Stephanie Moyerman

Harvey Mudd College Mathematics 2006

Thesis Proposal: A Mathematical Model of Exchange Bias
Thesis Advisor: Prof. James C. Eckert
Second Reader: Prof. Andrew J. Bernoff

Giant Magnetoresistance and Polarized Neutron Reflectometry Studies of Spin Valves with Pico-scale Antiferromagnetic Layers

The phenomenon of exchange bias, caused by exchange coupling between an antiferromagnet and a ferromagnet across their common interface, results in a shifted ferromagnetic hysteresis loop and an increased coercivity of the system. Spin valves are magnetic devices that exploit exchange bias, and are currently used as computer read heads. To date, the patent on spin valves has earned over 20 billion dollars, making this an incredibly profitable industry. However, much of the physics governing exchange bias is still poorly understood, making optimization of such devices incredibly difficult.

Current theories of exchange bias have successfully addressed several issues: How does interfacial coupling occur? How do these systems serve to increase coercivity? What is the origin of temperature dependence? These questions are all answered by Mark Stiles' current model of exchange bias. More information can be found here:

An Introduction to Exchange Bias written by Mark Stiles

This model assumes an infinitely thick antiferromagnetic layer, although exchange bias has been shown to occur in systems with atomic monolayer thicknesses. Further, this model addresses the origin of temperature dependence, but fails to investigate how temperature affects performance. My thesis builds upon this existing model to incorporate both of these issues.

Finally, subsequent field cycles performed on an exchange biased system are marked by a decreasing shift in the hysteresis loop and weakened pinning at the interface. This is known as the training effect. This effect becomes incredibly important for spin valves, as training the device will irreparably damage its performance. My thesis will also include a theoretical investigation of the training effect.

I have been fortunate to conduct research at both Harvey Mudd and the National Institute of Standards and Technology, where Giant Magnetoresistance and Polarized Neutron Reflectometry measurements of spin valves have been performed. These experimental data sets will be compared with the mathematical results of the model, in the hopes that theory and experimentation will pair successfully.

Resultant Publications

  1. Magnetic Structure Variations During GMR Training in Spin Valves with Pico-Scale Antiferromagnetic Layers, S. Moyerman, J. Eckert, J. Borchers, K. Perdue, M. Doucet, P. Sparks, M. Carey, accepted for publication in J. Appl. Phys., 2006
  2. Determining the Spin Dependent Mean Free Path in Co90Fe10 Using Giant Magnetoresistance, K.F.Shakespear, K.L.Purdue, M.J.Carey, S.M.Moyerman, J.G.Checkelsky, S.S.Harberger, A.C.Tamboli, P.D.Sparks, and J.C.Eckert, J. Appl. Phys. 97, 10C513(2005)