MnAs is a ferromagnetic metal whose thermodynamic ground state is the hexagonal NiAs-type structure. Density-functional calculations by Sanvito and Hill (2000) showed that a hypothetical zinc-blende phase of MnAs — the x=1x = 1 limit of the diluted magnetic semiconductor Ga1−x_{1-x}Mnx_xAs — is metastable by roughly an electron volt per formula unit, becomes half-metallic only when expanded to lattice constants of about 5.8 Å or more, and cannot be stabilized by isotropic compression or expansion, leaving its growth an open challenge. Later theory proved that coherent epitaxy never thermodynamically stabilizes zinc-blende MnAs, while nonequilibrium molecular-beam-epitaxy routes have in fact produced the phase in constrained forms: nanodots on GaAs(001), nanoclusters embedded in GaAs, and ultrathin films on GaAs or InP. However, the predicted half-metallicity has never been directly demonstrated: photoemission of zinc-blende MnAs nanodots shows no Fermi edge, in conflict with the calculated metallic majority-spin channel, and indirect magnetotransport claims of half-metallic behavior remain unconfirmed. The surviving open problem is to stabilize zinc-blende MnAs in a form usable as a spin injector and to establish experimentally whether it is genuinely half-metallic.

Background

MnAs is a ferromagnetic metal with Curie temperature (the temperature below which a ferromagnet spontaneously magnetizes) near 318 K that crystallizes, like all manganese pnictides, in the hexagonal NiAs-type structure (space group P63/mmcP6_3/mmc). It can be grown epitaxially on GaAs in this structure, and the MnAs/GaAs interface is thermodynamically stable because both materials share the As sublattice, making the MnAs/Ga1−x_{1-x}Mnx_xAs/GaAs materials system a leading platform for semiconductor spintronics. Ga1−x_{1-x}Mnx_xAs is the canonical diluted magnetic semiconductor — a semiconductor in which a small fraction of the host cations is replaced by magnetic ions: Mn substituted on Ga sites supplies both localized magnetic moments and itinerant holes, and the alloy orders ferromagnetically above roughly x=0.005x = 0.005. Taking that alloy to the x=1x = 1 limit yields a hypothetical zinc-blende MnAs, in which Mn and As alternate on the two face-centered-cubic sublattices of the GaAs crystal structure (the 'zinc-blende' or 'sphalerite' structure). A half-metal is a material that is metallic for one spin orientation and semiconducting for the other, so that the electrons at the Fermi level are, in the ideal limit, 100 percent spin-polarized; such a contact is attractive as a source of spin-polarized current because ordinary metallic injectors transfer spin poorly into semiconductors, owing to the conductivity mismatch between metal and semiconductor. Full-potential linearized augmented plane wave (FLAPW) calculations — an all-electron implementation of density-functional theory — had predicted zinc-blende MnAs to be a half-metal, which motivated the study by Sanvito and Hill (2000). Sanvito and Hill (2000) performed spin-polarized plane-wave pseudopotential calculations in the local spin-density approximation (LSDA, the spin-polarized form of the local-density approximation to density-functional theory) and found: (i) the NiAs-type structure is lower in total energy than the zinc-blende one over the entire volume range, so zinc-blende MnAs is unstable at all equilibrium pressures and 'has never been successfully grown'; (ii) the zinc-blende equilibrium lattice constant is 5.6-5.7 Å, notably smaller than the FLAPW value of about 5.9 Å and than the 5.89 Å extrapolated from Ga1−x_{1-x}Mnx_xAs, a discrepancy they could only tentatively attribute to the pseudopotential construction or to differences in convergence and k-point sampling; (iii) within LSDA the zinc-blende phase is paramagnetic below about a=5.0a = 5.0 Å, ferromagnetic at intermediate lattice constants, and half-metallic only for aa greater than about 5.8 Å, with a saturation magnetic moment approaching 4 μB\mu_\mathrm{B} per Mn atom, but LSDA is known to underestimate equilibrium volumes, so the quantitative placement of these phase boundaries was uncertain; and (iv) the saturation magnetization of NiAs-type MnAs films on GaAs varies strongly with epitaxial orientation (about 416, 331, and 640 emu/cm3^3 for the two (001) growth variants and (111) growth), which the authors could only qualitatively attribute to substrate-imposed distortion of the hexagonal cell, noting that complicated surface reconstructions make a direct correlation with experiment difficult. Their central question — posed in the preprint title 'Can half metallic zincblende MnAs be grown?' — is whether this metastable, predicted half-metallic phase can be experimentally realized, and whether its predicted properties are physically real.

Problem Statement

Determine whether the metastable zinc-blende phase of MnAs — predicted by density-functional theory to be ferromagnetic and half-metallic (metallic in the majority-spin channel, gapped in the minority-spin channel, with a magnetic moment approaching 4 μB\mu_\mathrm{B} per Mn) for lattice constants aa expanded beyond about 5.8 Å, but unstable against the NiAs-type ground structure at all equilibrium volumes — can be realized and verified experimentally. Concretely: (1) synthesize and stabilize zinc-blende MnAs by a practical growth route, in a continuous, phase-pure zinc-blende form, as opposed to the nanodot, nanocluster, or ultrathin-film forms reported to date (with any residual second phase below the detection limit of the structural characterization employed) — or else demonstrate that no practical growth route, equilibrium or nonequilibrium, yields such a form — given that coherent epitaxy is now known not to lower the energy of the zinc-blende phase below that of the NiAs-type phase for any substrate lattice constant; (2) measure directly, or refute, its predicted half-metallic character — that is, demonstrate (or exclude) essentially complete spin polarization at the Fermi level — and reconcile the outcome with the observed absence of a Fermi edge in photoemission of zinc-blende MnAs nanodots, which conflicts with the calculated metallic majority-spin channel; and (3) establish how quantitative the LSDA-predicted (local spin-density approximation) volume- and strain-dependent magnetic phase sequence (paramagnetic →\to ferromagnetic →\to half-metallic, with the concomitant magnetic-moment and Curie-temperature trends) is for the constrained, matrix-strained forms in which the phase actually exists. Parts (1) and (2) are the open core and are each required; part (3) is a supporting target that supplies the quantitative electronic- and magnetic-structure reference against which any realized material is to be judged, not a separate deliverable that can resolve the task on its own.

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