Novel Half-Metallic and Spin-Gapless Heusler Compounds

This thesis is devoted to experimental studies of Heusler compounds CoFeCrAl, CoFeCrX (X = Si, Ge) and Mn2PtSn. These Heusler alloys present an interesting class of ferromagnetic materials for spintronic applications since they are predicted to be spin gapless semiconductors and have half-metallic properties with 100 % spin polarization at the Fermi level. In this thesis, the structural, magnetic, spin-polarization and electron- transport properties of the fabricated alloys were studied. CoFeCrAl thin films deposited on MgO exhibit nearly perfect epitaxy and a high degree of L21 Heusler order. All considered types of chemical disorder destroy the spin-gapless semiconductivity of Y-ordered CoFeCrAl, but B2 disorder and A2-type Cr-Fe disorder conserve the half-metallicity of the alloy. The transport spin polarization at the Fermi level is higher than 68%. Epitaxial thin films of inverse tetragonal Mn2PtSn were synthesized and show lattice constants a = 0.449 nm, and b = 0.615 nm. Magnetization measurements revealed an in-plane anisotropy energy of 10 Merg/cm3. CoFeCrSi and CoFeCrGe Heusler alloys crystallize in cubic L21 structures with a small site disorder. The CoFeCrSi alloy exhibited a high Curie temperature (TC = 790 K). Cubic CoFeCrGe decomposed into other compounds near 402 oC (675 K). A new tetragonal Co53.4Fe30.4Cr8.6Ge7.6 phase with lattice parameters a = 0.760 nm, c = 0.284 nm was studied with a series of tilted electron- diffraction measurements in this work. The (Co, Fe)-rich phase was found to embedded in the matrix of a Cr-rich phase. The results show promise for the implementation of CoFeCrX (X = Al, Si, Ge) and Mn2PtSn in future spintronics devices. Adviser: David J. Sellmye