Evaluation of Magnetic Flux Density Change for Vibration Energy Harvesting: Comparison Between Direct and Indirect Measurements

The magnetic flux density change
$\Delta B$
due to the inverse magnetostrictive effect is important to developing superior inverse magnetostrictive materials for the application of vibration energy harvesting. Although the
$\Delta B$
value can be evaluated indirectly from magnetization curves under constant stresses, it is desired to measure directly by applying dynamic stresses under constant magnetic fields. In this study, a measurement system that can measure
$\Delta B$
via indirect and direct methods was developed. Using this system, the
$\Delta B$
values of Fe–70 at.% Co alloy were measured using an as-rolled specimen with larger hysteresis and annealed specimen with smaller hysteresis. In the direct measurement of the annealed specimen, the
$\Delta B$
value increased with increasing magnetic fields, showing a maximum. The indirect
$\Delta B$
measurements also obtained very close values to the direct method. However, the direct-measured
$\Delta B$
value of the as-rolled specimen under dynamic tensile stress of 160 MPa showed a maximum of 0.35 T, which is less than 19% compared with that of the indirect-measured
$\Delta B$
value. The magnetic domain wall motion during direct measurements is suggested to differ from that during indirect measurements.