Exchange bias in Co/MnPt polycrystalline films on Si(100)/SiO₂ substrates with Ta underlayer

Structure and magnetic properties of sputter-prepared Co(5 nm)/MnPt bilayer films with Ta underlayer and various thicknesses of MnPt layer (t_MnPt) have been studied. Ta underlayer is effective in promoting the ordering of MnPt layer, flattening the surface and interface, and therefore leading to the improvement of the exchange bias field (H_E). Nevertheless, higher annealing temperature leads to the interdiffusion and decrease H_E. Besides, low H_E of 8 kA/m with higher coercivity for the sample with thinner t_MnPt = 10 nm might be related lower magnetic anisotropy energy for MnPt layer due to too thin antiferromagnetic layer, and therefore, H_E is increased to 13.8–20.6 kA/m with increasing t_MnPt to 30–100 nm. Larger grain size distribution and roughened interface found for the sample with thicker t_MnPt = 100 nm may result in reduced H_E. This study provides useful information to fabricate exchange-bias Co/L1₀-MnPt system.