Atomistic simulations of magnetic shape memory alloys

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Doctoral thesis (article-based)
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47, [49]
Dissertations / Laboratory of Physics, Helsinki University of Technology, 119
Magnetic shape memory (MSM) alloys are novel smart materials which exhibit magnetic field induced strains of up to 10 %. As such they have potential for many technological applications. Also, the strong magneto-structural couplings of the MSM effect make the phenomenon very interesting from a scientific point of view. In this thesis, materials and properties related to the MSM effect are studied with atomistic simulations. Main interest is in the known MSM alloy Ni-Mn-Ga around the Ni2MnGa stoichiometry. One pre-requisite for the MSM effect is the existence of a structural transformation in a magnetic material, and therefore some candidate materials are investigated from this perpective. Here, Ni2MnAl is found to have potential for further studies. The magnetic moment is seen to originate mainly from Mn in the Mn-containing alloys and the existence of different structural phases is ascribed to a band Jahn-Teller effect in the Ni band. This picture is confirmed by comparisons between theoretical and experimental neutron diffraction results. In Ni2MnGa the structural phase transformations are found to be driven by vibrational entropy at finite temperatures. The magnetic key property in the MSM effect is the magnetic anisotropy energy which is studied in Ni2MnGa. The tetragonal structure with c/a = 0.94 is magnetically uniaxial characterized by the first anisotropy constant, but in the presence of several twin variants only the second anisotropy constant may be observed in the measurements. Analysis of the microscopic origins of the magnetic anisotropy shows that Ni has the largest contribution to the magnetic anisotropy energy. Investigations of other structures show that in Ni2MnGa the shortest crystal axis is always the easy axis of magnetization. From other magnetic properties, the Curie temperatures of Ni2MnGa and Ni2MnAl are estimated on the basis of total energy calculations of spin spirals. Ni is found to have an important effect also on the Curie temperatures despite its smaller magnetic moment when compared to Mn. Non-stoichiometric compositions of Ni-Mn-Ga are studied within the rigid band approximation and with supercell calculations. In some cases the rigid band approximation describes the correct trends, but more insight into the alloying effects can be obtained from the supercell calculations. The most important result of these investigations is that in Mn-rich compositions the extra Mn atoms couple antiferromagnetically to the neighbouring Mn atoms. This result implies a decrease of the total magnetic moment with Mn-doping. Also, all the experimentally observed martensite phases are explained theoretically when the extra Mn is explicitly included.
magnetic shape memory, martensitic transformation, magnetic anisotropy
Other note
  • Ayuela, A., Enkovaara, J., Ullakko, K. and Nieminen, R. M., 1999. Structural properties of magnetic Heusler alloys. Journal of Physics: Condensed Matter 11, pages 2017-2026. [article1.pdf] © 1999 Institute of Physics Publishing Ltd. By permission.
  • Ayuela, A., Enkovaara, J. and Nieminen, R. M., 2002. Ab-initio study of tetragonal variants in Ni<sub>2</sub>MnGa alloy. Journal of Physics: Condensed Matter 14, pages 5325-5336. [article2.pdf] © 2002 Institute of Physics Publishing Ltd. By permission.
  • Enkovaara, J., Ayuela, A., Nordström, L. and Nieminen, R. M., 2002. Structural, thermal and magnetic properties of Ni<sub>2</sub>MnGa. Journal of Applied Physics 91, No. 10, pages 7798-7800. [article3.pdf] © 2002 American Institute of Physics. By permission.
  • Enkovaara, J., Ayuela, A., Nordström, L. and Nieminen, R. M., 2002. Magnetic anisotropy in Ni<sub>2</sub>MnGa. Physical Review B 65, 134422, 6 pages. [article4.pdf] © 2002 American Physical Society. By permission.
  • Enkovaara, J., Ayuela, A., Jalkanen, J., Nordström, L. and Nieminen, R. M., First principles calculations of spin spirals in Ni<sub>2</sub>MnGa and Ni<sub>2</sub>MnAl. Accepted for publication in Physical Review B.
  • Enkovaara, J., Heczko, O., Ayuela, A. and Nieminen, R. M., Coexistence of ferro- and antiferromagnetic order in Mn-doped Ni<sub>2</sub>MnGa. Submitted for publication in Phys. Rev. Lett.
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