= Solution
\b[Reversing a saturating longitudinal field gives approximately the same <strain> and the same <voltage> polarity relative to the original unmagnetized reference.] In the isotropic orientational model,
$$
\varepsilon^{\rm mag}_{ij}=\frac{3\Lambda}{2}
\left(m_im_j-\frac{\delta_{ij}}3\right).
$$
It is unchanged under $\mathbf m\to-\mathbf m$. Both positive and negative longitudinal saturation therefore contract the negative-magnetostriction <nickel> in the same axial direction. The resulting <stress> on the piezoelectric layer has the same sign.
During field reversal there can be a hysteretic transient as domains reorganize, and finite circuit leakage can alter the measured waveform. Those effects do not turn the saturated <strain> law into a response odd in the field.
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