Spontaneous creation of exchange bias in epitaxially self-assembled nanostructures driven by inhomogeneous structural transformation

Ternary silicide nanostructures were grown by solid phase epitaxy of Ni₈₀Fe₂₀ Permalloy on a vicinal Si(0 0 1) surface, with the aim of increasing magnetic anisotropy and stability in assemblies of epitaxial islands. The first nanostructures to form were ternary Fe(Ni)Si₂ disilicide islands crystallized in a CaF₂-prototype structure, with well-defined flat-top and ridge geometry above the substrate surface and pyramidally faceted interfaces below the surface. Under higher temperature and Permalloy coverage conditions, most of the ridge islands had their shape transformed into the more energetically favorable flat-tops. This shape transition, was driven by vertical interdiffusion and structural phase transformation at the top part of the nanostructures above the surface: the Fe-rich top parts transformed from the CaF₂-based γ-disilicide to a CsCl-based c-monosilicide, while the Ni-rich sub-surface part of the islands retained its original CaF₂-γ structure. The γ-phase is known to exhibit ferromagnetic order, whereas the c-phase has been identified as antiferromagnetic. As a result of such self-formed coupling between the antiferromagnetic top and ferromagnetic bottom part of the islands, an exchange bias was created, evident from magnetometry.