Energy conversion in Ni-Mn-Ga with asymmetrical bias magnetic field

We studied the mechano-electric energy conversion for Ni-Mn-Ga alloys with dynamic experiments under a bias magnetic field. At low and at high magnetic fields, the magneto-crystalline anisotropy energy and the Zeeman energy dominate the formation of magnetic domains, respectively. At lower fields and when the bias field is tilted against the twin boundary, the formation of 180 & DEG; magnetic domains reduces the net magnetization parallel to the load axis. However, at low strains and in a compressed state and when the bias field is tilted along the twin boundary, the majority of the volume saturates parallel to the load axis. Further, the evolution of the magnetic domains with increasing the magnetic field is different in each twin domain; a lower magnetic field is needed to eliminate magnetic domain boundaries in the twin domain with the direction of easy magnetization closely aligned with the field than in the twin domain with the direction of easy magnetization at a large angle to the field. Therefore, due to increased net magnetization parallel to the load axis, the magnetic structure generated at lower bias fields tilted parallel to the twin boundary is more favorable to maximize power conversion. However, a minimum bias field is required to expand the sample against the axial load and this field must be higher than the switching field. Therefore, in order to optimize electric power output, the energy conversion has to take place at lower bias magnetic fields and on samples with low twinning stress with the field direction inclined nearly parallel to the twin boundaries.