Magnons in ultrathin Fe/Co multi-layers on W(110) and Ir(111)

Probing the fundamental magnetic interactions in layered magnetic structures is
essential to the understanding of the properties of these systems. Such physical
quantities can be experimentally measured by probing the magnon dispersion
relation over the whole Brillouin zone. On this regard, high wavevector magnons
in ultrathin Fe and Co multilayers grown on W(110) and Ir(111) are studied using
spin polarized high resolution electron energy loss spectroscopy.
In layers of Fe and Co films deposited on W(110) the magnons are probed
along the [001] direction in the wavevector range of 0.25 inverse angstrom to 1.3 inverse angstrom.
Experimental results reveal two magnon modes which exhibit a clear dispersion as expected. The
results are described within the Heisenberg model. In the case of 1ML Co/1ML
Fe/W(110) structure, the interface Fe intralayer exchange coupling constants are
found to be enhanced when compared to 1ML Fe/W(110). The interlayer exchange
coupling was found to be weak (4.5 meV) relative to the 2ML Fe deposited on the
same substrate (7.6 meV). The antisymmteric Dzyaloshinskii-Moriya interaction
(DMI) was observed to be enhanced when compared to the 2ML Fe/W(110) system.
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Fe films with the thickness of 2ML and 3ML showed very soft magnons when they
were grown on Ir(111). This indicates rather small exchange coupling constants in
these systems. For the case of 2ML Co/1ML Fe/Ir(111) two magnon modes were
observed. Comparing the experimental results with those of ab initio calculations
it is observed that the exchange interaction in the Fe layer at the interface is very
weak, with a large antiferromagnetic contribution.
The results provide a deeper understanding of the magnetic interactions in layered
structures and may help to advance the understanding of the role of symmetric and
antisymmetric exchange interaction in low dimensional magnetism.