The effect of acoustic energy input from a piezoelectric actuator on the magnetic-field-induced strain (MFIS) of Ni-Mn-Ga ferromagnetic shape memory alloys (FSMAs) is observed to enhance the MFIS performance [1]. The threshold field required to induce twin boundary motion is reduced by this new technique by up to 1 kOe. The twinning-yield stress is also reduced by the technique by up to 0.5 MPa. The significant reduction in the twinning stress or the threshold field by the acoustic-assistance is beneficial to reduce the size of a FSMA actuator with a given output. To appreciate the benefit of the acoustic-assistance better, the stress and strain output of the FSMA actuation under acoustic-assist is studied.
In this work, the stress-biased, field-induced strain of a Ni₂MnGa single crystal (100-cut measuring 15 x 5 x 2 mm) is measured. A cyclic transverse field, driven at 0.01 Hz from 0 to +/- 5 kOe, is applied along the sample’s width direction. A constant compressive load is applied to one end of the sample (perpendicular to the field direction). A piezo stack is attached at the other end to generate a longitudinal vibration along its length. The sample displacement under varying constant biased stress is measured with and without piezo assist.
Both operating stress and strain outputs of the FSMA actuation is clearly enhanced by the piezo-assisted effect. Without the piezo-assist, the maximum reversible strain is 3% and appears only in a limited external stress range between 0.7 and 1 MPa. With the piezo-assist, the maximum reversible strain increases to 4.5% and appears in a broader stress output ranging between 0.4 and 1.2 MPa. Smaller magnetic energy is required for twin motion as the the twinning stress is reduced by the acoustic assist; the unused magnetic energy can be utilized to work against a larger external stress.

[1] B. Peterson et al., J. of Appl. Phys., 95, 6963-6964 (2004).

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1. Characterization of frequency dependent magneto-elastic properties of magnetic shape memory actuators
2. Selected fundamental and technical advances in magnetic shape memory alloys
3. Magnetic and structural properties of non-stoichiometric Ni-Mn-Ga Ferromagnetic Shape Memory Alloys