Ab initio investigation of FeAs/GaAs heterostructures for potential spintronic and superconducting applications

Ultrathin FeAs is of interest both as the active component in the recently identified pnictide superconductors and in spintronic applications at the interface between ferromagnetic Fe and semiconducting GaAs. Here we use first-principles density-functional theory to investigate the properties of FeAs/GaAs heterostructures. We find that the Fermi surface is modified from that characteristic of the pnictide superconductors by interactions between the FeAs layer and the As atoms in the GaAs layers. Regardless of the number of FeAs layers, the Fe to As ratio, or the strain state, the lowest-energy magnetic ordering is always antiferromagnetic, offering an explanation for the failure of spin injection across Fe/GaAs interfaces. However, such antiferromagnetic layers could be incorporated into heterostructures as an exchange-bias layer.

Figure 1

(a) Structure of MnP-type FeAs with ground-state AFM ordering indicated. (b) Structure of zinc-blende FeAs with arrows indicating ground-state AFM ordering.

Figure 2

(a) Calculated energy-volume curves for MnP-type and zinc-blende FeAs with various types of magnetic orderings. The volume shown is for one two-atom formula unit of FeAs. The ferromagnetic (FM) curve was calculated by constraining the total moment to 11${\mu }_B$ for four Fe ions. Antiferromagnetic ordering becomes stable above $V=35{\text{Å}}^3$ and remains more stable than FM with cell expansion. The MnP structure is stable over a large range of volumes. (b) Absolute value of magnetic moment per Fe for the zinc-blende structure with AFM ordering as a function of volume. Paramagnetic ordering is denoted by PM.

Figure 3

Spin-polarized density of states for hypothetical ferromagnetically ordered zinc-blende FeAs calculated at (a) the equilibrium volume of $38.5$ Å${}^3$ per two-atom formula unit and (b) at the experimental GaAs volume of 45 Å${}^3$. The Fermi level is set to 0 eV.

Figure 4

Fe${}_2$As in the antifluorite structure. The calculated ground-state magnetic ordering is shown with arrows.

Figure 5

(a) Structure of Fe${}_2$As in the Cu${}_2$Sb structure. (b) Ground-state magnetic ordering of Cu${}_2$Sb-type Fe${}_2$As.

Figure 6

Calculated energy versus volume (of one three-atom formula unit) for the Cu${}_2$Sb-type and antifluorite Fe${}_2$As, for various magnetic orderings. Both were found to have AFM ground states, as shown in Figs. 4 and 5. The volume was varied by uniform scaling of the calculated equilibrium lattice parameters. The Cu${}_2$Sb-type structure is stable across the whole range of volumes studied.

Figure 7

Zinc-blende-structure (FeAs)${}_n$:(GaAs)${}_m$ heterostructures studied in this work.

Figure 8

Calculated densities of states for zinc-blende-structure FeAs:GaAs heterostructures. The Fermi level is set to 0 eV in each plot and is indicated by the dotted line. The top panel shows bulk zinc-blende FeAs. Following from top to bottom are (FeAs)${}_1$(GaAs)${}_1$, (FeAs)${}_3$(GaAs)${}_1$, and (FeAs)${}_1$(GaAs)${}_3$. The total DOS is given by the solid black line. The orbital projected DOS for the Fe $d$ states is given by the dashed orange line.

Figure 9

(FeAs)${}_n$:(GaAs)${}_m$ heterostructures for antifluorite FeAs on zinc-blende GaAs.

Figure 10

Band structures and densities of states for paramagnetic antifluorite Fe${}_2$As/ zinc-blende GaAs superlattices. In all plots, the Fermi level is set to 0 eV. The total densities of states are given by the black line. The orbitally projected states of Fe $d$, As $p$ (from the FeAs layers), and As $p$ (from the GaAs layers) are shown by the dashed orange, green, and shaded blue lines, respectively. (a) Single-layer FeAs with single-layer GaAs. (b) Single-layer FeAs with double-layer GaAs.

Figure 11

Fermi surface of LaOFeAs and FeAs/GaAs heterostructures for various levels of electron doping relative to the Fermi level: (a) LaOFeAs, (b) $E_F$, (c) $E_F-0.1$ eV, (d) $E_F-0.2$ eV, and (e) $E_F-0.3$ eV.