Study On The Physical Properties Mn₂-based Heusler Alloy

Heusler alloys exhibit rich physical properties and many applicable functions. Two of the most important functions are ferromagnetic shape memory effect and half metallic properties. In this dissertation, we will try to develop new Heusler alloys with these two properties and investigate their structure and basic physical properties. We abstract the main content of this dissertation as following:Ferromagnetic Heusler alloy Mn2NiGa has been discovered, which exhibits a martensitic transformation around the room temperature with a large thermal hysteresis up to 50K and a lattice distortion as large as 21.3%. It was also found that Mn2NiGa has a Curie temperature up 588 K being much higher than that of studied Ni2MnGa, whose Curie temperature is about 370 K. The excellent two-way shape memory behavior with a strain of 1.7% was observed in the single crystal Mn2NiGa. The magnetic-field-controlled effect created a total strain up to 4.0% and changed the sign of the shape deformation effectively.Both experimental and theoretical studies have been carried out to investigate the structural and magnetic properties of Mn2NiGa alloys. We have found, instead of forming L21 structure where both A and C sites are occupied by Mn atoms, the alloy favor a structure where C site is occupied by Ni atoms and Mn atoms at A and B sites. The electronic structures of both cubic austenite and tetragonal martensite Mn2NiGa were calculated by self-consistent full-potential linearized-augmented plane-wave (F-LAPW) method. Austenite Mn2NiGa materials show ferrimagnetism due to antiparallel but unbalanced magnetic moments of Mn atoms at A and B sublattice. The magnetic moment of Mn atoms decrease greatly upon martensitic transformation to a tetragonal structure with a 50% reduction in Mn moments at A site and almost completely suppressed Mn moments at B sites. Consequently, martensite Mn2NiGa alloys show ferromagnetic coupling. Different magnetic orderings in martensite and austenite also lead to very different temperature dependence, with which the abnormal behavior of magnetization upon martensitic transformation can be understood. In the off-stoichiometric samples with composition between Ni2MnGa and Mn2NiGa, We show that additional Mn atoms that substitute for Ni atoms in Ni2MnGa have the same magnetic behaviors as Mn in Mn2NiGa phase, which successfully explains the dependence of the magnetization on Mn content. In order to improve the properties of this material and develop new ferromagnetic shape memory alloys, an experiment of partially substituting Ni in Mn2NiGa alloys with Fe or Co was carried out to examine the effect of Fe or Co in quaternary Mn50Ni25-xFe(Co)xGa25 alloys on their structure, magnetic properties, martensitic transformation, and mechanical properties. Some interesting results were found.First-principle FLAPW calculations have been carried out to study the electronic structure and magnetic properties of compounds with Hg2CuTi-type structure. It is found that the compounds with Z=Al, Si, Ge, Sn and Sb are half-metallic ferrimagnet. Experimentally, we successfully synthesized the Mn2CoZ (Z=Al, Ga, In, Ge, Sn, Sb) compounds. Using the XRD measurements and Rietveld refinements, we have confirmed that these new compounds form Hg2CuTi-type structure instead of conventional L21 structure. Based on the analysis on the electronic structures, we find that there are two mechanisms to induce the minority-spin band gap near the Fermi level, but only d-d band gap determines the final width of the band gap. The magnetic interaction is quite complex in these alloys. The competition between the intra-atomic exchange splitting and covalency mechanism dominate the formation and coupling of the magnetic moments. The Mn2CoZ alloys follow the Slater-Pauling rule: MH = NV ? 24 with the varying Z atom. It was further elucidated that the molecular magnetic moment, MH, increases with increasing valence concentration only by decreasing the antiparallel magnetic moment of Mn(C) while the magnetic moments of Mn(B) and Co are unaffected .We successfully synthesized some of Mn2-based Heusler alloys using the melt-spun method. Some basic physical properties of these alloys were also investigated primarily.