Regensburg 2000 – scientific programme

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SYMT: Martensitic Transformations and Shape Memory Effect

SYMT 2: Shape Memory Effect

SYMT 2.3: Invited Talk

Wednesday, March 29, 2000, 17:30–18:00, H16

Premartensitic and Martensitic Phase Transitions in Ni-Mn-Ga — •Ll. Mañosa and A. Planes — Departament d’Estructura i Constituents de la Matèria, Facultat de Física. Universitat de Barcelona, Diagonal 645. 08028 Barcelona. Catalonia (Spain).

Ni-Mn-Ga alloys close to the stoichiometric composition, exhibit unique magnetomechanical properties arising from the interplay between the magnetic and structural degrees of freedom. Indeed they are the only known Heusler (L2₁ structure) ferromagnetic alloys undergoing a martensitic transition. The magnetoelastic interaction results in peculiar behaviour of both the L2₁ phase and the martensitic phase.

The L2₁ phase exhibits pronounced temperature softening of the (1/3 1/3 0) phonon on the TA₂ branch. For those alloys with a martensitic transition well below the Curie point, there is a freezing of this phonon mode which gives rise to a micromodulated bcc structure at a temperature slightly above the martensitic transition. It has been shown that magnetoelastic interaction is responsible for such a freezing, which occurs via a weakly first order phase transition.

At low temperatures, large strains have been reported in unstressed crystals under magnetic fields. They arise form the motion of twin boundaries in the martensitic phase under the driving force of the Zeeman energy difference across the twin boundaries. The possibility of magnetically inducing large deformations is attractive for the potential use of these alloys as actuators.