Thin-Film Magnetoimpedance Structures Onto Flexible Substrates as Deformation Sensors

The magnetic and magnetoimpedance (MI) properties of a Permalloy-based thin-film structure, deposited onto a polymeric flexible substrate, are investigated as a function of the applied stress. The selected structure, a sandwich of NiFe/Ti multilayers with a central Cu layer, has been previously shown to produce extraordinary MI performance when deposited onto rigid substrates. Here, grown onto a cyclo olefin polymer substrate, it is evaluated as a deformation sensor and its properties analyzed in terms of the preparation conditions. A specially designed sample holder allows measuring simultaneously the hysteresis loop of the sample using magnetooptical Kerr effect and its MI using a network analyzer, while applying a controlled stress to the sample. The magnetoelastic anisotropy developed in the sample is longitudinal, revealing that the magnetostriction coefficient is positive, and competes with the transverse magnetic anisotropy induced during the sputtering deposition of the multilayer structure. The combined, effective anisotropy determines the shape of the hysteresis loop and the MI curves. We show how the evolution of the maxima of the MI curves with the applied stress allows a precise determination of the magnetostriction coefficient of the sample. The stress impedance is determined as the impedance change, measured at H = 0 Oe, as function of the applied stress. It displays a linear behavior with a corresponding gauge factor of 60, thirty times larger than the one of conventional metallic strain gauges.