Novel Half-Metallic and Spin-Gapless Heusler Compounds

Authors

Yunlong Jin, University of Nebraska-LincolnFollow

ORCID IDs

Yunlong Jin

First Advisor

David J. Sellmyer

Date of this Version

8-2017

Document Type

Article

Citation

Yunlong Jin, "Novel Half-Metallic and Spin-Gapless Heusler Compounds", University of Nebraska-Lincoln, 2017.

Comments

A DISSERTATION Presented to the Faculty of The Graduate College of the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Physics and Astronomy, Under the Supervision of Professor David J. Sellmyer. Lincoln, Nebraska: August, 2017

Copyright (c) 2017 Yunlong Jin

Abstract

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 (T_C = 790 K). Cubic CoFeCrGe decomposed into other compounds near 402 ^oC (675 K). A new tetragonal Co_53.4Fe_30.4Cr_8.6Ge_7.6 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. Sellmyer