Variations in strain amplitude and phase in a cylindrical specimen of Tb0.3Dy0.7Fe1.95 in an AC magnetic field

In the light of differences between magnetostrictive strains measured on the surface of Terfenol with strain gauges and from a dilatometer, an optical interferometer has been used to probe the spatial variations of the magnetostrictive strains across the plane surface of a polished disc (6 mm diameter, 1 mm thick) of grain-orientated Tb_0.27Dy_0.73Fe_1.95 DC and AC fields (~15 kA/m and ~440 A/m RMS, respectively) were applied normal to the disk surface parallel to the [112]. The oscillatory displacements and their phases (±2°) relative to the applied field were mapped at 1-mm intervals over the entire surface for frequencies of 4.5 kHz and 30 kHz. In either case the circumference is observed to be almost stationary, but the amplitude of movement increases toward the interior of the disc. Neither the variations in amplitude nor the phase changes are in keeping with predictions from the skin effect and are attributed in part to demagnetizing field effects and also to surface anisotropy, which hinders domain wall motion