Direct observation of AC field-induced twin-boundary dynamics in bulk NiMnGa

The actuation performance and twin-boundary velocity in bulk NiMnGa magnetic shape memory single crystals at various frequencies are investigated. A dynamic actuation experimental setup with the ability to apply mechanical stress and a high-frequency alternating magnetic field is developed in tandem with time-resolved optical microscopy. Reversible twin-boundary motion activated in the frequency range up to 600 Hz is directly observed. The maximum field-induced strain increases with actuation frequency. Strain hysteresis curves measured at different frequencies show similar onset fields of strain. The data imply that twin-boundary velocity is not the limiting factor on actuation performance. The results provide significant information on twin-boundary performance during high-frequency actuation.