Self-organized exchange-spring magnet in epitaxial β-Fe(Ni)Si₂/Si system

Self-organized formation of exchange-spring magnets in Permalloy-derived epitaxial silicide nanostructures, fabricated by deposition of Ni₈₀Fe₂₀ onto a vicinal Si(1 1 1) substrate, is reported. The crystal structure of bar-shaped silicide nanostructures decorating Si(1 1 1) surface steps, following thermally activated reaction between Ni, Fe, and Si surface atoms, was identified as ternary β-Fe(Ni)Si₂ phase, in well-defined (1¯11¯) Si || (2¯2¯0) β and [0 1 1] Si || [0 0 1] β orientation relations with the substrate. At the same time, chemical composition within the islands was Fe-rich and Fe-deficient in comparison with the original stoichiometric Permalloy (Ni₈₀Fe₂₀) and not uniform, with higher concentration of Ni at the island bottoms, close to the interface with Si(1 1 1), creating de facto “compositional interfaces” within the islands, though no physical interfaces could be detected in a high-resolution structural-crystallographic analysis by transmission electron microscopy. Analysis of temperature-dependent magnetization reversal loops revealed, that the thicker and magnetically soft top part of the islands and yet magnetically softer and thinner bottom part, were magnetically exchange-coupled via the above “virtual” interfaces. Micromagnetic simulations, consistent with exchange-spring magnet, had further corroborated this conclusion.