Reduced exchange coupling and hysteresis loops in two-phased magnetic nanosystem

The reduced exchange coupling has been incorporated in our micromagnetic calculations for the hysteresis loops of Nd2Fe14B/α-Fe and Sm40Fe60/Ni80Fe20 multilayers. Analysis shows that nucleation and pinning fields are sensitive to the value of the interface coupling constant Ji when the soft layer thickness Ls is small. Hysteresis loops have been obtained for a trilayer system with a soft α-Fe sandwiched between two Nd2Fe14B layers for different values of Ls and Ji. As Ji decreases, nucleation field decreases while the pinning field increases. In the meantime, the squareness of the loops is deteriorated, which results in smaller energy product. For thick soft layer the coercivity mechanism transforms from pinning to nucleation as the interface coupling decreases, and vice versa. The above calculations have been extended to a Sm40Fe60/Ni80Fe20 bilayer and compared with available experimental data. The theoretical loop is consistent with the experimental one when the value of Ji is taken as 10% of the bulk one, demonstrating that the interface coupling in the experiment is far away from perfect coupling.

Research highlights
► Hysteresis loops are calculated for hard/soft multilayers using micromagnetics.
► Weak and strong exchange couplings at the interface are adopted and compared.
► The analytical formula for the nucleation field is derived.
► The nucleation field falls and pinning field increases as interface coupling reduces.
► The Coercivity mechanism is determined by soft layer thickness and interface coupling.