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College Park, Maryland      June 6 - 10 , 2004

M2-A1 (10:30 AM): Neutron scattering studies of exchange bias in Fe3O4/CoO epitaxial thin films (Invited)

Y. Ijiri (Department of Physics and Astronomy, Oberlin College, Oberlin, OH 44074)

In recent years, there has been much interest in understanding and manipulating two-component magnetic systems that exhibit the phenomenon of exchange biasing. First discovered in the 1950ís, the effect typically refers to the biasing or shift of the hysteresis loop of a ferromagnetic (F) material due to exchange interactions with an antiferromagnetic (AF) material. Despite extensive work and its technological importance in data storage, many questions remain concerning the microscopic mechanisms, and as a result, there is a significant need for experiments designed to characterize the nature of the interfacial spins (F and AF) in buried interfaces.

Neutron scattering work in this area has provided important insights into exchange bias. In particular, in this talk, I will describe our results in investigating the spin structures in a series of exchange-biased Fe3O4/CoO epitaxial thin films. Focusing on the AF CoO spins, we have observed new easy axes distinct from bulk behavior and in addition, an increase in the effective ordering temperature (~ TN) for decreasing CoO thickness. At the same time, we have also found an unusual perpendicular coupling of the AF and F spins in this system. The field and temperature dependencies of the spin structures exhibit strong correlations to the macroscopic biasing phenomenon. In particular, the perpendicular coupling arrangement of the AF and F spins freezes in at the blocking temperature (TB) below which exchange biasing is observed. These results are interpreted with a model for biasing that highlights the role of anisotropic exchange, as first proposed by Dzyaloshinsky and Moriya.

This work has been done in collaboration with J.A. Borchers and R.W. Erwin (NIST), P.J. van der Zaag (Philips), and T.C. Schulthess (Oak Ridge National Laboratory).

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