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
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 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
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 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 Materialia 54, 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 Magazine 87 (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) (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