Magnetic, magnetoelastic and dynamical properties of matensitic Heusler alloys

• Antragsteller: Dr. Jürgen Neuhaus
• Fachliche Zuordnung: Materialwissenschaft
• Förderung: Förderung von 2006 bis 2011
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 28320193

This grant application is part of a consortium, which aims on a better understanding of thefundamental properties of magnetic field induced shape memory effect in Heusler alloys.Prof. M. Acet and coworkers explore the macroscopic magnetic and elastic behaviour of newSystems based on Ni-Mn Heusler alloys like Ni-Mn-Sn and Ni-Mn-ln, that are expected toshow a magnetic shape memory effect with potential technical applications. The guiding ideabehind this research is to properly tune the e/a ratio of the alloy by varying the composition.Prof. Enteis group supports the experiments by ab-initio calculations of the microscopic andmacroscopic properties. The validation of the calculations will be done by our detailedphonon measurements äs function of composition (i.e. e/a ratio), temperature and appliedmagnetic field. In addition, our group aims to the understand the kinetics of the twin boundarymotion under applied magnetic field and stress by elastic and inelastic neutron scattering.Using time dependant diffraction on rapidly cycled polvcrvstalline Ni2MnGa (Mn concentration28-30 at%) by means of varying the magnetic field in the martensitic phase, we intend toobserve structural and magnetic Bragg intensities äs function of time, i.e. the periodic fieldand applied stress. Alternatively, a slow change of magnetic field over minutes will be appliedto investigate the kinetics of twin growth induced by the increasing magnetic field. Thesestudies will be followed by measuring single crvstals under similar conditions. The crystalshave to be oriented with the habit plane of the twins parallel to the scattering plane. Byobserving the time dependant evolution of intensities and line shapes of the structural andmagnetic Bragg peaks, we aim to investigate in detail the size changes of the twins äs afunction of field amplitude and time.Dynamic precursor phenomena for the twinning process will be observed by the phononresponse in a sinqle variant of Ni2MnGa when transforming from one variant to another orfrom a single to a multi-variant configuration. Acoustic and, eventually, also optical phononsnear the Brillouin zone center are most sensitive to an overall weakness of the crystaltowards the transformation. Phonons of short wavelength may teil us whether periodicmodulations of the twins are favourable.The search for new material svstems with magnetic shape memory effect will beaccompanied by phonon measurements in both the martensitic and the austenitic phases.These measurements are the most sensitive way to validate the interactions äs deduced bythe ab-initio calculations.An understanding of the magnetic interaction is mandatory for the modelling of new MSMSystems, because the anisotropy of the magneto-elastic interaction drives the shape memoryeffect. Therefore, particular emphasis will be put on measuring the evolution of maqneticreflections and the magnon response under varying magnetic field and stress conditions.

• DFG-Verfahren: Schwerpunktprogramme
• Teilprojekt zu: SPP 1239: Änderung von Mikrostruktur und Form fester Werkstoffe durch äußere Magnetfelder
• Beteiligte Person: Professor Dr. Winfried Petry