Confined Magnons
2021
Magnetic structures are known to possess magnon excitations confined to their
surfaces and interfaces, but these spatially localized modes are often not
resolved in spectroscopy experiments. We develop a theory to calculate the
confined magnon spectra and its associated spin scattering function, which is
the physical observable in spectroscopy based on neutron and electron
scattering, and a proxy for Raman and THz optical experiments. We show that
extra anisotropy at the surface or interface plays a key role in magnon
confinement. We obtain analytical expressions for the confinement length scale,
and show that it is qualitatively similar for ferromagnets and antiferromagnets
in dimension d>= 2. For d=1 we find remarkable differences between
ferromagnetic and antiferromagnetic models. The theory indicates the presence
of several confined magnon resonances in addition to the usual magnons thought
to explain the excitations of magnetic nanostructures. Detecting these modes
may elucidate the impact of the interface on spin anisotropy and magnetic
order.
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