The vertical streaks in the polarised neutron
reflectivity measured using Crisp show correlated
magnetic roughness from terraces, explaining the
magnetic ordering shown in the inset.
View full-size image
There has been intense interest in the exchange-bias phenomenon in recent years due to its importance in technological applications such as read heads, sensors, and magnetic random access memory.
Bias fields arise within magnetic multilayers from the interaction between ferromagnetic (FM) and antiferromagnetic (AF) components, but these fields are an order of magnitude smaller than expected using simple microscopic models. Until now it has proved impossible to determine the magnetic structures at the interfaces between the layers on the nanoscale using neutrons due to signal limitations. Using Fe/Mn multilayers we have been able to reveal the magnetic ordering at buried interfaces with atomic resolution in a simple system with uncompensated moments with all spins aligned in the AF Mn atomic layer next to the FM Fe. Our measurements show completely unexpected orthogonal magnetic structures, which may arise from frustration of the interfacial interaction at terraces (see figure). The results readily explain why the bias fields are so small, and show that the microscopic magnetic ordering assumed in a variety of exchange-biased systems may have to be revised.
SJ Lee, JP Goff (University of Liverpool), GJ McIntyre (ILL), RCC Ward (University of Oxford), S Langridge, T Charlton, R Dalgliesh (ISIS)
Research date: December 2007
SJ Lee et al., Phys. Rev. Lett. 99(3) (2007) 037204
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