Towards New Half-metal And Magnetic Martensitic Transformation Materials For Multi-functional Applications Based On Heusler Alloys

Heusler alloys are a remarkable class of intermetallic materials that not only have an endless number of members,but also a vast variety of properties,ranging from half-metallic ferromagnets,superconductors,and topological insulators to shape memory alloys.In this thesis,we mainly focus on the designing and exploiting of new half-metal and magnetic martensitic transformation materials for multi-functional applications.The main contents of the thesis are as follows:First-principles calculation results indicate that LiMgPdSn-type quaternary Heusler alloys CoFeTiZ(Z = Ga,Si,Ge,Sn,P,As and Sb)exhibit half-metallic ferrimagnetism.The total spin moments for all the compounds under study as a function of the total number of valence electrons follow the half-metallic-type Slater-Pauling rule:Mt = Nv-24.The origin of the half-metallic band gap and the influence of main group element on stability of half-metallicity have been discussed.Single Heusler phases of CoFeTiGa1-xSbx(x = 0?1)alloys have been experimentally synthesized,the experiment magnetic moments were in agreement with the theoretical values.It is found that C1b-type Mn2Z(Z = Si and Ge)are half-metallic antiferromagnets.In particularly,Mn2Si exhibits a spin gapless semicondutor behavior in band structure,as well as antiferromagnetism,which has rarely found before.A semiempirical rule that Heusler alloys with 18,24 or 28 valence eletrons in unit cell should be the likeliest candidates for antiferromagnetic spin gapless semiconductors is summarized for reaching the scope of discovering more such kind of materials.Depression of magnetization,large saturation field,spin glass system and giant magnetoresistance were observed in as-melt-spun Ni50Fe21Ga29.These behaviors originate from the existence of nanoscale antiphase boundaries(APB),which distributes randomly,pins the magnetic domain wall strongly and depresses the magnetic ordering deeply.An observation of deformation-induced B2-y transformation through cold work was observed in ferromagnetic shape memory alloy Fe44Mn28Ga28.Because of the generated y phase,the specimen exhibits excellent cold-workability.Heat treatment recovers the B2 Heusler phase as well as magnetic field induced transformation and strain,which indiates FeMnGa is a promising candidate for application in actuator devices.Single phase polycrystalline DO19-type hexagonal Fe3_xMnxGa(x=0?3)alloys were experimentally synthesized.A cluster spin glass state and a giant exchange bias field were observed in hexagonal Mn2FeGa alloy.The large exchange bias is originated from the large exchange anisotropy between cluster spin glass phase and ferrimagnetic matrix.Transition from anomalous hall effect to topological hall effect was observed in hexagonal non-collinear magnet Mn3Ga.The appearance of the topological hall effect is attributed to the change of spin texture as a result of weak structural distortion from hexagonal to orthorhombic symmetry.These results reveal that Heusler alloys are an outstanding class of materials with a large variety of diverse properties.The search for new,multi-functional Heusler alloys are still a fruitful field of research in coming years.