Spintronics are new types of devices and materials that enable the use of electron spin for the next breakthrough in data processing, communication, and storage. These materials are important for generating, injecting, and transferring spin-polarized currents. A Spin gapless semiconductor (SGS) shows insulating behavior for one type of spin and semiconducting behavior with a diminished bandgap for the other. It is particularly of interest due to its unique benefits over other types of spintronics. Most known SGS candidates are Heusler compounds.

Disorders are common in Heusler compounds. Taking the first-prepared SGS of Mn2CoAl as an example, while ideally, Mn2CoAl forms an ordered XA type lattice (inversed Heusler), real synthesis conditions often lead to off-stoichiometry and disordering such as the L21 and A2 type (see Figure). Reproducible SGS behavior can only be achieved if the composition and heat treatment are manipulated to minimize detrimental defects. However, there is no knowledge on how these affect the phase and defect configuration in most SGS compounds. Taking advantage of a funded research project on Mn2CoAl compounds, we offer a project for REU students to explore a similar compound of Ti2MnAl. We will study how to manipulate the chemical composition and heat treatment to achieve a near-single phase with minimized atomic defects. X-ray and synchrotron techniques can provide this critical information. a) High-resolution synchrotron diffraction and Rietveld refinement will provide information on site occupancies. b) X-ray absorption spectroscopy (XAS), including both extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) can be employed. The coordination and bond distance of Mn atoms is different in the ideal XA configuration versus possible L21 or A2 configurations, which can be resolved by EXAFS. XANES, on the other hand, can provide information on the oxidation state of Mn, which is related to its magnetic ordering.

The specific research goals for students are to be able to:

·         Synthesis of polycrystalline Ti2MnAl compounds and assess the phase composition with house XRD and SEM-EDS.

·         Perform different heat treatments and examine how they affects the phase stability of Ti2MnAl.

·         Collect synchrotron and XAS experiment data and perform basic analysis to help solve atomic disordering in different specimens.

The specific learning goals for students are to be able to:

·         Understand different types of atomic disorders in Heusler compounds.

·         Learned to use XRD and proper software for phase identification.

·         Understand the principle and general procedure of Rietveld refinement for quantitative phase composition identification, as well as for helping to identify atomic site occupancy.

Understand the principle of XAS and its application in materials research