Study On The Electronic Structure Of The Magnetic Shape Memory Alloys

In this thesis we mainly investigate the electronic structure of some new functionalmaterials such as ferromagnetic-shape-memory-alloy (FSMA) and half-metal in some alloysystems, especially in Heusler-type alloys, by using first-principles calculations. Severalnew FSMAs and half-metals have been predicted, which can have potential applications inmany technical fields.The magnetic structure of Mn2NiZ (Z=As, Sb, Bi) has been studied. The total energydifference between the antiferromagnetic (AFM) and ferromagnetic (FM) states is rathersmall compared with other Mn2-based Heusler alloys. This makes it possible that, underproper conditions, both the FM and AFM state can be realized in them. The FM or AFMmagnetic structure is strongly related to the relative position of the minority energy gaps tothe Fermi level. This specific electronic structure makes it possible to design the energybands of this series of alloys by doping and alloying. Then transitions between the AFMand FM states under driven of temperature/magnetic field/pressure can be expected.The site preference, electronic structure and magnetism of a newly discovered Heusleralloy Mn2RuSn have been investigated. The Hg2CuTi-type structure is found to be the mostenergetically favored in Mn2RuSn. Mn2RuSn is a ferrimagnetic metal at equilibrium latticeconstant, but with2%contraction of the lattice, it becomes a half-metal with a spin momentof2μB. A tetragonal martensitic transformation is predicted in Mn2RuSn. The total momentof the tetragonal phase is only0.01μB, which will lead to a large ΔM between the saturationmoments of martensite and austenite. Finally, in tetragonal Mn2RuSn, a high spinpolarization ratio of93%is predicted. Accompanied with the zero total moment, Mn2RuSncan be of special interest for spintronics.The electronic structure and magnetism of Mn2RuZ (Z=Al, Ga, In, Ge, Sn, Sb) havebeen studied. Mn2RuSb is predicted to show half-metallic character. The total spinmoments Mtand the number of valence electrons Ztof which follow the Slater-Pauling ruleof Mt=Zt-24. The spin polarization ratio of Mn2RuSb is always100%from5.8to6.3.The spin polarization ratio of Mn2RuAl and Mn2RuGe is also high.Fe2MnGa and CoMnGe FSMAs have been prepared by ball-milling. The annealedFe2MnGa powder mainly composed of a bcc phase together with a small amount of fccphase. The Curie temperature of the bcc phase is290℃. The CoMnGe powder is a mixtureof the austenitic and martensitic phases, indicating the phase transition temperature is nearroom temperature. The lattice of CoMn (Ge, B) shows a small contraction.