Investigations On The Structure, Phase Transition And Magnetism In Doped Ni-X-Z (X=Mn, Fe; Z=Ga, In) Alloys And The Half-metallicity Of Co-Cr-Z

Heusler alloys are new functional materials due to their excellent performances. They have a lot of potential applications in many fields, such as the aerospace, medicine, and computer communication, etc. The physical properties of Ni-based Heusler alloys Ni-X-Z have been systemically studied in this dissertation. Several Heusler alloys have been synthesized by arc-melting and melt-spinning techniques. Their structures have been characterized by X-ray diffraction (XRD). Magnetic properties have been measured by vibrating sample magnetometer (VSM) and superconducting quantum interference device (SQUID), etc. Meanwhile, the half-metallic properties of half Heusler compound Co-Cr-Z have also been theoretically investigated based on the first-principle calculations. The main content and results are as following:First, we discuss the structure, magnetism and ordering transition of Mn2-xNiGa1+x (x=0-1). With Ga content increasing, the structure of the parent phase is transited from Hg2CuTi-type to Cu2MnAl-type Heusler gradually, which precipitates Ni2In-type hexagonal phase. The Curie temperature of the primary phase reduces gradually from 590 K in Mn2NiGa to about 220 K in Ga2MnNi due to the reducing exchange interaction between 3d electrons in the transition metal. The substitution of Mn by Ga has a significant influence on the coupling interaction among various atoms, which leads to the firstly increasing and then decreasing of the saturated magnetization at low temperature. The differential scanning calorimeter shows that the melting temperature decreases gradually as x increases. Meanwhile, the transition temperature from parent phase (B2) to Heusler phase decreases at first and increases latter.Next, we study the magnetic properties of Mn50Ni40In10-xSbx (x=0,1,3,5) alloys and the thermal history effect on the magnetization behavior and magnetic entropy change of Mn50Ni40In9Sb1. It indicates that the overall magnetization of austenite gradually decreases and that of martensite increases with the increase of Sb content. This change is mainly originated from the enhanced p-d hybridization between p orbit in main-group element and d orbit in transitional metal, which manipulates the magnetism of Mn50Ni40In10-xSbx alloys. The magnetization behavior, the critical magnetic field for martensite-to-austenite transformation and the magnetic entropy of Mn50Ni40In9Sb1 strongly depend on the thermal history effect. The continuous heating mode shows higher critical field and larger magnetic entropy change than that of discontinuous heating method. We also study the influences of Sb content on the structure, pahse transition and magnetism of Mn41Ni46In13-xSbx alloys. Substitution of In by Sb has a significant influence on the p-d hybridization interaction, leading to a reverse trend of the magnetization of austenite and martensite for increasing Sb content.Then, we investigate the structure and phase transition of Ni55-xCuxFe18Ga27. With increasing Cu content, the martensitic transformation temperature of arc-melting Ni55-xCuxFe18Ga27 alloys reduces gradually from 292 K to 125 K. Substitution of Ni by Cu has a significant influence on the exchange interaction among neighboring atoms in the transition metals, leading to increasing of Curie temperature, except for some individual cases. The melt-spinning samples display pure L21-type. The increasing lattice constant causes the decrease of d-d exchange interaction between 3d electrons in the transition metals, which plays an important role in reducing the martensitic transformation temperature. We also study the influences of Mn-doping and Cr-doping on the martensitic transformation of Ni55Fe18Ga27 alloy.At last, we calculate the band structure, magnetism and density of states of half Heusler compounds CoCrZ (Z=Si, Ge, Si0.5Ge0.5,Si0.5P0.5, P, As) and discuss the influence of main-group element on the half-metallic properties of CoCrZ.It is found that the replacement of Ge for Si in CoCrSi can adjust the position of the Fermi level, while the substitution of P for Si can effectively adjust the magnetism without disrupting its half-metallicity. Our results demonstrate that the electronic structure of CoCrZ is mainly dependent on the number of valence electrons of the main group element.