Abstract
Ultrasensitive magnetic field sensors at low frequencies are necessary for several biomedical applications. Suitable devices can be achieved by using large area magnetic tunnel junction (MTJ) sensors combined with permanent magnets to stabilize the magnetic configuration of the free layer and improve linearity. However, further increase in sensitivity and consequently detectivity are achieved by incorporating also soft ferromagnetic flux guides (FG). A detailed study of tunnel junction sensors with variable areas and aspect ratios is presented in this work. In addition, the effect in the sensors transfer curve, namely in their coercivity and sensitivity, as a consequence of the incorporation of permanent magnets and FG is also thoroughly discussed. Devices consisting of MgO-based MTJ with magnetoresistance levels of ~200 %, and incorporating thin film permanent magnets (CoCrPt) and CoZrNb flux guides, could reach sensitivities of ~2,000 %/mT at room temperature, in a non-shielded environment. The noise levels of the final device measured at 10 Hz yield 3.9 × 10−17 V2/Hz, leading to the lowest detectable field of ~49 pT/Hz0.5. This value is half of the best value we obtained with MTJ-based devices, and represents a step further towards integration in a biomedical device for magnetocardiography.
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Almeida JM, Freitas PP (2009) Field detection in MgO magnetic tunnel junctions with superparamagnetic free layer and magnetic flux concentrators. J Appl Phys 105:07E722. doi:10.1063/1.3077228
Cardoso S, Gameiro L, Leitao DC, Cardoso F, Ferreira R, Paz E, Freitas PP (2013) Magnetic tunnel junction sensors with pTesla sensitivity for biomedical imaging. Proc. SPIE microtechnologies 8763, smart sensors, actuators, and MEMS VI, 87631ª. doi:10.1117/12.2018070
Chaves RC, Freitas PP, Ocker B, Maass W (2007) Low frequency picotesla field detection using hybrid MgO based tunnel sensors. Appl Phys Lett 91:102504. doi:10.1063/1.2775802
Chaves RC, Cardoso S, Ferreira R, Freitas PP (2011) Low aspect ratio micron size tunnel magnetoresistance sensors with permanent magnet biasing integrated in the top lead. J Appl Phys 109:07E506. doi:10.1063/1.3537926
Chen JY, Feng JF, Coey JMD (2012) Tunable linear magnetoresistance in MgO magnetic tunnel junction sensors using two pinned CoFeB electrodes. Appl Phys Lett 100:142407. doi:10.1063/1.3701277
Edelstein AS, Fischer GA (2002) Minimizing 1/f noise in magnetic sensors using a microelectromechanical system flux concentrator. J Appl Phys 91:7795. doi:10.1063/1.1451901
Ferreira R, Wisniowski P, Freitas PP, Langer J, Ocker B, Maass W (2006) Tuning of MgO barrier magnetic tunnel junction bias current for picotesla magnetic field detection. J Appl Phys 99:08K706. doi:10.1063/1.2173636
Ferreira R, Paz E, Freitas PP, Wang J, Xue S (2012) Large area and low aspect ratio linear magnetic tunnel junctions with a soft-pinned sensing layer. IEEE Trans Mag 48:3719. doi:10.1109/TMAG.2012.2200468
Freitas PP, Costa JL, Almeida N, Melo LV, Silva F, Bernardo J, Santos C (1999) Giant magnetoresistive sensors for rotational speed control. J Appl Phys 85:5459. doi:10.1063/1.369975
Guedes A, Almeida JM, Cardoso S, Ferreira R, Freitas PP (2007) Improving magnetic field detection limits of spin valve sensors using magnetic flux guide concentrators. IEEE Trans Magn 43:2376–2378. doi:10.1109/TMAG.2007.893119
Guedes A, Patil SB, Cardoso S, Chu V, Conde JP, Freitas PP (2008) Hybrid magnetoresistive/microelectromechanical devices for static field modulation and sensor 1/f noise cancellation. J Appl Phys 103:07E924–07E933. doi:10.1063/1.2837661
Guedes A, Jaramillo G, Buffa C, Vigevani G, Cardoso S, Leitao DC, Freitas PP (2012) Towards picoTesla magnetic field detection using a GMR-MEMS hybrid device. IEEE Trans Magn 48:4115–4118. doi:10.1109/TMAG.2012.2203297
Guerrero R, Pannetier-Lecoeur M, Fermon C, Cardoso S, Ferreira R, Freitas PP (2009) Low frequency noise in arrays of magnetic tunnel junctions connected in series and parallel. J Appl Phys 105:113922. doi:10.1063/1.3139284
Ho MK, Ching T, Childress J, Fontana R, Katine J, Carey K (2004) Study of longitudinal stabilization using in-stack biasing. IEEE Trans Magn 40:189. doi:10.1109/TMAG.2003.821201
Ikeda S, Hayakawa J, Ashizawa Y, Lee YM, Miura K, Hasegawa H, Tsunoda M, Matsukura F, Ohno H (2008) Tunnel magnetoresistance of 604 % at by suppression of Ta diffusion in pseudo-spin-valves annealed at high temperature. Appl Phys Lett 93:082508. doi:10.1063/1.2976435
Ingvarson S, Xiao G, Parkin S, Gallagher W, Grinstein G, Koch R (2000) Low-frequency magnetic noise in micron-scale magnetic tunnel junctions. Phys Rev Lett 85:3289. doi:10.1103/PhysRevLett.85.3289
Jander A, Nordman CA, Pohm AV, Anderson JM (2003) Chopping techniques for low-frequency nanotesla spin-dependent tunneling field sensors. J Appl Phys 93:8382. doi:10.1063/1.1555975
Leitao DC, Gameiro L, Silva AV, Cardoso S, Freitas PP (2012) Field detection in spin valve sensors using CoFeB/Ru synthetic-antiferromagnetic multilayers as magnetic flux concentrators. IEEE Trans Mag 48:3847–3850. doi:10.1109/TMAG.2012.2195302
Marinho Z, Cardoso S, Chaves R, Ferreira R, Melo LV, Freitas PP (2011) Three dimensional magnetic flux concentrators with improved efficiency for magnetoresistive sensors. J Appl Phys 109:07E521. doi:10.1063/1.3556946
Mazumdar D, Liu X, Schrag BD, Shen W, Carter M, Xiao G (2007) Thermal stability, sensitivity, and noise characteristics of MgO-based magnetic tunnel junctions. J Appl Phys 101:09B502. doi:10.1063/1.2710953
Pannetier M, Fermon C, Le Goff G, Simola J, Kerr E (2004) Femtotesla magnetic field measurement with magnetoresistive sensors. Science 304:1648. doi:10.1126/science.1096841
Pannetier-Lecoeur M, Parkkonen L, Sergeeva-Chollet N, Polovy H, Fermon C, Fowley C (2011) Magnetocardiography with sensors based on giant magnetoresistance. Appl Phys Lett 98:153705. doi:10.1063/1.3575591
Prieto JL, Evetts JE, Blamire MG, Rouse N (2002) Development of an integrated magnetic sensor with linear output based on spin valves. J Appl Phys 91:8578. doi:10.1063/1.1447297
Stutzke NA, Russek SE, Pappas DP, Tondra M (2005) Low-frequency noise measurements on commercial magnetoresistive magnetic field sensors. J Appl Phys 97:10Q107. doi:10.1063/1.1861375
Trindade IG, Fermento R, Sousa JB, Chaves RC, Cardoso S, Freitas PP (2008) Linear field amplification for magnetoresistive sensors. J Appl Phys 103:103914. doi:10.1063/1.2936315
Trindade IG, Leitao DC, Pogorelov Y, Sousa JB, Chaves RC, Cardoso S, Freitas PP (2009) Control of hysteretic behavior in flux concentrators. Appl Phys Lett 94:073501. doi:10.1063/1.3081012
Tsang C, Krounbi M, Kasiraj P, Lee R (1992) Study of recessed MR sensors with unlaminated and multi-laminated flux-guides. IEEE Trans Magn 28:2289. doi:10.1109/20.179471
Wisniowski P, Cardoso S, Barradas N, Freitas PP (2008) Effect of free layer thickness and shape anisotropy on the transfer curves of MgO magnetic tunnel junctions. J Appl Phys 103:07A910. doi:10.1063/1.2838626
Yamada K, Maruyama T, Tatsumi T, Suzuki T, Shimabayashi K, Motomura Y, Aoyama M, Urai H (1990) Shielded magnetoresistive head for high density recording. IEEE Trans Magn 26:3010. doi:10.1109/20.102882
Yuan GL, Liu JM, Zhou L, Zhang ST, Chen XY, Liu ZG (2002) Film heterostructure with soft ferromagnetics to enhance low-field magnetoresistance. Appl Phys Lett 81:4073. doi:10.1063/1.1522130
Acknowledgments
D. C. Leitao is thankful to FCT for the grant SFRH/BPD/72359/2010. F. Cardoso acknowledges funding from IMAGIC EU-FP7-ICT-288381. INL acknowledges partial funding from the ON2 project from PO Norte. INESC-MN acknowledges FCT funding through the Instituto de Nanociência e Nanotecnologia (IN) Associated Laboratory.
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Cardoso, S., Leitao, D.C., Gameiro, L. et al. Magnetic tunnel junction sensors with pTesla sensitivity. Microsyst Technol 20, 793–802 (2014). https://doi.org/10.1007/s00542-013-2035-1
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DOI: https://doi.org/10.1007/s00542-013-2035-1