Pt Content Dependence of Magnetic Properties of CoPt/Ru Patterned Films

The magnetic properties of dot arrays made of CoPt/Ru perpendicular films (20 nm thickness) were examined as a function of Pt content. The CoPt dot arrays with a dot size D of 140 nm showed a single domain state, after removal of the applied field equal to H_r. H_r decreased from 5.2 kOe to 3.0 kOe as the Pt content decreased from 20 at% to 14 at%. The angular dependence of H_r for these dot arrays indicated coherent rotation of the magnetization during nucleation. The effective magnetic anisotropy, including the demagnetizing energy due to the dot shape, K_u ^eff, decreased as the Pt content decreased, resulting in the H_r reduction. The values of the switching volume for nucleation, V_sw , evaluated from the stabilizing energy barrier E₀, were a few percent of the dot volume. The switching diameter for nucleation, D_sw, increased slightly as the Pt content decreased, which was qualitatively in good agreement with the increase in the exchange length of magnetization. The value of E₀/k _BT (k_B is the Boltzmann constant and T is the absolute temperature) reduced as the Pt content decreased; however, E ₀/k_BT still had a high of 440 even at 14 at% Pt content. We successfully demonstrated the reduction of H_r for CoPt/Ru patterned films on reducing the Pt content, while simultaneously maintaining a high thermal stability. A calculation based on the experimental results suggested the potential recording density of CoPt/Ru dot arrays used for patterned media to be over 1 Tb/in²