| Recently a lot of new physical phenomena have been observed in Heusler alloys, such as strong ferromagnetism, the large magnetostriction, magneto-structural phase transition, as well as shape memory controlled by temperature and magnetism. Due to the above novel characters, this kind of compounds have a wide range of applications in magnetic recording, the realization of spin electronic devices(i.e., magnetic tunnel junction or the low-field magnetoresistive sensor), aerospace, medical, engineering and other fields. Also, because the Curie temperature of these alloys is mostly higher than room temperature, they are regarded as an important part of both condensed matter physics and materials science.Based on the above information, we investigated the Heusler alloys, which have both ferromagnetism and prospective thermoelastic martensitic properties. The crystalline NiFeMnGa and FeMnGa alloys were prepared by arc melting of high-purity metals under argon atmosphere. The ribbons were made from the ingots by melt spinning method.In Chapter 1, we summarized the latest research progress in the field of Heusler alloys and ferromagnetic shape memory alloys at home and abroad. The motivation and proposals are also presented in the end of this chapter.In chapter 2, it involves the main methods and principles of our experiments, including sample preparations, structure detection using X-ray Diffraction (XRD), four-point-probe Measurement for electrical transport, Superconducting Quantum Interference Device (SQUID) for magnetization, etc. Besides, the functionalities and significance of the experimental means in condensed matter physics and other related research fields are also presented in this chapter.In chapter 3, we study the crystal structure and magnetic properties of NiFeMnGa alloys. Structure phase of the sample was detected by XRD data. The L21 structure is observed in the whole composition range. The lattice constant increases slowly with the addition of Fe, from 5.824A in Ni2MnGa to 5.834A in Fe2MnGa.Curie temperature and saturated magnetization as a function of Fe content for Ni50-xFexMn25Ga25 melt-spun ribbons. Both decrease with Fe increasing due to the antiferromagnetic coupling between Fe-Mn atoms.In chapter 4, the magnetic and electrical transport properties of ternary Fe-Mn-Ga melt-spun ribbons are investigated. X-ray diffraction patterns of Fe2MnGa and Fe5oMn17Ga33 melt-spun ribbons (L21 structure) show that the lattice constant of Fe5oMn17Ga33 is larger than that of Fe2MnGa because of the larger atomic radius of Ga. The Curie temperature and saturated magnetization increase with the decreasing content of Mn. The electrical transport behaviors at low temperature show negative coefficient of resistance, while at high temperature, there is also an inflection point on the R-T curve. In Chapter 5, a summary is given for this work. |
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