Controlling the supermagnetic response of tetragonal α-FeSi ₂ nanoislands

We used solid phase (SPE) and reactive deposition (RDE) epitaxy to grow crystallographically identical – but morphologically different – islands. Tetragonal α-FeSi ₂ islands crystallized pseudomorphically in the α-FeSi ₂ (1 1 2)〈1 1 0〉||Si(1 1 1)〈1 1 0〉 and α-FeSi ₂ (1 1 0)〈1 1 1〉||Si(2 2 0)〈1 1 2〉 orientation relations, with flat (2 × 2)-reconstructed top facets. The SPE-grown islands self-ordered in the form of 1D chains decorating the vicinal Si(1 1 1) step bunch edges along a specific 11¯0 direction. The RDE-grown islands elongated along three equivalent 112¯ directions, and were typically shorter, narrower, and thinner than the SPE-grown ones. In both cases, vast majority of the islands evolved sufficiently close to one another to enable dipolar interactions. Analyzing response to applied magnetic field, we identified areas of the island perimeter rims as the source of uncompensated magnetic moments creating the superspins, lateral island shape anisotropy as the origin of magnetic anisotropy, and periodic 1D ordering of interacting SPE-islands as the origin of superferromagnetic order. In contrast, three-fold orientational domains of the smaller RDE islands showed behavior more consistent with a superspin glass state. The above comparison emphasizes importance of the number of atoms at the island perimeter rims, and the role of dipolar interactions between the islands, for the magnetic behavior of the system.