Exchange bias of NiFe/FeMn nanocaps

NiFe/FeMn bilayers are fabricated on two-dimensional closely packed arrays of monodisperse polystyrene spheres. Although the exchange field is inversely proportional to the NiFe layer thickness in both NiFe/FeMn nanocaps and continuous films, the former series have a smaller exchange field than the latter ones. The exchange field and the coercivity both have different evolution trends with the FeMn layer thickness for the two series of samples. More remarkably, the asymmetry of hysteresis loops is more pronounced for the NiFe/FeMn nanocaps. These phenomena are ascribed to the spatial distributions of the constituent layer thickness and the local normal direction along the curved surface.