A Constitutive Model for Magnetic Shape Memory Alloys That Includes a Tilted Magnetic Easy Axis

Magnetic shape memory alloys (MSMAs) are materials commonly used for actuation, sensing, and/or power harvesting applications. While the actuation response of MSMAs can be fairly accurately predicted by currently available constitutive models, the power harvesting and/or sensing performance is not predicted as well. This suggests that current models lack features related to the change in magnetization. One such feature that is known to exist, but is not present in any current model, is the natural offset of the magnetic easy axis from the short axis of the tetragonal martensitic unit cell of MSMAs. Experimentally, Scheerbaum et al. [1] observed that this offset angle is in the range of 2° to 6°. While this is a relatively small angle, it is expected to make a dramatic difference in the evaluation of the power harvesting output, as it creates favorable domains even when the field is applied perpendicular to the short axis of the unit cell. Therefore, to facilitate the design of MSMA based sensing and power harvesting devices, a continuum model for the magneto-mechanical response of MSMAs, that accounts for the magnetic easy axis offset from the short side of the unit cell is derived from thermodynamic requirements and evaluated in this work.