Stress Effects on Magnetic Properties of Flexible Magnetic Thin Film

Abstract

Spin-based electronic devices on polymer substrates have advantages of weight, thickness, and flexibility compared to rigid substrates. In particular, magnetic materials have the inverse magnetostriction effect, which is a characteristic that magnetic anisotropy changes with respect to an external stress. Therefore, the magnetic anisotropy of Co, Fe and CoFe alloys with positive magnetotrictive constant increased with respect to tensile stress, however, the magnetic anisotropy of the negative magnetostrictive Ni increased when the compressive stress applied. The magnetostrictive constants of NiFe and NiCo alloys are determined depending on the composition ratio of each materials in alloys. So far, conventional flexible magnetic electronic devices have focused on maintaining the inherent characteristics of devices when external stresses are applied, such as bending or stretching. However, after bending, the magnetic thin film in a flat state did not return to initial state due to the residual stress. Such irreversible process was able to accumulate stress in the magnetic thin film, and it was able to change the magnetic anisotropy and magnetization state simply through the repetitive bending. The change occurred in multi-layered magnetic structures such as a spin-valve as well as a single layer. The free and the pinned layers of spin-valve structure composed of iron-rich-NiFe and CoFe with positive magnetostrictive constants was deposited on the polymer film to be able to change as applying external stress. Thereafter, it is possible to increase (or decrease) the magnetoresistance ratio by applying repetitive downward bending in the transverse (or parallel) direction to the easy magnetic easy axis. In addition, the different change of the magnetic anisotropy in each layer was able to be induced by repetitive bending in the direction parallel to the magnetic easy axis of the spin-valve structure in which the free and the pinned layers were composed of iron-rich-NiFe and Ni with opposite magnetostrictive constants. In the flat state after bending, due to residual compressive stress, the magnetic anisotropy of the positive magnetostrictive free layer was weakened while that of the pinned layer with negative magnetostriction was enhanced, so that the magnetic configuration between the free and the pinned layers were orthogonal for use as a magnetic sensor. Furthermore, in this process, it is possible to improve the sensitivity by increasing the magnetoresistance ratio as well as control the operating field range as a sensor. The prototype magnetic sensor cell array designed through the repetitive bending verified a sensing capability by detection of magnetic microbeads. These attempts demonstrate that the manipulation of the magnetic anisotropy and the fabrication of magnetic sensor, which were possible only through the complicated patterning and long-term annealing process, could be implemented through the repetitive bending process.