Studies On The Design And Preparation Of The Functional Heusler Alloys

In this thesis we investigated the site occupation, electronic structure, magnetic properties and martensitic transformation in Mn₂-based and Ni₂-based Heusler alloys different compositions. The influences of different main-group elements or transition metal on these properties have been studied. Some new possible functional materials have been predicted.In Heusler alloys Mn₂RuZ（Z = Al, Ga, In, Sn, Sb）, the energy differences between the austenitic and martensitic phases have a positive relationship with their equilibrium lattice constants. Possible martensitic transformation has predicted in Mn₂RuZ（Z = Ga,In, Sn, Sb）. These alloys are all ferrimagnets and their magnetic properties are mainly determined by the antiparallel aligned Mn spin moments in both austenite phase and martensite phase.We investigated the electronic structure and magnetic properties of Heusler alloy Mn₂ RuSn by using the KKR-CPA-LDA method. It is found that, the Hg2CuTi-type structure with ferrimagnetic ground state is energetically favored, the ground state energy and the equilibrium lattice constants of Mn₂ RuSn increase with increasing degree of Mn（A）-Ru（C） atomic disorder. Mn（A）-Ru（C） disorder lower the total spin moment effectively, but the ferrimagnetic state is always retained.Heusler alloys Mn₂ RuGe and Mn₂ RuGa ribbons have been prepared by the melt-spinning method successfully. Experimental studies indicate there may be a degree of atomic disorder in the two samples. The Curie temperatures are 303 K for Mn₂ RuGe and 272 K for Mn₂ RuGa. Theoretical studies reveal a ferrimagnetic ground state and a quite high spin polarization ratio in the two alloys. While the saturation moments at 5K of Mn₂ RuGe and Mn₂ RuGa are smaller than the theoretical values, especially in Mn₂ RuGa. A precipitation of the secondary phase will occur in the two samples when annealing the ribbon samples at 1173 K. But annealing Mn₂ RuGa ribbon at 773 K can increase its saturation magnetic moment and Curie temperature significantly.Theoretical studies reveal that the site occupation of excess Ni follows the valence electrons rule in Mn₂Ni1.5Z0.5（Z = Sn, Sb） alloys. The stability of the magnetic structure of Mn₂Ni1.5Sb0.5 is very sensitive to the lattice constants. As the number of valence electrons of main group elements increasing, the stability of the martensite increases.The partial moment of Mn（D） changes from positive to negative after the martensitic transformation, which leads to the drastic decrease of the total moment and is favorable for magnetic field driven martensitic phase transformation.The study on Ni₂YGa（Y = Ti, V, Cr, Mn, Fe, Co） reveal that Ni₂ TiGa and Ni₂ VGa have paramagnetic ground states, while other alloys have ferromagnetic state. Their magnetic properties are mainly determined by Y spin moments. Martensitic transformation is predicted in Ni₂YGa（Y = Cr, Mn, Fe, Co） alloys. The magnetic moment difference between the austenite and martensite is very small. Experimenal results show that second FCC phase increases with increasing Cr content in Ni₂Cr1-xMnxGa（x = 0.1, 0.2, 0.3, 0.4）, but melt-spun method can effectively restrain the precipitation of FCC phase.The study of Ni₂MnY（Y = Ti, V, Cu, Zn, Ag, Au） reveal that Ni₂ MnV crystalizes in Hg2CuTi-type structure and has a ferrimagnetic ground state, while other alloys prefer the Cu2MnAl-type structure and have a ferromagnetic ground state. Possible martensitic transformation has been predicted in Ni₂ MnY alloys, and the total moments of them are mainly determined by Mn spin moments.