Ferromagnetic layers in a multilayered stack that are separated by non-magnetic spacer layers interact through different coupling mechanisms such as oscillatory interlayer exchange coupling by confined electronic states in the spacer layer, and the so called Néel “orange peel” coupling by magnetostatic interaction emerging from conformal interface roughness.
In addition to these effects, we address new coupling mechanisms that become important if the specimen is either morphologically or magnetically non-uniform on the nanoscale. The former could be realized by patterning into small elements. Magnetic non-uniformity already arises if magnetic domains are present. Such mechanisms include the interaction between ferromagnetic layers by local stray fields.
A suitable method to address such local coupling phenomena is the element-resolved imaging of magnetic domains using photoelectron emission microscopy (PEEM) with X-ray magnetic circular dichroism (XMCD) providing magnetic contrast. The element-sensitivity of XMCD can be turned to different magnetic layers, even to buried layers. It is such possible to study the local interaction between ferromagnetic layers containing different elements.
Example: FeNi/Cu/Co microstructures
The magnetic domains of the two ferromagnetic layers in FeNi/Cu/Co microstructures are coupled parallel, via exchange interaction and orange-peel coupling for all measured Cu thicknesses.
For increasing Cu thickness, an antiparallel alignment of the magnetization direction inside the domain walls (DW) of the two magnetic layers becomes favorable due to stray fields: At the DWs the dipolar coupling competes with exchange and orange-peel coupling. Such a behaviour has been found in 180° and head-on walls.
In the figure this effect is also observed in 90° domain walls: The direction of magnetization of the FeNi (Permally, Py) layer across the domain wall rotates through a locally opposite orientation compared to that of the Co layer before it turnes back to parallel. A similar behaviour has been already observed in 180° and head-on domain walls.
Detailed statistics of the DW widths at different Cu thicknesses in the two magnetic layers like the one shown above will reveal more information about the details of the coupling.
Stray fields originating from magnetic ripple in the Co layer lead to strong irregularities in the magnetization of the FeNi layer for tCu > 2.0 nm.
Publications about this research:
J. Kurde, J. Miguel, D. Bayer, J. Sánchez-Barriga,
F. Kronast, M. Aeschlimann, H. A. Dürr, and W. Kuch
Magnetostatic coupling of 90° domain walls in Fe19Ni81/Cu/Co trilayers
New J. Phys. 13, 033015 (2011).