We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened KKR method. In the paramagnetic phase the C1 b compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, Zt, per unit cell, like CoTiSb, are semiconductors, but when Zt > 18 antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in µB is just the difference between Zt and 2 × 9. While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the sp atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.

Using the full-potential screened Korringa-Kohn-Rostoker method we study the full-Heusler alloys based on Co, Fe, Rh and Ru. We show that many of these compounds show a half-metallic behavior, however in contrast to the half-Heusler alloys the energy gap in the minority band is extremely small due to states localized only at the Co (Fe, Rh or Ru) sites which are not present in the half-Heusler compounds. The full-Heusler alloys show a Slater-Pauling behavior and the total spin-magnetic moment per unit cell (Mt) scales with the total number of valence electrons (Zt) following the rule: Mt = Zt − 24. We explain why the spin-down band contains exactly 12 electrons using arguments based on the group theory and show that this rule holds also for compounds with less than 24 valence electrons. Finally we discuss the deviations from this rule and the differences compared to the half-Heusler alloys.

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to their high Curie temperatures and their structural similarity to binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of both Heusler families: the so-called half-Heusler alloys like NiMnSb and the full-Heusler alloys like Co2MnGe. Ab initio results suggest that both the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment Mt scales linearly with the number of the valence electrons Zt, such that Mt = Zt − 24 for the full-Heusler and Mt = Zt − 18 for the half-Heusler alloys, thus opening the way to engineer new half-metallic alloys with the desired magnetic properties.

We report systematic first-principles calculations for ordered zinc-blende compounds of the transition metal elements V, Cr, Mn with the sp elements N, P, As, Sb, S, Se, Te, motivated by recent fabrication of zinc-blende CrAs, CrSb, and MnAs. They show ferromagnetic half-metallic behavior for a wide range of lattice constants. We discuss the origin and trends of half-metallicity, present the calculated equilibrium lattice constants, and examine the half-metallic behavior of their transition element terminated (001) surfaces.

Abstract. Using a full-potential ab-initio technique I study the electronic and magnetic properties of the (001) surfaces of the half-Heusler alloys, NiMnSb, CoMnSb and PtMnSb and of the full-Heusler alloys Co 2 MnGe, Co 2 MnSi and Co 2 CrAl. The MnSb terminated surfaces of the half-Heusler compounds present properties similar to the bulk compounds and, although the half-metallicity is lost, an important spinpolarisation at the Fermi level. In contrast to this the Ni terminated surface shows an almost zero net spin-polarisation. While the bulk Co 2 MnGe and Co 2 MnSi are almost half-ferromagnetic, their surfaces lose the half-metallic character and the net spin-polarisation at the Fermi level is close to zero. Contrary to these compounds the CrAl terminated (001) surface of Co 2 CrAl shows a spin polarisation of about 84%.

We employ ab-initio electronic structure calculations to study 60 LiMgPdSn-type (also known as LiMgPdSb-type) quaternary Heusler compounds. All compounds obey the Slater-Pauling rule with diverse electronic and magnetic properties. 41 compounds are found to be half-metals, 8 spingapless semiconductors, and 9 semiconductors. CoVTiAl and CrVTiAl compounds are identified as ferromagnetic and antiferromagnetic semiconductors, respectively, with large energy gaps in both spin directions. All magnetic compounds are expected to have high Curie temperatures making them suitable for spintronics/magnetoelectronics applications. V C 2013 AIP Publishing LLC.

We present extensive first-principles calculations on the inverse full-Heusler compounds having the chemical formula X2YZ where (X = Sc, Ti, V, Cr or Mn), (Z = Al, Si or As) and the Y ranges from Ti to Zn. Several of these alloys are identified to be half-metallic magnets. We show that the appearance of half-metallicity is associated in all cases to a Slater-Pauling behavior of the total spin-magnetic moment. There are three different variants of this rule for the inverse Heusler alloys depending on the chemical type of the constituent transition-metal atoms. Simple arguments regarding the hybridization of the d -orbitals of neighboring atoms can explain these rules. We expect our results to trigger further experimental interest on this type of half-metallic Heusler compounds. PACS numbers: 75.50.Cc, 75.30.Et, 71.15.Mb The rise of nanotechnology and nanoscience during the last decade brought to the center of scientific research new phenomena and materials. Spintronics and magnetoelectronics compose one of the most rapidly expanding field in nanoscience. 1 Half-metallic magnetic compounds play a crucial role in this development. 2 These materials present usual metallic behavior for the one spin direction while an energy gap in the band structure is present in the other spin direction similarly to semiconductors. 3,4 The possibility of creating 100% spin-polarized current has triggered the interest on such compounds. 5 De Groot and collaborators in 1983 have initially suggested based on electronic structure calculations that NiMnSb, a semi-Heusler alloy, is a halfmetal 6 and since then several half-metallic compounds have been discovered. 7 Several aspects concerning the implementation of half-metallic alloys in realistic devices, like tunnelling magnetic junctions or giant magnetoresistive junctions and spin-injectors, have been discussed in literature. [8][9][10] The family of Heusler alloys incorporates more than 1000 members almost all crystalizing in a close-packed cubic structure similar to the binary semiconductors. 11 Most of them are metals exhibiting diverse magnetic phenomena. The lattice is a f.c.c. with four equidistant sites as basis along the diagonal of the unit cell. 3 There are two families of Heusler alloys. The semi-(or half-)Heuslers have the chemical formula XYZ where the sequence of the sites is X-Y-void-Z. The X and Y are transitionmetal elements and Z is a sp-element and the structure is known as the C1 b lattice. The second subfamily consists the full-Heusler compounds with the chemical formula X 2 YZ. When the valence of the X is larger than Y, the atomic sequence is X-Y-X-Z and the structure is the well known L2 1 one with prototype Cu 2 MnAl. 12 When the valence of the Y elements is the largest, the compounds crystallize in the so-called XA structure, where the sequence of the atoms is X-X-Y-Z and the prototype is Hg 2 TiCu. 12 The latter alloys are also known as inverse Heusler compounds. Several inverse Heuslers have been studied using first-principles electronic structure calculat...

We study the exchange interactions in half-metallic Heusler alloys using first-principles calculations in conjunction with the frozen-magnon approximation. The Curie temperature is estimated within both mean-field (MF) and random-phase-approximation (RPA) approaches. For the halfHeusler alloys NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In this case the MF and RPA estimations differ strongly. The RPA approach provides better agreement with experiment. The exchange interactions are more complex in the case of full-Heusler alloys Co2MnSi and Co2CrAl where the dominant effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms and between Co atoms at different sublattices. For these compounds we find that both MF and RPA give very close values of the Curie temperature slightly underestimating experimental quantities. We study the influence of the lattice compression on the magnetic properties. The temperature dependence of the magnetization is calculated using the RPA method within both quantum mechanical and classical approaches.