Magnetic Domains and Twin Boundary Movement of NiMnGa Magnetic Shape Memory Crystals

Abstract

Time‐resolved metallographic optical microscopy techniques are used together with magnetic domain imaging to clarify the interaction between magnetic domains and twin boundary (TB) motion in magnetic shape memory NiMnGa single crystals. The magnetic field and stress induced magnetic domain formation is imaged by a magneto‐optical indicator film technique. Reversible TB motion is visualized up to high actuation speeds. From domain observation at adjacent crystal surfaces the fundamental volume magnetic processes during strain and field induced TB motions are derived. For magnetic field induced structural reorientations a concurrent absence of magnetic domain wall motion is found. In contrast, for strain induced reorientations processes, a complete rearrangement of the magnetic domain structure by the moving TB is observed. Dynamic actuation experiments on TB motion reveal non‐linear time effects on TB mobility. In addition to training effects, the maximum field induced strain increases with actuation speed. Both effects can be interpreted as the interaction of moving twin boundaries with local non‐fixed defects. The summarized results provide key information for the understanding of the connection of magnetic and crystallographic domains in magnetic shape memory alloys and for the optimization of devices for future technical applications.

Space-, time- and spin-resolved photoemission

“…Therefore, a high time resolution accompanied by a moderate lateral resolution (several 100 nm) has first been realized in optical techniques, such as magneto-optical microscopy (Kerr/Faraday microscopy) [39,40] and magneto-optical scanning near-field microscopy (SNOM) [41]. In a time-resolved PEEM, however, the lateral resolution may be better by almost an order of magnitude [42].…”

Microstructural and In Situ Lorentz TEM Domain Characterization of As-Quenched and γ’-Precipitated Co49Ni30Ga21 Monocrystal

“…The same paper makes the hypothesis of the presence of residual austenite in the martensitic material after quenching and ageing. The theory, however, did not receive full support in either work [25] or in the research described above.…”

“…Studies on the domain structure of Co-Ni-Ga alloys can be divided due to observation methods: magnetic force microscopy (MFM), and light or electron microscopy. Light microscopy is still the most frequently used technique that allows many phenomena occurring in shape memory alloys to be described [25]. Observations of the Co 48 Ni 22 Ga 30 single crystal using polarized light microscopy, called Kerr microscopy, showed that in a martensitic state each martensitic plane is divided into 180 • magnetic domains and the twin boundaries are 90 • magnetic domain walls of the Bloch type [26].…”

Magnetic Domain Structure and Magnetically-Induced Reorientation in Ni-Mn-Ga Magnetic Shape Memory Alloy

“…If MOIF is freely put on the ferromagnetic sample and it visualizes the stray field occurring from underlining magnetic domain structure [7,8]. shows the initial configuration and sample orientation.…”