The effect of the magnetic properties of the lower layer, of the antiferromagnetically coupled (AFC) media on thermal stability and the signal-to-noise ratio (S/sub 1F//N/sub d2F/) was studied using a Monte-Carlo simulation and the Langevin equation. Thermal stability was the highest when the largest M/sub s2/ value was used within the limits of achieving antiferromagnetic coupling between the magnetic layers, and when the value K/sub u2/ was less than the value of the upper layer. S/sub 1F//N/sub d2F/ increased linearly with a decrease in the anisotropy field H/sub k2/ at the rate of 0.5 dB per 100 kA/m.