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Change of microstructure and shape of solid materials by external magnetic fields
DFG Priority Programme 1239 |
| A2: Magnetic, magnetoelastic and dynamical properties of martensitic Heusler alloys: new materials |
| Our consortium Magnetic, magneto-elstic and dynamical properties of martensitic Heusler alloys aims on a better understanding of the fundamental properties of magnetic field induced shape memory effect in Heusler alloys. Prof. M. Acet and coworkers explore the macroscopic magnetic and elastic behaviour of new systems based on Ni-Mn Heusler alloys like Ni-Mn-Sn and Ni-Mn-In, that are expected to show a magnetic shape memory effect with potential technical applications. The guiding idea behind this research is to properly tune the e/a ratio of the alloy by varying the composition. Prof. Entels group supports the experiments by ab-initio calculations of the microscopic and macroscopic properties. The validation of the calculations will be done by our (Dr. Jürgen Neuhaus, Prof. Winfried Petry) detailed phonon measurements as function of composition (i.e. e/a ratio), temperature and applied magnetic field. In addition, our group aims to the understand the kinetics of the twin boundary motion under applied magnetic field and stress by elastic and inelastic neutron scattering. Using time dependant diffraction on rapidly cycled polycrystalline Ni2MnGa (Mn concentration 28 - 30 at%) by means of varying the magnetic field in the martensitic phase, we intend to observe structural and magnetic Bragg intensities as function of time, i.e. the periodic field and applied stress. Alternatively, a slow change of magnetic field over minutes will be applied to investigate the kinetics of twin growth induced by the increasing magnetic field. These studies will be followed by measuring single crystals under similar conditions. The crystals have to be oriented with the habit plane of the twins parallel to the scattering plane. By observing the time dependant evolution of intensities and line shapes of the structural and magnetic Bragg peaks, we aim to investigate in detail the size changes of the twins as a function of field amplitude and time. Dynamic precursor phenomena for the twinning process will be observed by the phonon response in a single variant of Ni2MnGa when transforming from one variant to another or from a single to a multi-variant configuration. Acoustic and, eventually, also optical phonons near the Brillouin zone center are most sensitive to an overall weakness of the crystal towards the transformation. Phonons of short wavelength may tell us whether periodic modulations of the twins are favourable. The search for new material systems with magnetic shape memory effect will be accompanied by phonon measurements in both the martensitic and the austenitic phases. These measurements are the most sensitive way to validate the interactions as deduced by the ab-initio calculations. An understanding of the magnetic interaction is mandatory for the modelling of new MSM systems, because the anisotropy of the magneto-elastic interaction drives the shape memory effect. Therefore, particular emphasis will be put on measuring the evolution of magnetic reflections and the magnon response under varying magnetic field and stress conditions.
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Project leaders:
Mehmet Acet, Eberhard Wassermann
Experimentalphysik
Universität Duisburg-Essen
Lotharstrasse 1
D-47048 Duisburg
Phone: ++49 203 379 2023 (Acet)
Fax: ++49 203 379 2098
E-mail:
Phone: ~2283 (Wassermann)
E-mail: |
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