Ferromagnetic Heusler alloys can be used in combination with semiconductors to create spintronic devices. The materials have cubic crystal structures, making it possible to grow lattice-matched heterojunctions by molecular beam epitaxy. However, the development of devices is limited by the difficulty of growing epitaxial semiconductors over metallic surfaces while preventing chemical reactions, a requirement to obtain abrupt interfaces and achieve efficient spin-injection by tunneling. We used a solid-phase epitaxy approach to grow crystalline thin film stacks on GaAs(001) substrates, while preventing interfacial reactions. The crystallized Ge layer forms superlattice regions, which are caused by the migration of Fe and Si atoms into the film. X-ray diffraction and transmission electron microscopy indicate that the trilayers are fully crystalline, lattice-matched, and have ideal interface quality over extended areas.

We report a systematic lattice dynamics study of the technologically important Fe 3 Si/GaAs heterostructure for Fe 3 Si layer thicknesses of 3, 6, 8, and 36 monolayers. The Fe-partial phonon density of states obtained by nuclear inelastic scattering exhibits up to a twofold enhancement of the low-energy phonon states compared to the bulk material for layer thicknesses of 8 monolayers and below. First-principles calculations explain the observed effect by interface-specific phonon states originating from the significantly reduced atomic force constants and allow for achieving a comprehensive understanding of the lattice dynamics of epitaxial strain-free interfaces.

Reducing the material sizes to the nanometer length scale leads to drastic modifications of the propagating lattice excitations (phonons) and their interactions with electrons and magnons.