Influence of Chemical Ordering on Field-Induced Strain in Ni-Mn-Ga Alloys

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TECHNICAL: Ferromagnetic shape memory alloys (FSMAs) have been extensively studied, particularly since the observation of magnetic-field-induced strains of 0.2% in Ni2MnGa crystals at -150C in 1996. More recently strains of 6% and 9.6% have been observed at room temperature in off-stoichiometry, martensitic compositions having tetragonal 5-M structure and orthorhombic 7-M structures, respectively. The mechanism for these large field-induced strains is believed to be twin boundary motion driven by magnetic energy differences and torques across the twin boundaries for certain applied field directions. While phenomenological theories exist that account for the general field dependence of the strain and magnetization, the microscopic mechanisms driving the twin boundary motion are not yet well understood. It is also not clear whether, and if so, how, the chemical order in stoichiometric compositions and chemical order in off-stoichiometric compositions affects the nucleation or kinetics of the partial dislocations, which control twin-boundary motion. This grant supports a multi-faceted experimental and modeling approach to address these issues. Neutron scattering is carried out at NIST (or Los Alamos) to determine the statistical site disorder in typical 5-M and 7-M compositions. These compositions always have excess Mn and deficiencies in Ga with Ni generally ranging from 48 to 51%. This determines where the excess Mn atoms prefer to reside, on Ni or Ga sites. An attempt will be made to identify species that accumulate at twin boundaries and that may account for the observed magnetic field or stress-induced creep observed in these materials. The effects of excess Mn occupation of Ni or Ga sites on saturation magnetization and magnetic anisotropy are studied with differential magnetometry. The Mn atoms in this intermetallic Heusler composition carry the majority of the magnetic moment. Thus, it is expected that whether Mn atoms occupy Ni or Ga sites, they will be closer to other Mn atoms and hence there could be a magnetic moment decrease. The reduced site symmetry in the partially disordered compositions may also lead to a reduction in the magnetic anisotropy, which is critical to field induced twin boundary motion. The application of a small acoustic signal to a Ni-Mn-Ga crystal, directed to induce a shear motion parallel to the twin planes has a significant enhancing effect on magnetic-field-induced twin boundary motion. These changes are investigated with changing stoichiometry. The experimental results are interpreted in terms of evolving microscopic models of partial-dislocation-mediated twin-boundary motion. NONTECHNICAL: This research is expected to have broad impact on the understanding and potential applications of Ni-Mn-Ga and other FSMA, active materials. Further, the education and outreach impact of the research will extend well beyond the training of graduate students. We make significant use of undergraduate students in our research and have extensive worldwide (China, Finland, Ukraine, Spain) and US (UCSD, ASU, Iowa State, UMd) collaborators; many of them will be involved in parts of the proposed work. Finally, our group has been honored with numerous invitations to speak at international conferences on the results of our work in this field (Dresden, Singapore, Cancun, Cardiff, and numerous locations in the US). This provides valuable opportunities for wider dissemination of our results.