Bilayers of antiferromagnetic and ferromagnetic films

   The magnetic coupling at the interface between an antiferromagnetic and a ferromagnetic layer is  of highest fundamental and technological importance due to the encountered phenomena ("Exchange Bias"). Single crystalline bilayers of Co (ferromagnet) and FeMn (antiferromagnet) can be prepared by molecular beam epitaxy on Cu(001) in order to study the interaction at well-defined interfaces.

  The image shows the magnetic domains in an ultrathin Co layer after growth on an FeMn wedge. The FeMn thickness increases from bottom to top, as indicated at the left axis. Below 10 atomic layers (monolayers, ML) thickness, the FeMn layer is paramagnetic at room temperature, above 10 ML it is antiferromagnetic. A stunning change in the Co domains is observed at that thickness: While they are relatively large at FeMn thicknesses below 10 ML, on top of antiferromagnetic FeMn Co domains become very small.


   These small domains are metastable, as can be shown by heating the bilayer. At 450 K (FeMn is now paramagnetic below ≈ 17 ML) Co domains merge together to form larger domains. After cooling back to room temperature, the domain pattern remains the same, however, the magnetization direction is everywhere rotated by 45°, as indicated by arrows in some domains.

   A possible explanation for that behavior are topological domains at the surface of the antiferromagnetic FeMn layer, which are reproduced in the Co domain pattern during growth. If the direction of the in-plane component of antiferromagnetic spin axis changes by 90° at each monoatomic step, exchange averaging over adjacent terraces leads to the observed 45° change in the Co magnetization direction.


Publications:
Physical Review B 65, 140408(R) (2002)
Physical Review B 66, 064419 (2002)
Physical Review B 67, 094419 (2003)
Journal of Magnetism and Magnetic Materials 261, L1 (2003).
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