Study On Structures, Thermal Stabilities And Magnetic Properties Of Ni-Mn(Fe, Co)-Ga(In)Ferromagnetic Shape Memory Alloys By Ab-initio Calculations

Ferromagnetic shape memory alloys (FSMAs) are novel smart materials which exhibit magnetic field induced strains of up to10%. These alloys possess excellent physical properties and great development potential for devices, especially served as actuators, sensors and magnetic micro-electro-mechanical systems. Also, the strong magnetostructural couplings of the FMSM effect make the phenomenon very interesting from a scientific point of view. In the present work, a series of first-principles calculations have been performed within the framework of the Density Functional Theory (DFT) using the Vienna Ab initio Software Package (VASP).The purpose of the present dissertation is:(1) to investigate the effects of substitution of Mn by Fe or Co, or substitution of Ga by In on crystal structures, phase stabilities and magnetic properties in stoichiometric Ni2Mn(Fe, Co)-Ga(In) alloys;(2) to study the preferred site occupation and magnetic structures of the various point defects, and the effects of various point defects on the thermal stability of the parent phase and martensitic transition temperature in off-stoichiometric Ni2Mn(Fe, Co)-Ga(In) alloys;(3) to discover the effects of Co addition on crystal structure, Curie temperature and magnetic properties of the Ni8-xMn4Ga4Cox (x=0,0.5,1,1.5and2).Firstly, the equilibrium lattice parameters, thermal stabilities, magnetic properties, electronic Density Of States (DOS) and difference charge density of the parent phase of the stoichiometric Ni2XY (X=Mn, Fe, Co; Y=Ga, In) have been systematically calculated. The lattice parameters of the parent phase decrease gradually with the increase in the atomic number of X, whereas the bulk modulus displays an opposite tendency. Substitution Ga by In will lead the lattice parameters incease and bulk modulus decreases. The formation energy investigation shows a destabilization tendency if Mn atom is substituted by Fe or Co; the substitution of Ga by In will also lead the system unstable.With the increase in the atomic number of X, the total and partial magnetic moments decrease gradually; the down spin total DOS at the Fermi level is gradually increasing, whereas those of the up spin part remains almost unchanged. Thus the decrease in the difference of the electronic number between up-spin part and down-spin part gives rise to the decrease in the magnetic properties in these alloys. The partial DOS is dominated by the Ni and X3d states in the bonding region below the Fermi level. The down spin X3d states dominates the antibonding region. The strong bond between the neighboring Ni atoms in the Ni2MnGa is replaced by the bond between Ni and X atoms in other alloys.Secondly, for the off-stoichiometric Ni2XY (X=Mn, Fe, Co; Y=Ga, In), the formation energies and magnetic structures of the several kind of defects (atomic exchange, antisite, vacancy) were calculated. In the antistie type defect, the Y and Ni antisites at the site of the X sublattice (YX and NiX) have the comparable lowest formation energies. Among all the antisite defects, the atomic substitutions on the Ga sublattice (NiY and XY) correspond to the highest formation energy. Therefore, Y constituent can stabilize the structure of the parent phase, whereas the effect of X constituent on the structural stability of the parent phase is opposite. The highest vacancy formation energies are obtained at the Y sublattice, confirmed again the effect of Y constituent on the stabilization of the parent phase. This information is of great importance to guide composition regulation during the fabrication process of these alloys.For most cases of the site occupation, the excess atoms of the rich component directly occupy the site (s) of the deficient one (s), except for Y-rich Ni-deficient composition. In the latter case, the defect pair (YX+XNi) is energetically more favorable. The variation in atomic moments is much larger for Ni than for X in defective Ni2XY. The value of Ni magnetic moment sensitively depends on the distance between Ni and X atoms.The last but not the least, the effect of Co addition on the properties of Ni8-xMn4Ga4Cox (x=0,0.5,1,1.5and2) FMSM alloys were systematically investigated. The added Co atoms prefer to occupy the Ni sites. With the increase in the Co content, the equilibrium lattice parameters for the parent phase decrease regularly, whereas for the martensite phase, the lattice parameters a increases and c decreases, thus leading to a decreased tetragonality c/a ratio. The formation energies show a destabilization tendency with the increase in the Co content for both the paramagnetic and the ferromagnetic austenite. The total energy difference between the paramagnetic and the ferromagnetic austenites increases, which results in the rise of TC when Ni is replaced by Co. With the increase in the Co content, the down-spin total DOS around the Fermi level gradually decrease, whereas those of the up spin part remains almost unchanged, which gives rise to the increase in the total magnetic moment.The present dissertation aimed at providing explanations to the experimental observations of Ni-Mn (Fe, Co)-Ga (In) alloys; revealing physical nature of the properties; and laying a solid foundation for the improvement of the performances of the present alloys and for the design of new promising ferromagnetic shape memory alloys.