Experimente an der Neutronenquelle des Hahn-Meitner-Instituts, Berlin.

• Research unit: EU RFP
• Project number: 96.0436-3

Key words (English)

Martensitic transformation; soft phonons; neutron scattering

Alternative project number (English)

EU project number: ERBFMGECT950

Research programs (English)

EU-programme: 4. Frame Research Programme - 10.1 Stimulation of training and mobility

Short description (English)

See abstract

Partners and International Organizations (English)

Hahn-Meitner Institut

Abstract (English)

The objective of the project was to investigate the structure and the dynamics (phonons) of Ni2MnGa by neutron scattering.
Ni2MnGa is the only known Heusler alloy that performs martensitic transformations in the ferromagnetic phase. The transformation is preceded by a strong softening of the transversal TA2-phonon. The martensitic transformation temperature varies strongly with stoichiometry and can be adjusted between ~200 and ~400K (whereas the Curie temperature varies only between ~350 and 380K). Previous studies were performed on samples were the martensite trans-formation temperature was much lower than the Curie temperature. In this study we investigated a sample were the martensite phase transition temperature and the Curie-temperature differs only by 10K. In this situation magneto-elastic interactions are expected to be most pronounced.
We investigated the structure and the phonons of this sample at the triple-axis spectrometer V2 (FLEX) at the Hahn-Meitner Institute in Berlin. Additionally, measurements of the magnetisation and the specific heat of the alloy were performed.
Most important results of the study:
The temperature dependence of the phonon softening in the high temperature phase is weaker than in the samples investigated in previous studies.
In contrast to the results of the previous experiments the phonon branch becomes soft in a very broad region (which might be a result of short coherency lengths of the soft phonons)
An additional intermartensitic phase transition was found at ~60 K below the martensite transition, however, this transformation was not observed in the measurements of the magnetisation and the specific heat.
Precursers of the Bragg-reflections can be found at least up to 30K below or above the phase transition temperatures.
This study was performed in collaboration with Dr. P. Vorderwisch (Hahn-Meitner Institut, Berlin) and Prof. Dr. V. V. Kokorin (Institute of Magnetism, Kiev, Ukraine).

References in databases (English)

Swiss Database: Euro-DB of the
State Secretariat for Education and Research
Hallwylstrasse 4
CH-3003 Berne, Switzerland
Tel. [phone]
Swiss Project-Number: 96.0436-3