Density-functional theory calculation of electronic and magnetic properties of hexagonalV∕Ruthin films and superlattices

Abstract

Stimulated by recent experimental evidence for the formation of ultrathin close-packed films of vanadium (V) and also indication of ferromagnetic order in epitaxial $\mathrm{V}∕\mathrm{Ru}$ multilayers, we have performed first-principles electronic structure and total energy calculations to study the stability and magnetism of the metastable hcp and fcc structures of V and also the effects of the surface and interface on these properties. The systems include ultrathin films of a few monolayers of V grown on hcp (0001) ruthenium (Ru) substrates and $\mathrm{V}∕\mathrm{Ru}$ superlattices as well as bulk bcc, fcc, and hcp structures. Our investigations are based on density-functional theory with local density approximation plus generalized gradient corrections. Both the frozen-core projector augmented-wave approach and the all-electron full-potential linearized augmented-plane-wave method are utilized. First, our calculations show that all three bulk structures are nonmagnetic at their respective minimal energy lattice constants. However, all the three structures transit first to a low-spin ferromagnetic phase and then to a high-spin phase as the lattices expand. Second, we find that thin films on hcp (0001) Ru with one monolayer of V in either hcp or fcc stacking sequence have a magnetic moment of the order of $1{\ensuremath{\mu}}_{B}$. A thin film of three monolayers of V in fcc stacking sequence is weakly ferromagnetic, though the other thin films with more than one monolayer of V are essentially nonmagnetic. A free-standing film of three V monolayers in hcp stacking sequence is also ferromagnetic with sizable magnetic moments. We also find that the ${\mathrm{V}}_{2}(\mathrm{hcp})∕{\mathrm{Ru}}_{6}(\mathrm{hcp})$ and ${\mathrm{V}}_{3}(\mathrm{hcp})∕{\mathrm{Ru}}_{5}(\mathrm{hcp})$ superlattices exhibit ferromagnetism with a small total magnetic moment of a few tenths of ${\ensuremath{\mu}}_{B}$. Lattice relaxation has the trend of decreasing the magnitude of the magnetic moments. All the other ${\mathrm{V}}_{n}∕{\mathrm{Ru}}_{m}(\mathrm{hcp})$ superlattices ($n=1,4,5$ and $m=5,6$ as well as $n=3$ with fcc or bcc stacking sequence) are essentially nonmagnetic. Finally, our calculations show that the stacking sequence has significant effects on the formation of stable atomic magnetic moments of V and is also important for the energetic stability of these systems.