We present first-principles density functional calculations of the electronic structure, magnetism, and structural stability of 378 XYZ half-Heusler compounds (with X = Cr, Mn, Fe, Co, Ni, Ru, Rh; Y = Ti, V, Cr, Mn, Fe, Ni; Z = Al, Ga, In, Si, Ge, Sn, P, As, Sb). We find that a "Slater-Pauling gap" in the density of states, (i.e. a gap or pseudogap after nine states in the three atom primitive cell) in at least one spin channel is a common feature in half-Heusler compounds. We find that the presence of such a gap at the Fermi energy in one or both spin channels contributes significantly to the stability of a half-Heusler compound. We calculate the formation energy of each compound and systematically investigate its stability against all other phases in the Open Quantum Materials Database (OQMD). We represent the thermodynamic phase stability of each compound as its distance from the convex hull of stable phases in the respective chemical space and show that the hull distance of a compound is a good measure of the likelihood of its experimental synthesis. We find low formation energies and mostly correspondingly low hull distances for compounds with X = Co, Rh or Ni, Y = Ti or V, and Z = P, As, Sb or Si. We identify 26 18-electron semiconductors, 45 half-metals, and 34 near half-metals with negative formation energy, that follow the Slater-Pauling rule of three electrons per atom. Our calculations predict several new, as-yet unknown, thermodynamically stable phases which merit further experimental exploration -RuVAs, CoVGe, FeVAs in the half-Heusler structure, and NiScAs, RuVP, RhTiP in the orthorhombic MgSrSi-type structure. Further, two interesting zero-moment half-metals, CrMnAs and MnCrAs, are calculated to have negative formation energy. In addition, our calculations predict a number of hitherto unreported semiconducting (e.g., CoVSn, RhVGe), half-metallic (e.g., RhVSb), and near half-metallic (e.g., CoFeSb, CoVP) half-Heusler compounds to lie close to the respective convex hull of stable phases, and thus may be experimentally realized under suitable synthesis conditions, resulting in potential candidates for various semiconducting and spintronics applications.

First-principles calculations of the electronic structure, magnetism and structural stability of inverse-Heusler compounds with the chemical formula X2YZ are presented and discussed with a goal of identifying compounds of interest for spintronics. Compounds for which the number of electrons per atom for Y exceed that for X and for which X is one of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, or Cu; Y is one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn; and Z is one of Al, Ga, In, Si, Ge, Sn, P, As or Sb were considered. The formation energy per atom of each compound was calculated. By comparing our calculated formation energies to those calculated for phases in the Inorganic Crystal Structure Database (ICSD) of observed phases, we estimate that inverse-Heuslers with formation energies within 0.052 eV/atom of the calculated convex hull are reasonably likely to be synthesizable in equilibrium. The observed trends in the formation energy and relative structural stability as the X, Y and Z elements vary are described. In addition to the Slater-Pauling gap after 12 states per formula unit in one of the spin channels, inverse-Heusler phases often have gaps after 9 states or 14 states. We describe the origin and occurrence of these gaps. We identify 14 inverse-Heusler semiconductors, 51 half-metals and 50 near half-metals with negative formation energy. In addition, our calculations predict 4 half-metals and 6 near half-metals to lie close to the respective convex hull of stable phases, and thus may be experimentally realized under suitable synthesis conditions, resulting in potential candidates for future spintronics applications.

To make a useful STT-MRAM (spin-transfer torque magnetoresistive random-access memory) device, it is necessary to be able to calculate switching rates, which determine the error rates of the device. In a singlemacrospin model, one can use a Fokker-Planck equation to obtain a low-current thermally activated rate ∝ exp(−E ef f /k B T ). Here the effective energy barrier E ef f scales with the single-macrospin energy barrier KV , where K is the effective anisotropy energy density and V the volume. A long-standing paradox in this field is that the actual energy barrier appears to be much smaller than this. It has been suggested that incoherent motions may lower the barrier, but this has proved difficult to quantify. In the present paper, we show that the coherent precession has a magnetostatic instability, which allows quantitative estimation of the energy barrier and may resolve the paradox.

Compounds of Fe, Ti and Sb were prepared using arc melting and vacuum annealing. Fe2TiSb, expected to be a full Heusler compound crystallizing in the L21 structure, was shown by XRD and SEM analyses to be composed of weakly magnetic grains of nominal composition Fe1.5TiSb with iron-rich precipitates in the grain boundaries. FeTiSb, a composition consistent with the formation of a half Heusler compound, also decomposed into Fe1.5TiSb grains with Ti-Sb rich precipitates and was weakly magnetic. The dominant Fe1.5TiSb phase appears to crystallize in a defective L21-like structure with iron vacancies. Based on this finding, a first-principles DFT-based binary cluster expansion of Fe and vacancies on the Fe sublattice of the L21 structure was performed. Using the cluster expansion, we computationally scanned > 10 3 configurations and predict a novel, stable, non-magnetic semiconductor phase to be the zero-temperature ground state. This new structure is an ordered arrangement of Fe and vacancies, belonging to the space group R3m, with composition Fe1.5TiSb, i.e., between the full-and half-Heusler compositions. This phase can be visualized as alternate layers of L21 phase Fe2TiSb and C1 b phase FeTiSb, with layering along the [111] direction of the original cubic phases. Our experimental results on annealed samples support this predicted ground state composition, but further work is required to confirm that the R3m structure is the ground state.

Dipole-coupled nanomagnetic logic (NML), where nanomagnets (NMs) with bistable magnetization states act as binary switches and information is transferred between them via dipole-coupling and Bennett clocking, is a potential replacement for conventional transistor logic since magnets dissipate less energy than transistors when they switch in a logic circuit. Magnets are also 'non-volatile' and hence can store the results of a computation after the computation is over, thereby doubling as both logic and memory-a feat that transistors cannot achieve. However, dipole-coupled NML is much more error-prone than transistor logic at room temperature [Formula: see text] because thermal noise can easily disrupt magnetization dynamics. Here, we study a particularly energy-efficient version of dipole-coupled NML known as straintronic multiferroic logic (SML) where magnets are clocked/switched with electrically generated mechanical strain. By appropriately 'shaping' the voltage pulse that generates strain, we show that the error rate in SML can be reduced to tolerable limits. We describe the error probabilities associated with various stress pulse shapes and discuss the trade-off between error rate and switching speed in SML.The lowest error probability is obtained when a 'shaped' high voltage pulse is applied to strain the output NM followed by a low voltage pulse. The high voltage pulse quickly rotates the output magnet's magnetization by 90° and aligns it roughly along the minor (or hard) axis of the NM. Next, the low voltage pulse produces the critical strain to overcome the shape anisotropy energy barrier in the NM and produce a monostable potential energy profile in the presence of dipole coupling from the neighboring NM. The magnetization of the output NM then migrates to the global energy minimum in this monostable profile and completes a 180° rotation (magnetization flip) with high likelihood.

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The stress-induced switching behavior of a multiferroic nanomagnet, dipole coupled to a hard nanomagnet, is numerically studied by solving the stochastic Landau-Lifshitz-Gilbert equation for a single-domain macrospin state. Different factors were found to affect the switching probability in the presence of thermal noise at room temperature: 1) dipole coupling strength, 2) stress levels, and 3) stress withdrawal rates (ramp rates). We report that the thermal broadening of the magnetization distribution causes large switching error rates. This could render nanomagnetic logic schemes that rely on dipole coupling to perform Boolean logic operations impractical whether they are clocked by stress or field or other means.

We show that when two Heusler alloys are layered in the [001], [110], or [111] directions for various thicknesses to form a superlattice, the Slater-Pauling rule may still be satisfied and the resulting superlattice is often half-metallic with gaps comparable to or larger than those of its constituents. In addition, uniaxial magnetocrystalline anisotropy is induced because of the differences in the electronic structure of the two Heuslers in the superlattice. Various full-full, full-half, and half-half Heusler superlattices are studied, and potential half-metallic superlattices with perpendicular magnetocrystalline anisotropy are identified.