1. Fe1.5TiSb : A New Slater-Pauling Heusler Compound
N.Naghibolashrafi, 1,4 S. Keshavarz, 1,2 J. Romero, 1,2 K. Munira, 1
D. Mazumdar, 1 P. LeClair, 1,2 W. Butler, 1,2 A. Gupta,1,3C.Wolverton,5 V. I. Hegde5
1) Center for Materials for Information Technology, MINT, University of Alabama, Tuscaloosa, AL 35487
2) Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487
3) Department of Chemistry, , University of Alabama, Tuscaloosa, AL 35487
4) Materials Science Program, University of Alabama, Tuscaloosa, AL 35487
5) Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208
INTRODUCTION
MICROSTRUCTURAL ANALYSIS
Half-metallic ferromagnets are ideal materials for many spintronics applications, including STT-RAMs and CPP-GMRs.
We performed an extensive survey of 270 number of L21 alloys, space group (No.225), which are also candidates for Slater-Pauling Half-metals.
In a search for new half-metallic materials using DFT calculations, we determined that the Fe2TiSb compound would be both half metallic and stable with an L21 structure. Though experimentally it was observed that the compound decomposes into a dominant Fe1.5TiSb phase and iron. This interested us into analyzing this dominant phase.
Our Density Functional Theory (DFT) calculations shows Fe1.5TiSb to be a Slater-Pauling semiconductor and also the most stable phase among all other possible compounds between half and full Heusler compositions.
Arc melting of the elements in an argon atmosphere was the sample processing method. Samples were remelted at least 7 times to ensure proper mixing of all the elements.
The samples were then cut and put in quartz tube under vacuum and then sealed. They were heat treated in many different cycles.
As the result of Metallography and SEM analysis, Fe2TiSb appeared to decompose into Fe1.5TiSb grains with some iron-rich phase precipitating in the grain boundaries.
The very same decomposition occurred in the FeTiSb compound with the alloy basically separating into Fe1.5TiSb grains and a Titanium /Antimony rich Sb1.8Ti1.5Fe which could be the same Fe0.5TiSb compound found stable in the enthalpy calculations.
The Fe1.5TiSb alloy composition showed only grains with the target stoichiometry and a paramagnetic behavior both of which are consistent with theory predications showing the stability and semiconductor behavior of this phase.
The discussion will be focused on this dominant phase from this point onwards.
Formation of grains, correct stoichiometry and the uniform distribution of L21 phase of Fe1.5TiSb;900˚C/7D are confirmed through study of the microstructure by microscopy methods, such as optical microscope, SEM, EDAX, EBSD phase mapping, as shown below.
In EBSD phase mapping, the experimental lattice constant of Å was considered and it was seen that Fe2TiSb has grains of Fe1.5TiSb grains with less than 10% vol. of iron precipitates.
Magnetic hysteresis curves of these sample at room temperature and low temperature (5 K) were taken by VSM-QD Dynacool system. The magnetic behavior is essentially paramagnetic with a weak magnetic contribution. The conductivity observed also might correspond to the low predicted band gap.
The Fe1.5TiSb phase seems to be a dominant phase observed in the full- and half-Heusler region of Fe-Ti-Sb alloys. The structure seems to be an L21 with two iron vacancies, therefore it can be considered as a “ layered” Heusler in which consecutive layers of C1b And L21 are stacked upon one another.
The material is predicted to be stable among all other possible stoichiometries and a Slater-Pauling semiconductor, both of which are observed in the Fe-Ti-Sb system experimentally.
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We would like to thank the National Science Foundation (NSF) for providing funds and support for the advance of this project. The funding is provided under NSF initiative for Designing Materials to Revolutionize and Engineer our Future (DMREF).
MAGNETIC AND ELECTRICAL PROPERTIES
THEORETICAL ANALYSIS OF Fe1.5TiSb SYSTEM
Presumed Fe1.5TiSb L21 structure with two iron vacancies
C1b Structure: Half-Heusler
L21 Structure: Full-Heusler
CONCLUSION
STRUCTURE OF Fe1.5TiSb
XRD analyses were performed using a Bruker D8 system with a Co-Kα1 source. The samples were polished and rotated to ensure the minimization of the surface and size effects on the measurements.
CaRine Crystallography software was used to simulate the expected XRD pattern for a L21 Fe2TiSb material with a lattice constant of Å. There were some traces of iron present in the pattern.
The Fe1.5TiSb compound itself shows no trace of iron precipitate and only rings belonging to a cubic L21 phase while observed under TEM selected area diffraction mode.
The Rietveld refinement performed through the Match! Software shows a good fit with the proposed L21 model with a χ2 factor of 0.9 and an average RBragg factor of 0.1.
All this indicates that most probably the crystal structure of Fe1.5TiSb is a cubic L21 phase with two iron vacancies in the tetrahedral sites as shown above. These experimental results are consistent with theoretical predictions mentioned before.
EXPERIMENTAL APPROACH
REFRENCES
ACKNOWLEDGEMENT