Magnetism And Phase Transition In Ni-Mn Based Heusler Alloys

In recent years,Ni-Mn-based ferromagnetic shape memory alloys have become the hotspot in the field of material science and condensed matter physics.These alloys not only exhibit conventional shape memroy effect controled by temperature,by also show field driven martensitic transformation.Owing to the strong coupling between the lattice and magnetism,a number of changes is observed in the vicinity of martensitic transformation:the lattice parameter or the volume of the cell,a peak in specific heat,the change of magnetic state causing the elastic deformation of the lattice,and the change of magnetic order degree leading to the increase or decrease of magnetic entropy.Because of the field driven martensitic transformation,some interesting phenomena,such as the magnetocaloric effect,magnetoresistance effect,magnetostriction effect,have been reported near martensite transition temperature in these alloys.Moreover,these alloys have been found to show exchange bias effect at low temperature,due to the coexistence and competition of ferromagnetic and antiferromagnetic exchange interactions.This article is mainly devided into three parts:(1)The phase transitions,magnetism and exchange bias effect in Ni43Mn46Sn11-xAlx alloy.In Ni-Mn-based ferromagnetic shape memory alloys,cell volume is an important fact affecting the martenstic trasformation and magnetism.In Ni43Mn46Sn11-xAlx alloys,with the increase of Al content,cell volume decreases,and the martensitic transformation temperature increases gradually,indicating the increase of martensitic transformation temperature by lattice contraction.The saturation magnetization of Ni43Mn46Sn11-xAlx alloy material gradually decreases with increase of Al content,which implies that Al dopping can enhance the antiferromagnetic interaction.Furthermore,we investigate both field-cooled and zero-field-cooled exchange bias in Ni43Mn46Sn11-xAlx alloy,and found that alloys exhibit obvious field-cooled field-cooled,while only several alloys show zero-field-cooled exchange bias.These results imply that zero-field-cooled exchange bias may be achieved by adjusting ferromagnetic and antiferromagnetic interactions through modulation lattice.(2)Phase transitions and multifunctional properties in Mn50Ni38Co3Sn9 alloys.In Ni-Mn-based alloys,the magnetism is mainly supplied by Mn,so improving the Mn content will help increase the magnetization,and enhance the multifunctional properties.We systematically studied the phase transntions,field-induce transition,magnetocaloric effect,magnetoresistance,and magnetostrictive effect in Mn50Ni38Co3Sn9 alloy.We found that the crystal structure at room temperature is a mixture of the martensite tetragonal L10 structure and the Hg2CuTi structure,suggesting that martensitic transition temperature is close to room temperature.This alloy not only exhibits large magnetocaloric effect near room temperature,but also large magnetoresistance and magnetostrictive effect.At 276 K,the magnetic entropy change reached a maximum of 24.2 J/kg·K in the field of 50 kOe.At 270 K,the maximum magnetic strain is equal to 6525.8 ppm.At 275 K,the maximum magnetoresistance is-30.7%.(3)Study on the magnetic modulation and exchange bias effect of Ni50-xFexMn36Gai4 alloy.We studied the crystall structure,phase transition,and exchange bias effect in Ni50-xFexMn36Ga14 alloys.It is found that the mother Ni50Mn36Ga14 alloy exhibits a spin glass gound state,showing zero-field exchange bias effect with exchange bias field of 1228 Oe.The substitution of Ni by Fe enhances the ferromagnetic exchange interaction,and increases the spin glass transition temperature.For x=16,the ground state transforms to a reentrant spin glass.All Ni50-xFexMn36Ga14(x=0,6,10,12,16)alloys exhibit exchang bias effect in field cooling condition.However,zero-field exchange bias was gradually surppressed by Fe dopping,and disappears completely for x=16.