Nanostructured giant magneto-impedance multilayers deposited onto flexible substrates for low pressure sensing
© Fernández et al; licensee Springer. 2012
Received: 10 January 2012
Accepted: 23 April 2012
Published: 23 April 2012
Nanostructured FeNi-based multilayers are very suitable for use as magnetic sensors using the giant magneto-impedance effect. New fields of application can be opened with these materials deposited onto flexible substrates. In this work, we compare the performance of samples prepared onto a rigid glass substrate and onto a cyclo olefin copolymer flexible one. Although a significant reduction of the field sensitivity is found due to the increased effect of the stresses generated during preparation, the results are still satisfactory for use as magnetic field sensors in special applications. Moreover, we take advantage of the flexible nature of the substrate to evaluate the pressure dependence of the giant magneto-impedance effect. Sensitivities up to 1 Ω/Pa are found for pressures in the range of 0 to 1 Pa, demostrating the suitability of these nanostructured materials deposited onto flexible substrates to build sensitive pressure sensors.
The GMI material used in this investigation is obtained by sputtering deposition from permalloy (Fe20Ni80), titanium and copper targets onto both glass and cyclo olefin copolymer (COC) substrates. The samples are deposited in the form of elongated strips 0.5 mm wide and 10 mm long using metallic masks (Figure 1b) under a magnetic field of 20 kA/m applied in the plane of the film to induce a well-defined transverse magnetic anisotropy. The background pressure was 3 × 10 -7 mbar and the Ar pressure during the deposition was 3.8 × 10-3 mbar. The power used for the FeNi target was 100 W with a deposition speed of VFeNi = 26 nm/min and 60 W for titanium and cooper targets with a deposition speed of VTi = 4 nm/min and VCu = 25 nm/min.
COC is a transparent and flexible polymer that is used, for example, to fabricate micro-fluidic systems. Recently, a sensor to measure the magnetic-particle concentration in continuous flow has been proposed based in a COC micro-chamber with a GMI magnetic sensor underneath . An obvious upgrade of such a system will be to deposit the sensing GMI material directly onto the COC material, as described in this work. To magnetically characterize the prepared samples, the hysteresis loops along the sample length were measured by vibrating sample magnetometry (VSM). The impedance measurements were performed using radio frequency (RF) techniques by gluing the sample with a conductive silver paint between two micro-strip lines with 50 Ω of characteristic impedance. The impedance was deduced from the scattering parameter S11 measured by a network analyzer using an RF input power of 0 dB (that corresponds to an excitation of about 1 mA across the sample) after proper calibration and mathematical subtraction of the test fixture contributions. Details of the measuring procedure can be found elsewhere . The test fixture containing the sample is placed inside a pair of Helmholtz coils to provide the variable magnetic field. To study the performance of the samples deposited onto the COC substrates as pressure detectors, different weights were placed over a rectangular glass (14 × 18 mm) situated onto the sample, reaching a maximum pressure of 4 Pa. In this way, the sample's complex impedance Z was measured at different pressure values as a function of the external magnetic field in a frequency range of 300 kHz to 300 MHz. In this range, the ferromagnetic resonance effects are still unimportant, and quasi-static processes dominate the MI behavior . The GMI ratio was defined with respect to the magnetically saturated sample in the maximum applied field of Hsat = 12 kA/m. Also, GMI sensitivities were calculated by differentiating with respect to the magnetic field H.
Results and discussion
In our experimental set-up, the applied magnetic field is continuously monitored, and its value is maintained stable using feedback control with an accuracy of 0.8 A/m. To demonstrate the stability of the pressure sensor against small changes in the applied field, both graphs (Figure 5a,b) display data corresponding to the impedance measured at different but close (8 A/m apart) values of the applied magnetic field. The results show that the impedance variation due to field fluctuations is very small (typically about 0.4%, 2% in the worst case). It is important to state that, for comparison, similar pressure measurements have been performed on the sample deposited onto glass. No observable pressure dependence has been found in this case.
We have demonstrated first that excellent GMI response can be obtained from nanostructured multilayers deposited onto a flexible polymeric substrate. These magnetic nanostructures can be useful for a number of applications as detection of magnetic micro- and nanoparticles in micro-fluidic chambers that are fabricated using such materials . On the other hand, we have studied the response of the sample deposited onto the flexible substrate to the applied pressure. The GMI curves display great changes as a function of pressure. At zero magnetic field, the impedance increases monotonically with an average slope of 0.4 Ω/Pa. When biased at the point of maximum field sensitivity, the sample presents a pressure sensitivity of about 1 Ω/Pa between 0 and 1 Pa. Focusing on possible pressure sensor applications, we have checked that the pressure dependence is quite stable when small magnetic field variations are present.
Mr. Eduardo Fernández got his degree on physics at the University of the Basque Country in 2008. He is currently doing his PhD about magnetic field microsensors based on giant magnetoimpedance at the Department of Electricity and Electronics in the same university.
Dr. Galina V. Kurlyandskaya graduated from Ural State University, Ekaterinburg, Russia. She started her research work in 1983 at the Institute of Metal Physics UD RAS. She obtained her PhD in Physics of Magnetic Phenomena in 1990 and advanced Doctor of Science degree in 2007. Dr. Kurlyandskaya received an advanced training at the Institute of Applied Magnetism, University of Oviedo, University of the Basque Country, University of Dusseldorf named under Heinrich Heine, ENSCashan, University of Maryland and Ural State University A.M. Gorky. Her main research areas are fabrication, magnetic and transport properties of amorphous and nanostructured materials, magnetic domains, resonant and non-resonant magnetoabsorption, and magnetic sensors and biosensors.
Dr. Alfredo García-Arribas got his PhD degree at the Universidad del País Vasco (Spain) in 1996. He is currently with the Departamento de Electricidad y Electrónica in the same university. He is mainly interested in soft magnetic properties of materials and their application to magnetic sensors.
Dr. Andrey V. Svalov graduated from Ural State University, Ekaterinburg, Russia were he also obtained PhD in Physics of Magnetic Phenomenon in 2002. Dr. Svalov received advanced training at Ural State University, University of Oviedo and The Basque Country University UPV-EHU. For a long time, his main research activities were related to preparation and characterization of thin magnetic films and multilayers with special focus of Rare Earth containing nanostructures.
The authors wish to thank the financial support from the Spanish and Basque governments (grant no. IT-347-07) and ACTIMAT projects respectively.
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