at approximately 15 nm. The largest room temperature value of coercivity, H-c=850 Oe, was found for a CoAg 70:30 volume percent alloy. The concentration of maximum coercivity is usually associated with the percolation threshold, x(p), of the ferromagnetic element and is much higher than the similar to 50 vol % value normally observed in thick film and bulk granular ferromagnets. Values of x(p) less than or equal to 0.65 were found for 30 nm thick films increasing up to x(p) similar to 0.75 for 10 nm films. This behavior is explained as a shift in the percolation limit to higher values associated with the reduced dimensionality of very thin films. This shift implies that the volume fraction of the ferromagnetic component should be larger than its bulk counterpart if granular ferromagnets are to be used as thin film media for magnetic recording. The temperature dependence of H-c was also studied and coercivities exceeding 1800 Oe have been found at 50 K. The presence of a thin antiferromagnetic oxide layer is manifested as a shift in the low temperature hysteresis loops due to the anisotropic exchange coupling interaction with ferromagnetic Co. (C) 1998 American Institute of Physics.