Misfit-energy-increasing dislocations in vapor-deposited CoFe/NiFe multilayers

TLDR

Molecular dynamics simulations of NiFe/CoFe/Cu multilayers reveal dislocation structures that, unlike conventional misfit dislocations, increase mismatch strain energy rather than reduce it, mirroring experimental observations. The study reports simulations demonstrating edge dislocation formation at interfaces of vapor‑deposited (111) NiFe/CoFe/Cu multilayers. During (111) deposition, adatoms randomly occupy fcc or hcp sites, creating fcc/hcp domains whose boundary dislocations allow site shifts that produce missing atoms and planes in subsequent layers. These dislocations can generate tensile stress in fcc films, but their likelihood rapidly decreases under energetic deposition conditions.

Abstract

Recent molecular dynamics simulations of the growth of $[\{\mathrm{Ni}}_{0.8}{\mathrm{Fe}}_{0.2}/\mathrm{Au}]$ multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited (111) [NiFe/CoFe/Cu] multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the (111) surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.