Exchange bias: a microscopic understanding of exchange anisotropy in Fe/MnF2 bilayers

A microscopic understanding of the Ferromagnetic/Antiferromagnetic (F/AF) direct exchange coupling
or exchange biasing has been elusive for the over 45 years since its was discovered. In part, the almost
exclusive use of hysteresis loops to study the phenomenon has limited our understanding. A new experimental
technique was developed to study the exchange coupling between a ferromagnet and antiferromagnet [Appl.
Phys. Lett. 69,3932-3931 (1996)]. This new technique has enjoyed considerable success in explaining many
general exchange bias features using Co/CoO as a model system [J. Appl. Phys. 87, 6418-20 (2000), J. Appl.
Phys. 89, 7543-5, (2001), and Phys. Rev. B 65(RC) 180406-10(2002)].
We will review the models of exchange bias and then apply them to our work on Fe/MnF2 bilayers. In
this work we used variations of the AMR technique to study the angular dependence of the exchange anisotropy
in Fe/MnF2 bilayers. We were able to phenomenologically describe the anisotropy energy as a combination of
unidirectional, uniaxial, threefold and fourfold components[Phys. Rev. B 65(RC), 100402, (2002)] and thus
explain the anomalous magnetization reversal asymmetry discovered in this system [Phys. Rev. Lett. 84, 3986
(2000)]. Since our initial effort, we have been able to explain the phenomenological energy terms using a
microscopic model which sums over all lattice sites in both the ferromagnet and antiferromagnet [Phys. Rev. B
68, 054430 (2003)]. The microscopic model includes terms for the interfacial exchange coupling,
uncompensated spin density in the AF, the AF spin-canting energy, and domain walls in both the F and AF.
Application of the model to the Fe/MnF2 bilayer experimental data allows one to separately determine the
fraction of uncompensated interfacial spins in the AF layer and the interfacial exchange coupling energy for the
first time. An understanding of the spatial distribution of the microscopic energies allows for a simplification of
the energy in which the physics is transparent.
This work supported by the University of Minnesota MRSEC

E. Dan Dahlberg received the B.S. and M.S. in physics from the University of Texas at Arlington in 1970 and 1972, respectively, and the Ph.D. from University of California, Los Angeles, in 1978. He joined the faculty at the University of Minnesota in 1980 and is currently a professor of physics and an Institute of Technology Distinguished Professor. He is the director and principal investigator of the Magnetic Microscopy Center (MMC) at the university. His recent research is on exchange bias, the physics of magnetic tunnel junctions, and noise in magnetic devices. Some of his previous research includes magnetotransport and other magnetic properties of thin films and multilayers, the dynamics of spin glasses, nonequilibrium superconductivity, the thermodynamics of magnetic superconductors, and the quantum Hall effect.

Prof. Dahlberg is a member of Sigma Pi Sigma (1971) and a Fellow of the American Association for the Advancement of Science (1995) and the American Physical Society (1996). He was an Alfred P. Sloan Foundation Research Fellow (1981-1985). He served as general chair of the 2001 Joint Magnetism and Magnetic Materials (MMM)-Intermag Conference and as an editor of several MMM and MMM-Intermag conferences. He was divisional councilor to the Division of Condensed Matter Physics of the American Physical Society (1999-2002), a member of the Executive Board of American Physical Society (2001-2002), and Vice President of the International Union of Pure and Applied Physics (IUPAP) (2006-2009).