Temperature dependence of unidirectional anisotropy effects in the Permalloy-FeMn systems

Abstract

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.