Field-induced domain wall motion of amorphous [CoSiB/Pt]_N multilayers with perpendicular anisotropy

Amorphous CoSiB/Pt multilayer is a perpendicular magnetic anisotropy material to achieve high squareness, low coercivity, strong anisotropy, and smooth domain wall (DW) motion, because of the smoother interface compared with crystalline multilayers. For [CoSiB(6Å)/Pt (14Å)]_N multilayers with N=3, 6, and 9, we studied the field-induced DW dynamics. The effective anisotropy constant K₁^eff is 1.5×10 ⁶erg/cm³ for all the N values, and the linear increment of coercive field H_c with N gives constant exchange coupling J. By analyzing the field dependence of DW images at room temperature, a clear creep motion with the exponent μ=1/4 could be observed. Even though the pinning field H_dep slightly increases with N, the pinning potential energy U_c is constant (=35 k_BT) for all the N values. These results imply that the amorphous [CoSiB/Pt]_N multilayers are inherently homogeneous compared to crystalline multilayers. For N≤6, the pinning site density ρ_pin is less than 1000/μm², which is about 1 pinning site per the typical device junction size of 30×30 nm². Also, the exchange stiffness constant A_ex is obtained to be 0.48×10^-6erg/cm, and the domain wall width is expected to be smaller than 5.5nm. These results may be applicable for spin-transfer-torque magnetic random access memory and DW logic device applications.