The effect of magnetic field orientation on the open-circuit voltage of Ni-Mn-Ga based power harvesters

Ni-Mn-Ga is a ferromagnetic alloy that can exhibit the shape memory effect or superelasticity in the presence of a magnetic field. The behavior of the material is largely due to its microstructure, which is thought to be made of tetragonal martensite variants, each exhibiting an innate magnetization aligned approximately with the short side of the unit cell. Because the reorientation strain can be induced and recovered by either magnetic field or mechanical stress, it can be induced at frequencies larger than 1 kHz, which makes the material suitable for high-frequency actuation, sensing, or power harvesting applications. This paper investigates the power harvesting capability of Ni-Mn-Ga wrapped with a pick-up coil under a bi-axial magnetic field. In this work, both experimental tests and numerical simulations are used to identify the optimal direction of the externally applied magnetic field in order to achieve maximum open-circuit voltage output from a particular Ni-Mn-Ga based power harvester. Results suggest that significantly more power can be achieved with the bias field applied at an angle of 10°-20° off the perpendicular to the coil axis and the compressive stress. We believe that this increased power output is due to the saturation of domain walls due to a small component of the magnetic field along the direction of the coil.