We have systematically investigated exchange coupled films of sputter deposited Ni80Fe20/FeMn and FeMn/Ni80Fe20 and obtained films with high exchange bias and low coercive forces. The variation of film properties with deposition conditions as well as with the permalloy and FeMn thicknesses have also been studied. The results demonstrated a strong dependence of exchange bias effect (HUA) on the amount of γ-phase FeMn, the amount of impurities, as well as the abruptness of the transition profile at the interface. In addition, our results have also raised a number of new questions concerning the magnetic state of the interface and the origin of the interface coupling phenomena.

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We have systematically studied the temperature dependence of the exchange-induced unidirectional anisotropy field (HUA) for the Permalloy-FeMn system from room temperature up to ∼220 °C. Results show that HUA(T) decreases monotonically with T, vanishing around some critical temperature (Tc) which varies with sample configurations, film thicknesses, and deposition conditions, and in the best case is close to the Néel temperature of the bulk FeMn. The Permalloy/FeMn and the FeMn/Permalloy systems differ in their HUA(T) behavior, indicating a basic difference in the interface structures. Our results suggest a model where the interface comprises an ensemble of exchange paths, each with its own ordering temperature (To). The net HUA may then be regarded as an averaged behavior of all these To components. Using a special cooling procedure, we are able to study the distribution and behavior of these To components, with results showing that the FeMn/Permalloy system does exhibit a narrower and higher-valued distribution of To components. This result, together with the HUA(T) and Auger profile results, suggest that the FeMn/Permalloy system has a fundamentally sharper and more ideal interface than the Permalloy/FeMn system.

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