Study On The Phase Transformation,Magnetic Properties And Transport Properties Of Co-V Based And CoCuMnSb Heusler Alloys

Heusler alloys are a remarkable class of intermetallic materials,initially discovered by Fritz Heusler in 1903,have grown into a family of more than 1500members,synthesized from combinations of more than 40 elements.As an important category of members for Heusler family,most of Co-based Heusler alloys have attracted much attention,due to their 100%spin polarization at the Fermi surface,relatively higher Curie temperature and adjustable magnetic moment.Especially,many of Co-based Heusler alloys exhibit high thermodynamic stability,as well as extremely excellent lattice matching to some of nonmagnetic semiconductors or insulators.Some of these alloys can maintain high spin polarizability even in the disordered B2 phase.Recently,a martensitic transformation from a highly ordered L2₁-type cubic structure to a D0₂₂ tetragonal martensitic structure has been established in Co-based Heusler alloys with off-stoichiometric or substitution of sp elements.The metamagnetic martensitic transformation associated with the magnetostrain and the considerable magnetic entropy change of about 9.6 J/kg K have been obtained under the magnetic field change of 3 T in Co₅₀V₃₄Ga₁₆ Heusler alloy.Considering the unique feature and limited solubility of Co-V-Ga system alloys,under the scheme of sp or transitional ferromagnetic elements substitution for corresponding elements,the polycrystalline alloys were prepared by arc melting method,and the multiple physical properties were tested by XRD,PPMS,etc.The effects of sp or Fe elements on the crystal structure,thermodynamic stability,magnetism,electronic structure and atomic spin polarization of the Co-V-Ga system alloys were investigated insight from the first-principles calculations.Finally,we designed a heterostructure and magnetic tunnel junction based on quaternary Co Cu Mn Sb Heusler alloy,and studied the transport properties of the devices with the method of non-equilibrium Green's function,which provide a theoretical foundation for device development.Concretely,the main research contents can be summarized as follows:?1?A series of Co₂VGa_1-xSb_x?x=0,0.25,0.5?polycrystalline alloys were prepared by arc melting method after comprehensive analysis of the physical properties of the Co-V-Ga ternary Heusler alloys.The crystal structure,magnetic properties and electronic structure characteristics of Co-V-Ga-Sb quaternary alloy were systematically studied.The results indicate that the studied alloy is still stabilized in highly ordered cubic structure and preserves 100%spin polarization under the condition of 25%Ga replaced by Sb atoms.Furthermore,the origin of the gap in the minority band stems from the spin splitting of Co-Co atoms,which is really different from Co₂VGa alloy.Martensitic transformation was observed at about room temperature when half number of Ga is substituted by Sb.Moreover,the substitution of Sb for Ga increases the magnetic moment,especially increase the magnetic moments of V atoms.?2?Based on the previous work,in order to increase the magnetic moments and probing the multifunctional features of this category of alloys,we further increase the V content by simultaneously replacing Ga atoms by V and Sb atoms.The results of structural analysis by XRD and PPMS indicate that the Co₅₀V₃₃Ga_17-xSb_x?x=0,1,2?series alloys are stable in L2₁ cubic structure at room temperature,and both the Co₅₀V₃₃Ga₁₆Sb and Co₅₀V₃₃Ga₁₅Sb₂ alloys behave martensitic transformation characteristics.However,the metamagnetic martensitic transformation,shape memory effect and the large magnetic entropy change are only observed in Co₅₀V₃₃Ga₁₆Sb alloy.The maximum strain reaches 0.33%at 100 K under the magnetic field of 3 T.These unique features indicate that the potential applications in shape memory and sensor device are expected.?3?For further improvement of the magnetic properties and structural stability of these Co-V-Ga system alloys,Fe-substituted Co₂VGa alloys have been investigated based on the first-principles calculations.The results indicate that the alloys exhibit thermodynamic stability when Fe partial substitution for Co,V or Ga atoms.The structure is stable in highly ordered L2₁cubic structure in the case of Fe partial substitution for Co atoms.Unfortunately,in this case,the magnetic moment must be sacrificed to maintain high spin polarizability.It was very meaningful that the magnetic moment can be enhanced and spin polarizability will be preserved under the condition of Fe partial substitution for V atoms.Especially,the martensitic transformation is also predictable in Co₂V_0.5Fe_0.5Ga alloy.Coincidentally,high spin polarizability at the Fermi level also can be proved in Fe partial substitution for Ga atoms,and Co₂VFe_0.5Ga_0.5 alloy also provides the character of martensitic transformation.All the above excellent features indicate that a series of Co-V-Fe-Ga alloy is expected to be multifunctional materials.?4?The Co Cu Mn Sb?CCMS?Heusler alloy was fabricated by arc-melting method.The CCMS/Ga Sb heterostructure and CCMS/Ga Sb/CCMS magnetic tunnel junction?MTJ?were designed based on the structural features of CCMS.The architecture exhibits effective spin filtering and the MTJ produces a dramatic magnetoresistance ratio.These outstanding properties mean that CCMS should be applicated in spintronics.This research also provides strong theoretical support for experimental design and device fabrication.