IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 11, NOVEMBER 2014 6201304 Improved Magnetic Tunnel Junctions Design for the Detection of Superﬁcial Defects by Eddy Currents Testing Filipe A. Cardoso 1 , Luís S. Rosado 2,3 , Fernando Franco 1,2 , Ricardo Ferreira 4 , Elvira Paz 4 , Susana F. Cardoso 1,2 , Pedro M. Ramos 5 , Moises Piedade 2,3 , and Paulo J. P. Freitas 4 1 Instituto de Engenharia de Sistemas e Computadores-Microsistemas e Nanotecnologias, Lisbon 1000-029, Portugal 2 Instituto Superior Técnico, Universidade de Lisboa, Lisbon 1049-001, Portugal 3 Instituto de Engenharia de Sistemas e Computadores-Investigação e Desenvolvimento, Lisbon 1000-029, Portugal 4 International Iberian Nanotechnology Laboratory, Braga 4715-330, Portugal 5 Instituto de Telecomunicações, Instituto Superior Técnico, Universidade de Lisboa, Lisbon 1049-001, Portugal In the last decade, magnetoresistive sensors attracted great interest for integration in eddy current-based non-destructive testing due to their high sensitivity and signal to noise ratio in a large range of frequencies (from dc to hundreds of megahertz). In this paper, a sensor composed of several magnetic tunnel junction (MTJ) elements in series is optimized and included in a custom probe conﬁguration for the detection of superﬁcial defects. Since the signal magnetic ﬁelds are very low, a ﬁnite element modeling simulation was supporting the sensor design optimization. The MTJ chips were microfabricated, assembled with the excitation coils and used in experimental measurements of defects 400 μm wide and 500 μm deep. The experimental results obtained showed very good agreement with the simulations. Index Terms—Differential probe, eddy currents testing, magnetic tunnel junction (MTJ), magnetoresistance, non-destructive testing (NDT), sensor optimization. I. I NTRODUCTION M AGNETORESISTIVE (MR) sensors have become a widespread technology included in many industrial [1] and commercial [2] devices. Good sensitivities and spatial resolution and principally its compatibility with conventional microelectronics have contributed to its increasing popularity. In some applications, MR sensors are replacing inductive coil sensors enabling operation at lower frequencies. In particular, for eddy currents testing, these sensors enabled the use of lower excitation frequencies and the detection of defects located deeper inside a conducting material [3]. Another advantage of MR sensors rely on their high-spatial resolution (down to nanometer scale) when compared with inductive sensors (down to 1 mm). This advantage has been recently used for the detecion of very small surface defects with anisotropic MR (AMR) [4], [5] and giant MR (GMR) [6], [7] sensors. This non-destructive testing (NDT) method relies on the induction of electrical currents on metallic blocks, which may be distorted in the presence of defects or other features in the block. The MR sensor is sensitive to small variations in the resulting magnetic ﬁeld, therefore detecting defects. Several probe conﬁgurations employing MR sensors have been proposed. One possible probe conﬁguration comprises a GMR sensor positioned coaxially with an excitation coil [6] and set to measure the out of plane magnetic ﬁeld component. In other conﬁgurations, GMR sensors were set to measure a null component of the magnetic ﬁeld when no defect is present [7]. This design decision allows effectively exploring Manuscript received March 7, 2014; revised April 14, 2014; accepted May 15, 2014. Date of current version November 18, 2014. Corresponding author: F. A. Cardoso (e-mail: fcardoso@inesc-mn.pt). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/TMAG.2014.2326959 the available electronic dynamic range of the measurement instrument improving the signal to noise ratio (SNR) at which a defect can be detected. Other reports assessed the charac- terization of superﬁcial defects [8]. In most of the reported experiments, commercially available MR sensors have been used [9]. These sensors, which are typically GMR sensors, are usually offered in few packaging options and measurement conﬁgurations, which may not be the most suited to be included in eddy current probes. Previously, it was shown that the geometrical parameters of magnetic tunnel junction (MTJ) sensors [10] can have a signiﬁcant impact on the defects detection. These sensors also show much higher sensitivities to magnetic ﬁelds when compared with AMR and GMR sensors [11], which facilitate the detection of very low-magnetic ﬁeld variation that are often observed in NDT applications. Finally, it was also demon- strated that a sensor including a series of MTJ elements show improved SNRs under a homogeneous magnetic ﬁeld [12]. Nevertheless, in NDT applications based on eddy currents and in particular when detecting surface defects, the magnetic ﬁeld sensed by each MTJ elements of a series can be very inhomogeneous. In this paper, this inhomogeneous magnetic ﬁeld variations were ﬁrst simulated assuming a solid block with a surface defect. Then, the number of MTJ sensing elements in series on each sensor was optimized, aiming maximum signal. Finally, a probe including the optimum sensor conﬁguration was microfabricated and tested on an aluminum block with a defect 400 μm wide and 500 μm deep. II. PROBE OPTIMIZATION The ﬁnite element modeling (FEM) tool CST EM studio was applied to simulate the induced currents and magnetic ﬁeld being measured by the sensors. In this modeling tool, 0018-9464 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.