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Magnetic and structural properties of non-stoichiometric Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

Patricia Lazpita 1Jose Manuel Barandiarán 1Jon Gutierrez 1Marc Richard 2Samuel M. Allen 2Robert O'Handley 2

1. Department of Electricity and Electronic, University of Basque Country (UPV/EHU), P. Box 644, Bilbao 48080, Spain
2. Massachusetts Institute of Technology, Department of Materials Science and Engineering (MIT), Cambridge, Boston, MA 02139, United States


Ferromagnetic shape memory alloys are able to develop large magnetic field induced strains, up to 10% in non stoichiometric Ni-Mn-Ga alloys, making them very interesting for basic studies and applications. Structural and magnetic transition temperatures of those alloys present a strong dependence on slight departures from the stoichiometry, as does the mobility of twin boundaries responsible for the large magnetic field induced strains.

In this work we study four non stoichiometric Ni-Mn-Ga polycrystalline alloys with compositions of 43-52 at. % nickel, excess manganese and deficient in gallium, and a single crystal of composition Ni52Mn26Ga22. Those compounds are of technical interest due to the observed large room temperature magnetic field induced strains. We report the characterization of martensitic transition temperatures and determination of martensitic phase structure of these alloys in order to complete previous neutron diffraction results on the structures of the austenitic phases.

Calorimetric and magnetic measurements determined the martensitic transition and Curie temperatures of the alloys (As =331 K and TCurie=366 K for 52 at. % nickel alloy). Nickel defective alloys present a martensitic transition region broader than excess nickel ones. This fact is directly related to the structures of the respective martensitic phases. Neutron powder diffraction analysis confirmed orthorhombic martensitic structures for nickel defective alloys (cell parameters for 49 at. % nickel are a=4.217Å, b=4.213Å and c=5.548Å), and tetragonal for excess nickel ones (cell parameters for 52 at. % nickel are a=4.208Å and c=5.561Å). In the last case the crystallographic structure of the martensitic phase was also obtained on a single crystal with the same composition, trained to get a single variant, and it agrees with the determined in the powder sample.


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Related papers

Presentation: Poster at E-MRS Fall Meeting 2007, Symposium E, by Patricia Lazpita
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-14 11:11
Revised:   2009-06-07 00:44