Nanoscale Magnetic Materials - Magnetic Domains

Magnetic shape memory alloys (ended project)

MSMA domains small

Dynamic metallographic and magneto-optical polarization microscopy of magnetic shape memory alloy systems (completed)

Time-resolved metallographic optical microscopy techniques together with magnetic domain imaging are used to clarify the interaction between magnetic domains and twin boundary motion in magnetic shape memory alloy single crystals. The magnetic field and stress induced magnetic domain formation can be imaged by a magneto-optical indicator film technique. Thereby reversible twin boundary motion can be visualized up to high actuation speeds. 

For instance, from domain observations at adjacent crystal surfaces we have derived the fundamental volume magnetic processes during strain and field induced twin boundary motion. One of the main results is the discovery of magnetic field induced structural reorientations without concurrent magnetic domain wall motion. On the contrary, for strain induced reorientations processes a complete rearrangement of the magnetic domain structure by the moving twin boundaries is observed. 

From dynamic actuation experiments on twin boundary motion we found that the field induced strain increases with actuation speed, resulting in non-linear time effects on twin boundary mobility. The findings can be interpreted as the interaction of moving twin boundaries with local non-fixed defects. The results provide key information for the understanding of the connection of magnetic and crystallographic domains in magnetic shape memory alloys as well as for the optimization of devices for future technical applications.

MSM project at DFG Gepris

References 

  1. A. Neudert, Y. W. Lai, R. Schäfer, M. Kustov, L. Schultz, J. McCord, Magnetic Domains and Twin Boundary Movement of NiMnGa Magnetic Shape Memory Crystals, Advanced Engineering Materials 14, 601–613 (2012)
  2. Y.-W. Lai, R. Schäfer, L. Schultz, J. McCord, Volume magnetic domain mirroring in magnetic shape memory crystals, Applied Physics Letters 96, 22507 (2010)
  3. N. Scheerbaum, Y.W. Lai, T. Leisegang, M. Thomas, J. Liu, K. Khlopkov, J. McCord, S. Faehler, R. Traeger, D.C. Meyer, L. Schultz, O. Gutfleisch, Constraint-dependent twin variant distribution in Ni2MnGa single crystal, polycrystals and thin film: An EBSD study, Acta Materialia 58, 4629-4638 (2010)
  4. M. Thomas, O. Heczko, J. Buschbeck, Y.W. Lai, J. McCord, L. Schultz, S. Fähler, Stray field induced actuation mode of freestanding magnetic shape memory films, Advanced Materials, 21, 3708, (2009)
  5. Y.-W. Lai, R. Schäfer, L. Schultz, J. McCord, Direct observation of AC field-induced twin-boundary dynamics in bulk NiMnGa, Acta Materialia 56, 5130 (2008)
  6. Y.W. Lai, N. Scheerbaum, D. Hinz, O. Gutfleisch, R. Schäfer, L. Schultz, J. McCord, Absence of magnetic domain wall motion during magnetic field induced twin boundary motion in bulk magnetic shape memory alloys, Applied Physics Letters 90, 192504 (2007)