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We have performed a comprehensive study of Exchange Bias in Antiferromagnetic (AF) Fluoride/Metallic Ferromagnets (F) bilayers and nanostructures. To arrive at a comprehensive understanding of the phenomenon we have performed global (magnetization, Kerr effect, magnetotransport, ferromagnetic resonance), and local (magnetic circular dichroism, polarized neutron diffraction) magnetic measurements and combined these with detailed quantitative structural (X-ray and neutron diffraction) and growth studies. These studies where complemented by micromagnetic and Monte Carlo calculations to understand the role of various parameters in exchange bias and to clarify the importance of the various possible mechanisms.
We discovered many unexpected surprises, such as large exchange bias in fully compensated surfaces, positive exchange bias, and reversal asymmetries. The overall picture that is emerging is that many phenomena are operational simultaneously even in the simplest exchange bias systems such as the one studied here. Domain walls and uncompensated spins in both the F and the AF, uncompensated spins and anisotropy in the AF, the interfacial coupling, inhomogeneities and roughness at the interface, and the detailed crystal structure of the constituents all play a major role. The overall picture that emerges is that pinned uncompensated spins in the bulk AF, coupled to pinned uncompensated spins at the interface, provide the unidirectional anisotropy needed for exchange bias. This together with interfacial inhomogeneities, interfacial coupling and the various anisotropies can explain the large variety of apparently disconnected phenomena. The origin and exact nature of the uncompensated spins remains a major unsolved issue.
I will describe very recent attempts at beating the superparamagnetic limit and modification of the magnetism in magnetic nanostructures.
Work supported by DOE and AFOSR
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