Acoustic-assist effect on magnetic threshold field and twinning-yield stress of Ni-Mn-Ga single crystals

Abstract

Ni-Mn-Ga based ferromagnetic shape memory alloys (FSMAs) have been shown to exhibit large magnetic-field-induced strain (MFIS) up to 10% for frequencies up to 1 kHz. However, a magnetic threshold field of 150-300 kA/m is typically required to achieve MFIS, which will require a large electromagnet. The application of a small acoustic signal to the FSMA crystal during MFIS has been shown to reduce the threshold field by up to 80 kA/m and increase the stress and strain outputs. We systematically study the threshold field and the twinning-yield stress of Ni-Mn-Ga single crystals with and without acoustic assist. The acoustic-assist technique helps to reduce the required threshold field and twinning-yield stress of the samples studied here by 60%. The effectiveness of the acoustic assistance in enhancing twin-boundary motion increases with increasing acoustic drive frequency up to a limiting frequency of 2-5 kHz. The magnitude of the acoustic-assist effect appears to be limited by the performance of the piezoelectric stack used in this experiment. The acoustic assistance is a result of a bipolar stress wave that enhances twin-boundary motion in either direction. For a given piezodrive frequency and displacement amplitude, the stress-wave theory provides an estimate of the expected acoustic stress-wave magnitude, and this acoustic stress value is consistent with the observed decrease in twinning-yield stress. © 2008 American Institute of Physics.