The Strain And Magnetic Entropy Change Of Ferromagnetic Shape Memory Alloy NiMnGa

Ni-Mn-Ga alloys are intermetallic compounds possessing the characteristic of ferromagnetism and thermoelasticity of martensitic transition. They are not only having the two-way shape memory effect controlled by magnetic field, but also possessing large magnetic-field-induce strain. It is found that their martensitic transformations exhibit various characteristics. Depending on the different composition, the materials also have the intermartensitic transformation and premartensitic transition, except the martensitic transformation. In some materials, martensitic transformation and intermartensitic transformation can happen simultaneously. The magnetic transition and martensitic transformation also could happen simultaneously, accompanying large magnetic entropy change. Therefore, NiMnGa alloys are hopeful for magnetic refrigeration materials excepte for magnetic driven and sensor materials.Different single crystals are grown by Czochralski technique in this thesis. And their magnetic-field-induced strain, shape memory effect, magnetic entropy change and the intermartensitic transformation are studied by various measurements. Some experimental results are obtained as follows:The single crystal Ni₅₁Mn_25.5Ga_23.5 is grown. A large spontaneous two-step shape memory effect with the strain along [001] high to -1.62% is found without a biasing magnetic field. When a magnetic field 1.2T is applied along [001] direction, the transiton strain is enhanced to -2.8%. When the sample is cooled in a dc magnetic field applied perpendicular to the measuring direction of strain, a large and reversible magnetic-field-induced strain up to -1.5% is obtained. This value of -1.5% is approximately two times larger than that detected in zero field cooling. The further analysis indicates that the large transformation strain and magnetic-field-enhanced strain should be attributed to the preferential orientation of the martensitic variants induced by twin boundary motion along the direction of magnetic field.The concurrence of magnetic and martensitic transitions are foud in Ni₄₆Mn₃₅Ga₁₉ single crystal by ac susceptibility and magnetization measurement. Moreover, according to magnetization measurement and Maxwell relation, the magnetic entropy change is caculated under the magnetic field from 0.5T to 1.6T. A large magnetic entropy change of 13.8 J/kg K is obtained under the field of 1.6T, and the temperature of magnetic entropy change is above the room temperature. The peak value of ?ΔS increases linearly with the field at a rate of 9.0 J/kg K T. Furthermore, the alloy exhibits spontaneous and magnetic-field-controlled shape memory effect with the transition strain of -0.89% and -1.90%.A two-step complete thermoelastic martensitic transformation, martensitic transformation and intermartensitic transformation, in Ni_53.2Mn_22.6Ga_24.2 single crystal was studied. The martensitic and intermartensitic transformations exhibit the same transition character under the magnetic field, and this alloy exhibits a magnetic-field-controlled shape memory effect. Moreover, the stress--strain measurements of the single crystal under the magnetic field indicate that the magnetic field not only has an effect on the stress needed for reorientation of the variants during the martensitic transformation, but also makes the initial irreversible transition become reversible.According to growing characteristic of single crystal, magnetics theory and mechanism of shape memory alloy, the above results are analyzed theoretically and explained.