The crystal and magnetic microstructure of Ni-Mn-Ga alloys

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Doctoral thesis (article-based)
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Helsinki University of Technology doctoral theses in materials and earth sciences, 6
The crystal structure and magnetic domain patterns of Ni-Mn-Ga alloys are studied in the present thesis work. The crystal structure of the martensitic phases in these alloys is highly dependent on the chemical composition. Single crystal X-ray diffraction shows that five-layered martensite is approximately a tetragonal phase with c < a, seven-layered martensite is orthorhombic and non-modulated martensite is a tetragonal type with c > a. Powder X-ray diffraction refinement suggests that five-layered martensite is a modulated structure with its basic structure having a monoclinic lattice, with the lattice parameter a being slightly different from the parameter b. Two-dimensional X-ray scattering distribution and electron diffraction confirmed that there are two shuffling systems with two modulation wave vectors. The interface, i.e. the macrotwin boundary, is formed between these two domains, which have a nearly orthogonal microtwin plane. This interface consists of two constituent elements, a step and a crossing. The layered martensite, which can be viewed as a periodic microtwin sequence, is not perfect; aperiodic plane faults and other-than nominal periodic microtwins are definitely present. In a multi-variant martensite, each martensitic band consists of internal twins. The 180° magnetic domains arise from the major internal twin variant, which is observed both by Type I and Type II magnetic contrast with a scanning electron microscope. The minor internal twin variants show a zigzag pattern when the c-axis is out of plane. In a two-variant state sample, the 180° magnetic domains follow the c-axis in each twin variant and continue to the neighbouring twin variant by a 90° domain wall, which coincides with the twin boundary. In a single-variant state the 180° magnetic domains are parallel to the c-axis and run through the whole observed surface. Optical observation of the magnetic domain pattern reveals that there is a surface relief associated with the magnetic domains. This surface relief causes the (011) twins to appear as a zigzag pattern when projected on the (010) plane. Such magnetic domain associated surface relief is due to the less strained surface as compared to the bulk during the magnetic shape memory phenomenon. In this thesis work it is also found that the application of an excitation voltage of 20-30kV and the Kα line for Ga are critical factors for obtaining a reliable chemical composition for Ni-Mn-Ga alloys using energy-dispersive spectrometer and wavelength-dispersive spectrometer analysis. It is discovered that there are two shuffling systems and the interface between them consists of step and crossing elements in five-layered martensite. It is revealed for the first time in Ni-Mn-Ga alloys that surface relief is associated with the magnetic domains. This provides a new opportunity to observe the magnetic domain patterns with an optical microscope.
Ni-Mn-Ga alloys, magnetic shape memory effect, martensitic transformation, magnetic domains, X-ray diffraction, high-resolution transmission electron microscopy, EDS analysis, WDS analysis, twin boundaries, microtwins
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