# Investigation of Energy Harvesting Capabilities of Metglas 2605 SA1
## 2 Magnetic Behavior
### Result and Discussion  
Firstly, the anisotropy of Metglas 2605 SA1 was assessed. The sample denoted \"A\" was cut in RD (rolling direction) and the one denoted \"B\" in TD (transverse direction). Both samples were made of a single layer, which increased the stress level to 60 MPa. The characterization result presented in Figure 5 showed a negligible anisotropy. Therefore, the sample mentioned in the following discussion is the sample cut in RD. The measurements of magnetic hysteresis curves of Metglas 2605 SA1 under different compressive and tensile stresses are depicted in Figure 6. These results demonstrate that the coercive field showed low sensitivity to the tensile stresses, and, considering the range of applied magnetic excitation field, could be considered negligible. The results also highlight that the hysteresis behavior has a limited impact on the enclosed area between two curves corresponding to different stresses. Overall, the Metglas experiences a very  
Figure 4: Setup for the measurement of Metglas 2605 SA1 under compressive stress. Figure 5: Tensile stress influence on characterization of sample A (rolling direction) and sample B (transverse direction). limited hysteresis effect over the full stress range. Therefore, the hysteresis effect of the materials is ignored in the following discussion. It could be noted that Metglas 2605 SA1 showed a non-negligible linear part in the considered magnetic excitation range. In particular, the tensile stress increased the slope around zero flux density point while the compressive stress apparently decreased the slope. To put it another way, the elongation (resp. contraction) of the sample yielded an increase (resp. decrease) of the low-regime permeability, in accordance with the physical mechanisms behind magnetostriction. This low-field equivalent permeability thus ranged from \(600\mu_{0}\) for a stress of \(-30\) MPa to approximately \(8000\mu_{0}\) for 30 MPa. At zero stress, this permeability was around \(5000\mu_{0}\). As shown in Figure 6, at the same magnetic field level, the induced magnetic flux variation caused by compressive stress was twice that of the one caused under tensile stress. Hence, in terms of relative variation, compressive stress showed higher magnetostrictive activity than tensile stress. Therefore, the effect of stress on the variation of magnetic flux density is not linear, which has also been observed in other materials such as FeCo-2V [40] and FeSiNO [40]. While strong nonlinearity in the form of the hysteresis effect is not considered, as it could be considered negligible, soft nonlinearities arose as the magnetic responses started saturating above 500 A/m. Similarly, the application of tensile stress tended to widen the range of excitation field where the response can be considered linear. Such a behavior is again consistent with the previously exposed physical effects, as application of tensile (resp. compressive) stress yields a rotation of magnetic domains so that they are in the same (resp. perpendicular) axis as that of the applied field. Yet, no clear and pronounced saturation was observed in the range of the excitation field of interest. Unsurprisingly, the results also demonstrate a trade of between low-excitation field equivalent permeability and the wideness of the linear zone. Noticeably, the magnetization tended to converge for all mechanical stresses for excitation fields around 4 kA/m. While such results are qualitatively predictable, as the fully saturated magnetic state is independent from mechanical excitation, these results give relevant quantitative considerations for practical application. Indeed, this results in a closed Ericsson cycle, so that the ultimate energy density under a constant stress would not change even with a higher stress level. Therefore, for Metglas 2605 SA1, a higher magnetic field than 4 kA/m is unnecessary. From these measurements and observations, a model was developed as this investigation focuses on the characterization and prediction of energy density under different stress levels.