Magnetization reversal via perpendicular exchange spring in FePt/FeRh bilayer films

We present a theoretical analysis of the magnetization reversal process in a bi-layer structure with hard and soft (with metamagnetic transition) magnetic layers on an example of an FePt/FeRh bi-layer. The latter leads to the formation of a new type of exchange spring which results in a significant reduction of the switching field in the temperature range of the metamagnetic (from antiferromagnetic to ferromagnetic state) transition in an FeRh layer. Analytic expressions for nucleation and switching fields are presented along with results of numerical micromagnetic simulations. The reduction of the switching field due to the metamagnetic transition is controlled by the following microscopic parameters: (i) the interfacial exchange coupling parameter J(12); (ii) saturation magnetization of the FeRh layer in a ferromagnetic phase; (iii) the metamagnetic transition temperature. The switching field dependence on the J(12) parameter is shown to saturate quickly as it approaches the bulk exchange interaction value which has been evaluated using first-principles method used also to verify the electronic nature of the metamagnetic transition. Theoretical results are discussed in the context of recent experimental observations.