Influence of chemical ordering on field-induced strain

1. Influence of chemical ordering on field-induced strain in Ni-Mn-Ga Ferromagnetic shape memory alloys Robert C. O’Handley, Massachusetts Institute of Technology, DMR 0530829 Background: Ni50+xMn25+yGa25-x-y ferromagnetic shape memory alloys show 6% (tetragonal) or 10% (orthorhombic) field-induced strain. But threshold field for actuation ranges from 1 to 4 kOe; yield stress from 0.5 - 2 MPa. Anelasticity suggests defect mobility. Could stress fields play a role In twin-boundary pinning? Results Our neutron diffraction experiments (at Grenoble & Los Alamos ) show: For x  0, excess Mn occupies Ni and Ga vacancies For x > 0, Ni occupies Mn sites, excess and displaced Mn go to Ga sites (2) (1) These site occupations allow accurate calculation of observed magnetic moments; Mn-Mn nearest neighbors couple antiferromagneticly (1) Experiments underway to probe correlation of large strain in > 50% Ni alloy with enhanced threshold field, Time dependent actuation.

2. Influence of chemical ordering on field-induced strain in Ni-Mn-Ga Ferromagnetic shape memory alloys Robert C. O’Handley, Massachusetts Institute of Technology, DMR 0530829 Significance: Ni-Mn-Ga ferromagnetic shape memory alloys show potential as active materials, transducers, positioners, mechanical energy absorbers. But threshold field for actuation ranges from 1 to 4 kOe; yield stress from 0.5 - 2 MPa. Two copanies are currently exploring applications. Heusler alloys are also of interest as spin filters, where site selection affect magnetic and other properties. Collaborations: In addition to the primary student, Marc Richard (now a postdoc) on this program, another of our grad students, Ratchatee Techapiesancharoenkij, has carried out neutron diffraction measurements at Los Alamos. Further, the diffraction in Grenoble was assisted by the group of Barandiarán (particularly Patricia Lázpita) of Univ. of Basque Country, where Marc has visited to analyze the data. These international and transnational research experiences are invaluable for student development. Results of this research are used in the graduate class on Magnetic Materials at MIT, often attended by undergraduate students and students from other universities. Relevant publications 1) “Crystal structure and transformation behavior of Ni–Mn–Ga martensites”, M. Richard, J. Feuchtwanger, D.Schlagel, T. Lograsso, S.M. Allen, R.C. O’Handley, Scripta Materialia54, 1797 (2006). 2) “Chemical ordering in off-stroichiometric Ni–Mn–Ga ferromagnetic shape-memory alloys studied with neutron diffraction”, M. L. Richard, J. Feuchtwanger, P.Lazpita, J.M. Barandiaran, J. Gutierrez, B. Ouladdiaf and C. Mondelli, S. M. Allen, R.C. O’Handley, Philosophical Magazine87 (23), 3437, (2007). 3) "Martensite Transformation in Ni-Mn-Ga Ferromagnetic Shape-Memory Alloys", Marc Richard, Jorg Feuchtwanger, Patricia Lazpita, J.M. Barandiaran, Samuel Allen, and R. O'Handley, Metallurgical and Materials Transactions A 38 (4) 777-780 (2007) Relevant invited talks 1) Euro. Magn Sensors and Actuators, Bilbao, Spain, July 3, 2006 2) Czech/Slovak Intern’l Conf on Magnetism, Kosice Slovakia, July 9, 2007 3) EMRS, Warsaw, Poland, Sept. 18, 2007 4) MRS, Boston, MA, Nov. 26, 2007