Studies On The Electronic Structures And Physical Properties Of The Heusler Alloys With The Special Functions

The rich physical properties and appliable functions are being developed in the Heusler alloy family. The Heusler alloy family is a huge treasure house of searching for new functional materials and has exhibited great potential applications in magnetomechanical,magnetoelectric and magnetocaloric fields. In this dissertation, we developed several new functional materials with Heusler structure and investigated their basic physical and functional properties. The main results are as follows:The ferromagnetic shape memory effect has been discovered in CoMnGa alloy with half-Heusler structure. A huge change of volume and magnetic structure transition occur during the process of first order phase transition but the lattice symmetry is unchanged. The Co-Mn antisites induced the ferro-ferrimagnetic transition(Fo-Fi-T) in CoMnGa alloy by adjusting the driving force of phase transition and the energy barrier between two phases.The content of the Co-Mn antisites can be adjusted by different annealing and quenching process. The first order phase transition with the ferro-ferrimagnetic transition(Fo-Fi-T),and the unchanged lattice symmetry was realized in CoMnGa alloy with proper Co-Mn antisites.We investigated the martensitic transformation of the L21-type Co2TiSb1-x Snx alloys using first-principles calculations. The L21-type Co2TiSb1-x Snx(x=0.25, 0.5) alloys are successfully synthesized by the melt-spun method.The thermal-elastic martensitic transformation was experimentally observed in Co2TiSb0.75Sn0.25 alloy.We predicted that Co-Mn-V-Al alloy is a fully-compensated half-metallic ferrimagnet in CoVMnAl-type atomic arrangement with 25%~50% Co-Mn antisites using band structure calculations. The CoVMnAl-type atomic arrangement with about 30% Co-Mn antisites was successfully synthesized. The measurements of magnetic properties and electronic transport properties confirmed the half-metallicity and fully-compensated ferrimagnetism. The half-metallicity and ferrimagnetism in Co-Mn-V-Al compounds originate from the antisite between the Co and Mn atoms, which overcomes a troubling problem of antisite destroying the half-metallicity. To induce half-metallicity and ferrimagnetism by antisite was proposed as a new way to search for half-metallic material in Heusler alloys.We designed the Fe-based single atomic chains in the semiconductive CoTiSb matrix by continuously substituting Fe for Ti, Ti-Sb or Co atoms. The single atomic chains of Fe-Sb and Fe-vacancy are predicted to have a 100% spin polarization and form a very small single spin channel nanorod. The single atomic chain of Fe-Fe shows a spin-gapless characteristic.A very peculiar spin-gapless semiconductor(SGS): Mn2.25Co0.75Ga0.5Sn0.5 Heusler alloywere developed. The Mn2.25Co0.75Ga0.5Sn0.5 alloy has a highly ordered atomic arrangement,a magnetic moment of 2μB and a high Curie temperature of about 720 K. An ordinary Hall effect(OHE) which is two orders of magnitude larger than any other magnetic or nonmagnetic material, was found in Mn2.25Co0.75Ga0.5Sn0.5 Heusler alloy. The ordinary Hall effect generates a Hall voltage opposite to the anomalous Hall effect which leads to the change of total Hall voltage from negative to positive with increasing magnetic field. The Mn2.25Co0.75Ga0.5Sn0.5 alloy has a low conductivity of 13.7S/cm and a linear positive magnetoresistance at 7K. With increasing temperature, the Mn2.25Co0.75Ga0.5Sn0.5 alloy shows a double crossover magnetoresistance from positive to negative which is a feature of spin-gapless material.