Magnetic properties of thin hard/soft-stacked dot arrays with a large uniaxial magnetic anisotropy

Magnetic properties of hard/soft stacked dot arrays consisting of thin Co/Pt superlattice hard layers (3.6 nm in thickness, uniaxial magnetic anisotropy, Ku =1.3× 107 erg/ cm3) and Co soft layers, with dot diameters of 30-40 nm, were investigated as functions of Co soft layer thickness and the interfacial exchange coupling between the hard and soft layers. Pt was used as the control layer of the interfacial exchange coupling, and Co soft layers were sandwiched with Pt layers to induce surface anisotropy on the Co soft layers. The remanence coercivity, Hr, was 2.7 kOe for Co/Pt(3.6 nm)/Co(4 nm) stacked dot arrays and 3.2 kOe for Co/Pt(3.6 nm)/Pt(1.2 nm)/Co(3 nm) stacked dot arrays, and these values were less than half that of single hard layer dot arrays (7.1 kOe). Hr was nearly constant in the φ range from 0° to about 45° (φ is the applied field angle from the easy axis), and increased significantly as φ increased further, as theoretically predicted. Hard/soft dot arrays maintained a relatively large Ku due to the surface anisotropy of the Co soft layers. It was suggested that the ratio of magnetic energy to the thermal energy, Ku eff V/kT, for Co/Pt(3.6 nm)/Pt(1.2 nm)/Co(3 nm) hard/soft dot arrays was 1.5 times larger than that for Co/Pt(3.6 nm) single dot arrays because of the relatively large Ku.