Ferromagnetic Layer Thickness Dependence of Magnetic Domain Structure in Ultrathin Symmetric Ta/Pt/Co/Pt/Ta Multilayers

To minimize the total film thickness and to make symmetric layer structures, the ultrathin symmetric Ta(1 nm)/Pt(1.7 nm)/ Co( $t$ )/Pt(1.7 nm)/Ta(1 nm) multilayers, where the Co thickness ( $t$ ) was varied from 0.3 to 1.4 nm, were fabricated on Si substrate with a native oxide layer. The perpendicular magnetic anisotropy (PMA) was observed in the range of $t = 0.3-0.9$ nm. The coercive field of ~16 mT for $t =0.7$ nm and the domain wall velocity of ~ $100 \mu \text{m}$ /s for $t \le0.6$ nm were comparable with those in previous reports. Surprisingly, as the Co thickness approached 0.9 nm, the domain wall velocity dropped dramatically to ~ $0.1 \mu \text{m}$ /s, and the magnetic domain structure shifted from big-circular domains to small-bubble domains. A change from PMA to easy-plane magnetic anisotropy around $t =0.95$ nm could be explained using the classic Kittel’s model. Because of their good PMA, such ultrathin symmetric multilayers could be used in various spintronic applications.