The Studies On Synthesis, Structure And Physical Properties Of Heusler Alloys With The High Content Of Mn

Heusler alloys are rich in physical property and applicable function. They have exhibited great scientific value and potential application in many high technology fields,such as information storage, magnetic sensor and magnetic refrigeration. With the discovery of the ferromagnetic shape memory effect in Mn₂ NiGa alloy, significant attentions have been attracted on the Hg2CuTi-type Heusler alloy. In this dissertation, we investigated systematically Mn₂ Ni Al alloy in theory and experiment. The synthesis, structure, magnetic properties, antisites and martensitic transformation of the Mn₂ NiAl alloy are studied in details. And the main results are as follows :The Mn₂ Ni Al alloys with pure b.c.c structure are successfully synthesized by the melt-spun method with a higher linear velocity of 34 m/s. However, there are some interatomic antisites in the melt-spun samples and a martensitic transformation is not observed in experiments. It is found that with the increase of velocity, the saturation magnetization increases, but the coercivity decreases. The first principles calculations show that the decrease of saturation magnetization originates from the Mn（B）-Ni（C） antisites.Mn₅₀Ni_25-xCox Al25（x = 1¹5） and Mn₅₀Ni₂₅CoxAl25-x（x = 1⁴） have been successfully synthesized by the melt-spun method. The magnetic measurement results show that the saturation magnetization increases with the increase of Co content. The martensitic transformation is also not observed in the Co-doped Mn₂ NiAl alloys. The first principles calculations show that the Co dopants do not change the magnetic structure of the Mn₅₀Ni_25-xCoxAl25. While, for Mn₅₀Ni₂₅CoxAl25-x alloy, the Co atoms are preferential to occupy the A site, the magnetic moment of Mn（A） is still antiparallel to that of Mn（B）, but Mn（B） and Mn（D） shows a ferromagnetic coupling.