Study On The Physical Properties Of Mn₅₀Ni_50-xSn_x Ferromagnetic Shape Memory Alloys

Ferromagnetic shape memory alloys （FSMAs） are new functional materials. They havelots of potential applications in many fields because of their good properties. In recent years,with the emergence of new materials, researchers have produced great interest in them.However, studies show that some of the FSMAs have some shortcomings. Researchers aremaking good efforts to develop new materials and investigate in detail their new properties soas to make up the deficiencies of the old materials. After the appearance of the Mn2NiGa alloy,researchers have made some calculations on the structures and the martensitic transformationof Mn2NiSn and other Mn₂Ni-based alloys on the basis of first-principles calculations. Theypredicted that Mn₂NiSn may be the next FSMA. However, there have so far been noexperimental studies of Mn2NiSn.A series of Mn₅₀Ni₅₀-x)Snxalloys were prepared by arc melting and melt spinningtechniques. And the alloys with different preparation conditions and compositions werestudied. The martensitic transformation temperature was measured by differential thermalanalysis （DTA） and differential scanning calorimeter （DSC）. The crystal structure wasdetermined from x-ray diffraction （XRD） patterns. Magnetism measurements were performedon a physical property measurement system （PPMS）. The intrinsic mechanisms of thestructural stability were calculated by first-principles calculations. Some properties ofMn₅₀Ni₅₀-xSnxwere studied by means of data analysis. The main research contents are asfollows:1. A series of Mn₅₀Ni₅₀-xSnx（0≤x≤25） alloys were prepared by arc melting technology.The experiment results show that all the samples were single phase with a Hg2CuTi type cubicstructure for the parent phase （austenitic phase） and a tetragonal structure for the martensiticphase. The valence electron concentration decreases when Sn is substituted Mn in theMn₅₀Ni₅₀-xSnxsystem. Accordingly, the martensitic transformation temperature decreases withincreasing Sn content, which is shown as the reduction by about72K of the martensitictransformation temperature by increasing the Sn concentration by1at.%. When the Sn content increases to12at.%, the martensitic transformation of the sample disappears and thesample is a cubic structure all the time. The value of the Curie temperature of the austeniticphase, marked as TCA, hardly changes with the Sn content. Thus, we can low the martensitictransformation temperature down below the TCA. In this way, the ferromagnetic shape memoryalloys Mn₅₀Ni₅₀-xSnx（9≤x≤11） are obtained. The thermal hysteresis decreases withincreasing Sn content in the Mn₅₀Ni₅₀-xSnxsystem. For Mn₅₀Ni40Sn10alloy, the thermalhysteresis is only8K. In addition，magnetic field induced martensitic transformation,magnetoresistance effect and Hall effect were observed in Mn₅₀Ni₄0Sn10alloy with dT/dH=2K/T and MR=37%.2. A series of Mn₅₀Ni₅₀-x-ySnxCoy（4≤x≤11，1≤y≤16） alloys were prepared by thecombination of arc melting and melt spinning techniques. The doping of the Co and thedecrease of the Sn can obviously enhance theTAC and the martensitic transformationtemperature of the system. Accordingly, the high temperature FSMA Mn₅₀Ni32Sn6Co12isobtained with a martensitic transformation temperature of about530K. Mn₅₀Ni31Sn5Co14hasthe second phase.3. Mn₅₀Ni_41-ySn₉Co_y（0≤y≤6） alloys possesses exchange bias effect at low temperature,and the exchange bias field and the coercivity depend on the Co content. The exchange biaseffect of the Mn₅₀Ni_50-x-ySnxCoyalloys is thought to originate from the interface exchangeinteraction between the spin glass phase and the ferromagnetic martensitic phase.4. The structural stability of the Mn₅₀Ni₅₀-xSnxsystem is analyzed on the basis offirst-principles calculations. With the help of the electron localization function and theelectron density, we find that the bonding characteristics and the electron density of theMn₅₀Ni₅₀-xSnxalloys are related to the Sn content. The band structure and density of statesshow that the electrons of Mn₅₀Ni25Sn₂₅are localized on a larger scale than Mn₅₀Ni31.25Sn18.75and Mn₅₀Ni₅₀. The splitting of the peak exactly at EFin the density of states pattern makeMn₅₀Ni25Sn25more stable than Mn₅₀Ni₅₀.