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 (A_s =331 K and T_Curie=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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