Nucleation field, hysteresis loop, coercivity mechanism and critical thickness in composite multilayers with perpendicular anisotropy

Analytical expression for the nucleation field has been derived for a hard/soft multilayer system with anisotropy perpendicular to the film plane, which depends on the soft thickness LsLs, the interface exchange coupling constant JiJi and the intrinsic material parameters. Both nucleation field and coercivity decrease as LsLs increases. For very small LsLs, the coercivity mechanism is pure nucleation and the hysteresis loops are square. As LsLs rises, the coercivity mechanism changes from nucleation to pinning gradually, where the hysteresis loops have to be calculated numerically. The critical thickness at which the mechanism varies has been discussed in detail on the basis of easy axis orientation and the interface exchange coupling constant.

► Hysteresis loops calculated with reduced exchange coupling at hard/soft interface.
► Analytical formula of the nucleation field derived.
► As soft layer gets thicker, coercivity mechanism changes from nucleation to pinning.
► Critical thickness determined analytically, very small.