Robust exchange coupling in bilayer exchange-spring thin films

Exchange coupling across the interface in polycrystalline exchange-spring thin films of L10 CoPt and hcp Co has been investigated with magnetic measurements and micromagnetic modeling. For thick enough Co films, the reversal in the second quadrant of the hysteresis loop is multiphase, with full reversal of the soft Co phase accomplished before reversal of the hard CoPt phase. Therefore the exchange field acting on the soft phase is determined as the field of maximum susceptibility in the soft-phase reversal portion of the second quadrant. Experimentally, it is found that the exchange field is a linear function of the inverse thickness of the soft phase. This result is in agreement with the exchange field calculated from the associated micromagnetic modeling only under the condition that the magnitude of the exchange coupling constant between spins across the CoPt-Co interface is similar to that characterizing the bulk CoPt. Experimental and modeling-derived results do not agree for decreased interfacial exchange coupling. This result is surprising, because it was expected that the condition of the interface between the exchange-coupled film layers would have a dominant effect on the magnetic reversal of the system; the presence of expected atomic disorder or chemical impurity at the interface should severely degrade the exchange coupling.