Computational Investigation of Heusler Compounds for Spintronics Application

Heusler compounds have become a large family of binary, ternary, and quaternary compounds that shows a variety of diverse properties and attracted tremendous scientific interest in the spintronics. The extensive tunability of the Heusler compounds through chemical substitutions makes the large potential for the family.

In this dissertation, we first 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. 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.

The next set of systems studied was inverse-Heusler compounds. First-principles calculations of the electronic structure, magnetism and structural stability of 405 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 and Y are each one of the 3d elements, Sc-Zn, and Z is one of the group IIIA-VA elements: 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.

In the following the discovery, a series of half-metallic half-Heusler alloys are combined with MgO to create Heusler-MgO junctions. The electronic and magnetic properties of junctions are investigated. The strong oxidation between metal and oxygen atoms causes the systems with pure YY interfaces to be the most stable cases. We concluded that the uniaxial anisotropy can be induced in Heusler layers next to MgO. Type of interface layers determines the half-metallicity and the anisotropy (in-plane or perpendicular) in the Heusler-MgO junctions. The capacity to keep half-metallicity and perpendicular magnetic anisotropy (PMA) in NiMnSb/MgO and CoTiSn/MgO junction with MnMn interface layer makes it be potential candidates as electrode layers in Spin Transfer Torque Random Access Memory (STT-RAM) devices.

Finally, using first-principles calculations of structural and magnetic properties, we identify several promising ferrimagnetic inverse Heusler half-metal/near half-metals can be the ideal candidate for hosting ultra-small, fast, and room temperature stable skyrmions.