NEW INVERSE-HEUSLER MATERIALS WITH POTENTIAL SPINTRONICS APPLICATIONS

Spintronics or spin-electronics attempt to utilize the electronic spin degree of freedom to make advanced materials and devices for the future. Heusler materials are considered very promising for spintronics applications as many highly spin-polarized materials potentially exist in this family. To accelerate materials discovery and development, The Materials Genome Initiative (https://www.mgi.gov/) was undertaken in 2011 to promote theory-driven search of new materials. In this thesis work, we outline our effort to develop several new materials that are predicted to be 100% spin-polarized (half-metallic) and thermodynamically stable by theory. In particular, two Mn-based Heusler families were investigated: Mn2CoZ (Z= Ga, Sb, Ge) and Mn2FeZ (Z=Si,Ge), where the latter is potentially a new Heusler family. These materials were synthesized using the arc-melting technique and their crystal structure was investigated using the X-ray diffraction (XRD) method before and after appropriate annealing of the samples. Preliminary magnetometry measurements are also reported. We first developed a heat-treatment procedure that could be applied to all the Mn-based compounds mentioned above. Mn2CoGa was successfully stabilized in the cubic inverse-Heusler phase with a=5.869 Å and magnetic moment of 2.007 /fu. This is in good agreement with prior literature reports [1]. However, cubic phases of Mn2CoSb and Mn2CoGe could not be stabilized within the annealing temperature range that is accessible in our lab. We successfully synthesized a cubic Mn2FeSi phase using an annealing procedure similar to Mn2CoGa. The measured cubic lattice parameter of Mn2FeSi was 5.682 Å. This is the first experimental report of this material to the best of our knowledge. Detailed analysis of relative intensities of different X-ray peaks revealed that the structure is most likely in an inverse Heusler phase, in agreement with theory. However, a substantial atomic-level disorder was also uncovered from XRD analysis that requires further investigation to understand its effect on its magnetism and half-metallicity. Mn2FeGe showed the existence of non-cubic phases that substantially weakened at high annealing temperatures