Stretchable Spin Valves on Elastomer Membranes by Predetermined Periodic Fracture and Random Wrinkling

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Journal Article
Advanced Materials, 2012, 24 (48), pp. 6468 - 6472 (5)
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The first highly stretchable and sensitive spin valve sensor on elastomeric membranes are demonstrated. The sensor elements exhibit stable GMR behavior up to tensile strains of 29% in in situ stretching experiments and show no fatigue over 500 loading cycles. This remarkable stretchability is achieved by a predetermined periodic fracture mechanism that creates a meander-like pattern upon stretching. Application fields such as smart skin1 and flexible or stretchable consumer electronics2–6 require a wide range of electronic components that are shapeable into nonplanar geometries after fabrication. Ideally, they should also be elastic and withstand many cycles of deformations without degrading in performance. Mechanical compliancy of thin films of naturally stiff materials can be achieved by different morphology transitions,6 that transfer large deformations of the substrate into small strains in the functional film, e.g., by wrinkling7, 8 or lithographically defined meander structures.9, 10 However, exploiting the full bandwidth and application potential of modern electronics requires implementation of stretchability into all types of electronic devices. For instance, there is rapid progress in fabrication of interconnects11 and opto-electronic devices12 with stretchabilities up to several tens of percents. During the last years, magnetic13, 14 as well as magnetoresistive15–18 structures were fabricated and characterized on bendable polymeric substrates. Recently, stretchable magneto-electronics has been introduced by the fabrication of a wrinkled magnetic sensor based on [Co(Py)/Cu] multilayers19, 20 revealing a giant magnetoresistive (GMR) effect; Py stands for the permalloy, Ni81Fe19 alloy. However, only moderate stretchability of the magnetic sensor of up to about 4% was achieved. Furthermore, the sensitivity of stretchable magnetic sensors to small magnetic fields, as required for applications such as wearable electronics,1 has to be substantially enhanced. These two aspects, increase of magnetic sensitivity and stretchability, call for the development of a novel technology platform. This task is addressed in the present work by using a spin valve (SV) magnetic sensor and by applying a predetermined periodic fracture method to substantially improve the stretchability. The synergy of the two approaches allowed us to achieve a stretchability of up to 29% with a top sensitivity of 0.8% Oe−1 in a magnetic field of 12 Oe. Thus, the present work represents the first realization of an elastically stretchable spin valve sensor. This technology is in particular promising for integration into large area hybrid electronic systems, e.g., wearable electronics, to be equipped with magnetic functionalities. To achieve stretchability of a magnetic sensor, the support has to be not only flexible but also elastic. The fabrication process is schematically shown in Figure 1a. For our studies we chose poly(dimethylsiloxane) (PDMS), which is prepared as a flat film (surface roughness <0.5 nm) on a rigid silicon handling wafer equipped with an antistick layer. Photolithography was performed on the PDMS rubber surface to define a stripe-like pattern (width: 1 mm; length: 16 mm) with four contacting pads for magneto-electronic characterizations (Figure 1c1). This renders the fabrication process compatible to current microelectronic structuring procedures. A 5-nm-thick Ta layer was used as a bottom buffer in the spin valve layer system on the rubber film. Under the used sputter conditions (see the Experimental Section), Ta grows with compressive stress. This stress relaxes into the soft rubber substrate, forming a random wrinkling pattern as revealed by atomic force microscopy (AFM) measurements (Supporting Information, Figure S1). From the AFM data the lateral expansion of the Ta buffer layer upon deposition was extracted to be 6.6%. Hence, the subsequent spin valve stack: Ta (2 nm)/IrMn (5 nm)/[Py (4 nm)/CoFe (1 nm)]/Cu (1.8 nm)/[CoFe (1 nm)/Py (4 nm)] is deposited onto a wrinkled surface without interrupting the vacuum; compositions of alloy targets are as follows: Ir19Mn81, Co90Fe10, Ni81Fe19. The complete layer stack resembles a conventional IrMn-based top-pinned SV (Figure 1b). The deposition is done in the presence of an in-plane magnetic field in order to introduce antiferromagnetic order in the IrMn and to set the direction of the exchange bias (EB) to the [CoFe/Py] bilayer underneath (pinned layer). If not stated differently, the exchange bias direction was chosen to be perpendicular to the long axis of the patterned sensor stripe (cross-pinned configuration), and the sample is stretched along the long axis of the stripe. The bottom [CoFe/Py] bilayer is referred to as the magnetic free layer and acts as a sensing layer in the stack.
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