Martensitic Transformation in the Shape Memory Alloy Ni₂MnGa

Project Responsible

Uwe Stuhr

Collaborations

Dr. P. Vorderwisch, Hahn-Meitner-Institut Berlin,
Prof. V.V. Kokorin, Institute of Magnetism, Kiev

Project

Ni₂MnGa is the only Heusler alloy that shows a martensitic transformation within the ferromagnetic phase.  The transition temperature depends strongly on composition and can be adjusted between 200 and 400K.  The transformation is preceded by a pronounced softening of the TA₂-phonon branch. 

In our neutron spectroscopic studies we investigated pre-martensitic phenomena in two single crystals with martensitic transition temperatures close to the Curie-temperature.   These samples are expected to be most sensitive on magneto-elastic effects. 

The martensitic transformation temperatures of the two investigated single crystals were 80K (#1) and less than 10K (#2) below the Curie-temperature.  The strong softening near q=1/3 (in reciprocal lattice units) of the TA₂-phonon in the cubic austenitic phase is shown in the phonon dispersion curves presented in Fig 1.  In Fig. 2 the temperature dependence of the softening of three samples with different transformation temperatures are shown.  The kink in the slope of the curve of sample #1 at the Curie-temperature demonstrates the importance of magneto-elastic interactions in this alloy. 

Fig. 1: Phonon dispersion curves of sample #1.  Note the strong softening of the TA₂-phonon when the martensitic phase transformation temperature T_M=284 K is approached.  The other branches do not show a significant temperature dependence.

Fig. 2: Temperature dependence of the TA₂-phonon energy at q=0.33 for different samples.  The kink in the curve at the Curie-temperature for sample 2 is caused by magneto-elastic interactions

Selected publications

U.Stuhr, P. Vorderwisch, V. V. Kokorin und P.-A. Lindgård,
Premartensitic Phenomena in the ferro- and paramagnetic phases of Ni₂MnGa,
Phys. Rev. B, 56, 14360-14365 (1997)

U.Stuhr, P. Vorderwisch and V. V. Kokorin,
Phonon softening in Ni₂MnGa with high martensitic transition temperature
Journal of Physics: Condensed Matter, 12, 7541-7545 (2000)