Studies On The Magnetism And Phase Transition Of MnNiAl Alloy

In this thesis,the structure,martensitic transition,exchange bias and magnetic properties of rich-Mn component Heusler alloys have been studied.On the basis of the Mn₂NiAl,Mn and Ni atoms are substituted with Co atom respectively.We study the effects of different doping element and content on the structure,martensitic transformation and exchange bias effect of the alloys.In addition,the Ni atom in the Mn₂NiAl alloy is gradually replaced by the Mn atom to get the Mn₃Al alloy.The structure,magnetic properties and martensitic transition of the alloys are studied by the combination of calculation and experiment.By comparing different methods,it is found that the Mn_2-xCo_xNiAl?x=0-1?series ribbons with pure bcc structure can be prepared by melt-spun techniques.The atomic occupancy of ribbons presents B2 structure.In the process of gradually doping Co atoms,the coupling effect between two magnetic phases disappear due to the increase of saturation magnetization M_S.The exchange bias effect of alloys will decrease and disappear.The addition of Co atom not only makes the magnetic phase change in the alloys,but also induces martensitic transition of the alloys.In other words,Co plays a role in regulating the temperature of martensitic transition.We use the melt-spun techniques to fabricate Mn₂Ni_1-x-x Co_xAl?x=0-0.7?ribbons with pure bcc structure and its atomic occupancy is B2 structure.With the increase of doping contents,the exchange bias effect of alloys will disappear,but appear with the change of saturation magnetization M_S.It is also proved that the strength of ferromagnetic phase will directly affect the coupling effect between two magnetic phases.However,doping Co does not change the superparamagnetic phase as the pinning phase,and there is no martensitic transition in this series alloys.The changes in the structure,magnetic properties and martensitic transition of Mn_2+xNi_1-xAl?x=0,0.25,0.5,0.75,1?series alloys are studied by the combination of calculation and experiment.It is found that martensitic transition can occur in this series of alloys except x=1.However,the driving force of martensitic transition?E decreases with the increasing Mn incorporation,and the martensitic transition can not occur at x=1.We prepare Mn_2+xNi_1-xAl?x=0,0.25,0.5,0.75,1?series ribbons.When x=0,0.25,0.5,0.75,the ribbons are pure bcc structure,and the atomic occupying is B2 structure.When x=1,the alloy ribbon is?Mn structure with a little miscellaneous phase.It is found that no martensitic transition occurs in the Mn_2+xNi_1-xAl?x=0,0.25,0.5,0.75,1?alloys.The difference results between calculation and experimental are due to the insufficient driving force of martensitic transition in the actual samples and the chaotic occupying of the Mn atoms.