Structural and electronic properties of Ni-Mn-Ga magnetic shape memory alloys

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In recent years there has been an increased demand for the development of smart and functional materials for integration into actuators, sensors and energy harvesting devices. Magnetic shape memory alloys represent a new type of multifunctional materials, which display a strong coupling between structural and magnetic degrees of freedom, accompanied by a strong magnetocrystalline anisotropy. Thus, exposed to sufficiently high magnetic fields, they show a macroscopic shape change. The effect arises from the reversible rearrangement of the martensitic microstructure. To date, Ni-Mn-Ga based alloys have demonstrated many remarkable properties, like 10% magnetic field induced strain and operating frequencies up to the lower kHz range, thus being of particular technological interest. In order to perform their rational design, the microscopic origin of their functional properties must be understood.

Since the discovery of the magnetic shape memory effect in Ni-Mn-Ga a strong effort has been first of all invested into improvement of the performance of bulk materials. However, the fundamental microscopic properties of these multifunctional materials were not intensively investigated from the experimental point of view. This thesis is devoted to the implementation of surface sensitive techniques for the investigation of structural and electronic properties of Ni-Mn-Ga magnetic shape memory alloys. There is also an emphasis on the fabrication of freestanding Ni-Mn-Ga thin film microstructures, relevant for device applications.

A fabrication process was developed to prepare freestanding single crystalline Ni-Mn-Ga film microstructures. This process is based on electron beam lithography, focused ion beam milling and wet chemical etching. A thermally induced change of the martensitic twin variant configuration was observed.

Temperature dependent scanning tunneling microscopy was used to investigate the surface structure of Ni-Mn-Ga alloys in both austenitic and martensitic phases. Film samples epitaxially grown on MgO(001) substrates and single crystal samples were studied. The microscopic surface structure was addressed from the micrometer scale down to the atomic level. Measurements revealed a well-ordered surface exhibiting a Mn-Ga termination. In the martensitic phase a distinct surface corrugation feature originating from the modulated nature of martensite was observed. Topography measurements demonstrate that martensitic Ni-Mn-Ga exhibits two distinctly different martensitic phases with respect to the nature of the modulation. On the one hand, sinusoidal modulation with incommensurate five-fold period was found for stoichiometric alloys. The periodic lattice distortion is accompanied by a charge density wave. On the other hand, a nearly seven-fold nanotwinned configuration with random stacking faults was identified for alloys with off-stoichiometric compositions. The structural modulation appears to be similar to the hierarchical microstructure predicted by the concept of adaptive martensite.

Electronic properties of the stoichiometric Ni2MnGa compound have been studied by means of angle-resolved photoemission spectroscopy. Fermi surface topology has been examined in the austenitic state. A nesting feature, which has been previously identified in theoretical calculations, has been observed. The nesting vector matches the modulation period of the martensitic structure. Spectra, obtained in the martensitic state, provide a convincing proof of the a pseudogap formation for the nested parts of the Fermi surface. These results are in line with the observation of a charge density modulation by STM and support the assumption of a Peierls like instability. Temperature dependent ultraviolet photoemission spectra demonstrate a similar transfer of spectral weight near the Fermi level across the martensitic transformation for off-stoichiometric and stoichiometric alloys.

It can be consistently accounted for the seemingly conflicting observations of two different Ni-Mn-Ga martensitic microstructures if a strong electron-lattice coupling scenario is considered.

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530 Physik
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Magnetic shape memory alloys, Scanning tunneling microscopy, Structural properties, Electronic properties, Phase transition, Epitaxial films
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ISO 690LAPTEV, Aleksej, 2015. Structural and electronic properties of Ni-Mn-Ga magnetic shape memory alloys [Dissertation]. Konstanz: University of Konstanz
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@phdthesis{Laptev2015Struc-31957,
  year={2015},
  title={Structural and electronic properties of Ni-Mn-Ga magnetic shape memory alloys},
  author={Laptev, Aleksej},
  address={Konstanz},
  school={Universität Konstanz}
}
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July 24, 2015
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Konstanz, Univ., Diss., 2015
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