Exchange bias on rippled substrates

The interface between ferromagnet (FM) and antiferromagnet (AFM) plays a crucial role for the exchange bias phenomenon. This is mainly because monoatomic steps, islands or point defects give rise to uncompensated spins at the interface which serve as the origin of exchange bias. The number of monoatomic steps and thus the magnitude of the induced uniaxial anisotropy can easily be varied by means of Ar+ ion erosion. Depending on the primary energy and the angle of incidence a rippled surface with a well defined periodicity (20 – 100 nm) and peak to valley height (2 – 5 nm) can be created. On top of this surface the exchange bias system 5 nm Ni81Fe19 coupled to 10 nm Fe50Mn50 has been prepared. The interface corrugation remained throughout the whole layer stack. By means of a field annealing cycle the exchange bias direction has been initialized either along or perpendicular to the ripple direction, which causes to align the uniaxial and unidirectional anisotropy contributions either parallel or perpendicular to each other. For both cases the magnetic easy- and hard-axes remain the same because the uniaxial anisotropy contribution is much larger than the unidirectional one. However, if the exchange bias is initialized along (perpendicular to) the ripple direction the easy-axis (hard-axis) magnetization reversal loop is shifted. In order to evaluate this behavior in more detail a complete angular dependency of the magnetization reversal behavior has been investigated in both cases and compared to simulations based on a Stoner-Wohlfarth coherent rotation model. A good agreement between experimental data and simulations is found.
M. O. L. acknowledges the financial support from the European Communities Human Potential Program NEXBIAS under Contract No. HPRN-CT2002-00296.