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College Park, Maryland      June 6 - 10 , 2004

M2-A2 (11:00): Magnetic coupling across antiferromagnetic layer in exchange-bias Co/FeMn/Py trilayer

W.-T. Lee (Spallation Neutron Source, Oak Ridge National Laboratory), G.P. Felcher (Materials Science Division, Argonne National Laboratory), F.Y. Yang (Department of Physics, Ohio State University), F. Zhu, C.L. Chien (Department of Physics & Astronomy, Johns Hopkins University)

Polarized neutron reflectometry (PNR) and magnetization measurements were used to study an unusual magnetic behavior found in trilayers of Co/FeMn/Py, where the soft ferromagnet permalloy and the hard magnet Co were separated by an antiferromagnet (AF) FeMn layer of thickness t. Each sample was studied for the effects of two different field-cooling procedures: The trilayer was initially subjected to a magnetic history at 425K, which is above the FeMn Neel temperature TN, such that the Co and Py magnetization were either (1) parallel or (2) opposite to each other in the field-cooling field. After field-cooled the sample below TN to room temperature, the measurement field was applied along the field-cooling axis. We found the hysteresis loop for a film depended on its field-cooling history. In films with t > 90 Ĺ, PNR studies showed that the Py and Co were magnetized collinearly to H as two independent films. The exchange-bias induced hysteresis loop shifts of the Py and Co can be individually identified. In contrast, in films with t < 90 Ĺ, the Py and Co magnetic vectors formed an angle that depended on both t and H. Such angular dependence was found regardless of the field-cooling history of the trilayer. We therefore concluded that the angular dependence arose not as a result of the field-cooling procedure, or a “freezing in” of the magnetic configuration below TN, but rather from a magnetic coupling of the Py and Co.

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