Boundary Motion And Thermal Hysteresis Of Ferromagnetic Shape Memory Alloy NiMnGa

Ferromagnetic shape memory alloys is a new magnetic driving functional material. Itis one of the focuses in the field of the condensed states and materials, because it is notonly having high-driving force and large magnetic field induced strain relative thetraditional shape memory alloys, but also having high response of magnetostrictivematerials. NiMnGa alloys are the typical ferromagnetic shape memory materials, theyhave largely magnetic-field-induce strain and two-way thermoelastic shape memoryeffect controlled by magnetic field, their martensitic transformations exhibit variousphysical properties. Mechanism of large magnetic field induced strain relating to themartensitic transformation, which obtain maximum in the temperature of the martensitictransformation has just completed. Depending on the appropriate composition, thismaterial shows also the premartensitic transition before occurring the martensitictransformation or intermartensitic transformation after occurring the martensitictransformation. Premartensitic transition and martensitic transformation, as well asmartensitic transformation and intermartensitic transformation can happensimultaneously in some materials. Due to unique physical properties, the ferromagneticshape memory alloys have a broad application prospects.Different single crystals are grown by Czochralski method in this work. Thephysical characteristics were measured by various measuring method, such asalternating-current magnetic susceptibility, differential scanning calorimetry (DSC),and X-ray. Main experimental results are exhibited as follows:Based on some experimental data and the thermodynamical theory, the differentenergies relative to the thermoelastic martensitic transformation, for threeoff-stoichiometric NiMnGa single crystal samples with the transformation temperaturebelow, near and above room temperature, respectively, were calculated. The influence ofthe different energies to the thermoelastic transformation behavior was discussed. Theresults show further that, for a completely thermoelastic martensitic transformation, theslope of the branch of the transformation loop comes from the elastic energy, and thethermal hysteresis results from the friction of interfacial motion between the parent phase and martensitic phase.Single crystal of Ni50.3Mn24.5Ga25.2were annealed and quenched using various heattreatments. Then the samples were measured with X-ray, DSC, and alternating-currentmagnetic susceptibility. It is found that the anneal treatment induces slightly low of themartensitic transformation temperature, greatly low of the premartensitic transformationtemperature, but the Courier temperature of the Ni50.3Mn24.5Ga25.2single crystal is notinfluenced. Moreover, low temperature quench induces the low of the Couriertemperature, high temperature quench has not influence the Courier temperature. Themartensitic transformation temperature and premartensitic transition temperature areinfluenced by the quench treatments. Furthermore, the magnetization curves at varioustemperatures of the Ni50.3Mn24.5Ga25.2single crystal were measured. It is found that themartensitic phase is hard to magnetization to saturation, which reflects that themartensitic phase has large magnetocrystalline anisotropy...