Two-dimensional (2D) magnet–superconductor hybrid
systems
are intensively studied due to their potential for the realization
of 2D topological superconductors with Majorana edge modes. It is
theoretically predicted that this quantum state is ubiquitous in spin–orbit-coupled
ferromagnetic or skyrmionic 2D spin–lattices in proximity to
an
s
-wave superconductor. However, recent examples
suggest that the requirements for topological superconductivity are
complicated by the multiorbital nature of the magnetic components
and disorder effects. Here, we investigate Fe monolayer islands grown
on a surface of the
s
-wave superconductor with the
largest gap of all elemental superconductors, Nb, with respect to
magnetism and superconductivity using spin-resolved scanning tunneling
spectroscopy. We find three types of islands which differ by their
reconstruction inducing disorder, the magnetism and the subgap electronic
states. All three types are ferromagnetic with different coercive
fields, indicating diverse exchange and anisotropy energies. On all
three islands, there is finite spectral weight throughout the substrate’s
energy gap at the expense of the coherence peak intensity, indicating
the formation of Shiba bands overlapping with the Fermi energy. A
strong lateral variation of the spectral weight of the Shiba bands
signifies substantial disorder on the order of the substrate’s
pairing energy with a length scale of the period of the three different
reconstructions. There are neither signs of topological gaps within
these bands nor of any kind of edge modes. Our work illustrates that
a reconstructed growth mode of magnetic layers on superconducting
surfaces is detrimental for the formation of 2D topological superconductivity.