- Email: [email protected]
CoFeB magnetic tunnel junctions
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e649–e651 www.elsevier.com/locate/jmmm
Differential conductance measurements of low-resistance CoFeB/MgO/CoFeB magnetic tunnel junctions S. Nishiokaa,, Y.V. Hamadaa, R. Matsumotoa, M. Mizuguchia, M. Shiraishia, A. Fukushimab, H. Kubotab, T. Nagahamab, S. Yuasab, H. Maeharac, Y. Nagaminec, K. Tsunekawac, D.D. Djayaprawiraa,c, N. Watanabec, Y. Suzukia,b a Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan c Electron Device Equipment Division, Canon ANELVA Corporation, Fuchu, Tokyo 183-8508, Japan
b
Available online 16 November 2006
Abstract We measured differential conductance spectra of magnetic tunnel junctions (MTJs) with thin MgO barrier and low-resistance area product. The spectra of MTJs with MgO barrier thicker than 1.05 nm were essentially the same except for slight decrease of contributions from low-energy excitations, such as magnons. The spectra of MTJ with 1.01 nm MgO barrier were thoroughly different from the MTJs with thicker barrier. The result reveals that an MTJ with very thin MgO barrier thickness has different conduction characteristics from those with thicker MgO barriers. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70. I; 73.61. r; 85.75.Dd; 75.47. m Keywords: Magnetic film and multilayer; Electric band structure; Electronic transport; Tunneling
1. Introduction Much attention has been paid on magnetic tunneling junctions (MTJs) due to its vast application to nextgeneration read heads for ultrahigh density hard disk drives and magnetoresistive random access memory [1,2]. Recently, MTJs showing a giant tunnel magnetoresistance (TMR) effect based on MgO tunnel barriers have been developed [3–7]. It is believed that the giant TMR effect, which provides over 100% TMR effect at room temperature, is a consequence of electron filtering effect brought by specific electronic structure of MgO (0 0 1) layer. This effect realizes 100% effective spin polarization when the MgO barrier is thick enough, i.e., high resistance–area (RA) product [8]. We have also succeeded in fabricating MTJs showing over 130% TMR ratio at fairly low RA of 1–2 O mm2 [9,10], which is useful for application to TMR Corresponding author. Tel.: +81 6 6850 6425; fax: +81 6 6845 4632.
E-mail address: [email protected] (S. Nishioka). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.773
heads. However, the transport mechanism of the low RAMTJs has not been clarified yet. Tunneling spectroscopy is a useful tool to investigate detailed spin-dependent tunneling processes of electrons in MTJs. Several studies on the tunneling spectroscopy of Al–O-based MTJs [11,12] and MgO-based MTJs [13–15] have been reported. We have reported quantitative results of tunneling spectroscopy for thick MgO barrier MTJs [14]. In this study, we measured the tunneling spectra of low RA CoFeB/MgO/CoFeB MTJs, which were developed for the purpose of application to TMR heads, to investigate the transport mechanism. 2. Experimental procedure The spin valve type MTJs were prepared using a magnetron sputtering system (Canon ANELVA C-7100) and were micro-fabricated using electron beam lithography method [6,9,10]. The fundamental structure of the film was as follows (nm): Ru(7)/Ta(8)/CoFeB(3)/MgO/CoFeB(3)/
ARTICLE IN PRESS e650
S. Nishioka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e649–e651
Ru(0.85)/CoFe(2.5)/PtMn(15)/Ta(3)/oxidized Si(1 0 0) substrate. We prepared five kinds of MTJs with different MgO barrier thicknesses (1.01, 1.05, 1.08, 1.12, and 1.8 nm). The MgO barrier layer was deposited by an rf sputtering, while the other layers were deposited by a DC sputtering method. The samples were annealed in a high vacuum at 360 1C for 2 h in a magnetic field of 8 kOe. We performed tunnel spectroscopy measurement using a cryostat with superconducting solenoids (OXFORD MagLab system2000). First, MR (magnetoresistance) curves were measured by a DC two-probe method at liquid helium temperature. Then, we measured second derivative conductance with a conventional lock-in method at liquid helium temperature. An AC modulation voltage was 4 mVp–p (p–p represents peak to peak) and the frequency was 1300 Hz. The measurements were performed under parallel and anti-parallel configurations of ferromagnetic CoFeB layers. 3. Results and discussion The TMR ratios and RA of five MTJs are shown in Table 1. Around 1.8 nm MTJs (represents MTJs with 1.8 nm of MgO thickness) showed high TMR ratio of 308% at 4 K. With decreasing MgO barrier thickness, TMR ratio and RA gradually dropped down to 170% and 1.2 O mm2 for the MgO thickness of 1.05 nm. At 1.01 nm, they dropped abruptly to 37% and 0.4 O mm2, respectively [10]. To compare the tunneling spectra of MTJs with different resistance, we normalized d2I/dV2 spectra by employing (d2I/dV2)/(dI/dV) [14]. Fig. 1 shows the (d2I/dV2)/(dI/dV) spectra measured at 4.3 K. In antiparallel configuration, the spectrum of 1.01 nm MTJs is thoroughly different from that of 1.8 and 1.12 nm MTJs. This reveals that the transport mechanism of 1.01 nm MTJs is different from MTJs with thicker barriers in spite of its significant TMR ratio of 37%. Around zero-bias voltage (below 10 mV), intense peaks were observed in both configurations. These peaks are often called ‘‘zero-bias anomaly’’. The origins of these peaks have been thought to be magnetic impurities. Above 10 mV, subtle peaks appear down to 1.12 nm. Since
Table 1 Resistance area product and MR ratio measured at 300 and 4 K for various MgO barrier thickness MgO thickness (nm)
Rp (O mm2)
MR (%) 4 K
300 K
1.8 1.12 1.08 1.05 1.01
120 2.3 1.8 1.2 0.4
308 217 204 170 37
— 170 150 130 35
those peaks change its intensities and positions depending on magnetization configuration of the electrodes, those are attributed to magnon excitations. In 1.01 nm MTJ these peaks were not observed. This significant change in the conduction property can be explained by taking existence of pin-holes in the thin MgO barrier into account. If there are pin-holes, electrons undergo Ohmic conduction through them and do not excite magnons since electrons cannot get enough excess kinetic energy to excite it. Therefore, we believe that this clear spectrum change suggests a contribution of pin-hole conductance in 1.01 nm MTJ. 4. Conclusion We performed differential conductance measurements for low-resistance MTJs with thin MgO barriers. In MTJs with MgO barriers thicker than 1.05 nm, the shapes of spectra were unchanged. The spectrum of MTJs with 1.01 nm-MgO thickness was very different from those of thicker MgO barrier MTJs. Therefore, we conclude that this MTJ should have different transport mechanism from conventional tunnel conduction. Acknowledgments This work was supported by the 21st Century COE program (G18) of the Japan Society for the Promotion of Science. We acknowledge Ms. M. Yamamoto of AIST for micro-fabrication of the samples. This study was partly supported by Research and Development of Nanodevices for Practical Utilization of Nanotechnology (Nanotech Challenge Project) of New Energy and Industrial Technology Development Organization (NEDO). References
Fig. 1. d2I/dV2 spectra of CoFeB/MgO (1.8, 1.12, and 1.01 nm)/CoFeB MTJs. Arrows with ‘‘ZB’’ and ‘‘Mag’’ stand for zero-bias anomaly and contributions of magnon excitations, respectively.
[1] T. Miyazaki, N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231. [2] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [3] S. Yuasa, A. Fukushima, T. Nagahama, K. Ando, Y. Suzuki, Jpn. J. Appl. Phys. Part 2 43 (2004) L588. [4] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.H. Yang, Nat. Mater. 3 (2004) 862. [5] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. [6] D.D. Djayaprawira, et al., Appl. Phys. Lett. 86 (2005) 092502. [7] S. Ikeda, et al., Jpn. J. Appl. Phys. 44 (2005) L1442.
ARTICLE IN PRESS S. Nishioka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e649–e651 [8] W.H. Butler, X.-G. Zhang, T.C. Schulthess, J.M. MacLaren, Phys. Rev. B 63 (2001) 054416. [9] K. Tsunekawa, et al., Appl. Phys. Lett. 87 (2005) 072503. [10] Y. Nagamine et al., Digest of Intermag 2006, DD-08, 2006. [11] J.S. Moodera, J. Nowak, R.J.M. van de Veerdonk, Phys. Rev. Lett. 80 (1998) 2941.
e651
[12] J. Murai, Y. Ando, M. Kamijo, H. Kubota, T. Miyazaki, Jpn. J. Appl. Phys. Part 2 38 (1999) L1106. [13] Y. Ando, T. Miyakoshi, M. Oogane, T. Miyazaki, H. Kubota, K. Ando, S. Yuasa, Appl. Phys. Lett. 87 (2005) 142502. [14] R. Matsumoto, et al., Solid State Commun. 136 (2005) 611. [15] G.-X. Miao, et al., J. Appl. Phys. 99 (2006) 08T305.
Journal of Magnetism and Magnetic Materials 310 (2007) e649–e651 www.elsevier.com/locate/jmmm
Differential conductance measurements of low-resistance CoFeB/MgO/CoFeB magnetic tunnel junctions S. Nishiokaa,, Y.V. Hamadaa, R. Matsumotoa, M. Mizuguchia, M. Shiraishia, A. Fukushimab, H. Kubotab, T. Nagahamab, S. Yuasab, H. Maeharac, Y. Nagaminec, K. Tsunekawac, D.D. Djayaprawiraa,c, N. Watanabec, Y. Suzukia,b a Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8568, Japan c Electron Device Equipment Division, Canon ANELVA Corporation, Fuchu, Tokyo 183-8508, Japan
b
Available online 16 November 2006
Abstract We measured differential conductance spectra of magnetic tunnel junctions (MTJs) with thin MgO barrier and low-resistance area product. The spectra of MTJs with MgO barrier thicker than 1.05 nm were essentially the same except for slight decrease of contributions from low-energy excitations, such as magnons. The spectra of MTJ with 1.01 nm MgO barrier were thoroughly different from the MTJs with thicker barrier. The result reveals that an MTJ with very thin MgO barrier thickness has different conduction characteristics from those with thicker MgO barriers. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70. I; 73.61. r; 85.75.Dd; 75.47. m Keywords: Magnetic film and multilayer; Electric band structure; Electronic transport; Tunneling
1. Introduction Much attention has been paid on magnetic tunneling junctions (MTJs) due to its vast application to nextgeneration read heads for ultrahigh density hard disk drives and magnetoresistive random access memory [1,2]. Recently, MTJs showing a giant tunnel magnetoresistance (TMR) effect based on MgO tunnel barriers have been developed [3–7]. It is believed that the giant TMR effect, which provides over 100% TMR effect at room temperature, is a consequence of electron filtering effect brought by specific electronic structure of MgO (0 0 1) layer. This effect realizes 100% effective spin polarization when the MgO barrier is thick enough, i.e., high resistance–area (RA) product [8]. We have also succeeded in fabricating MTJs showing over 130% TMR ratio at fairly low RA of 1–2 O mm2 [9,10], which is useful for application to TMR Corresponding author. Tel.: +81 6 6850 6425; fax: +81 6 6845 4632.
E-mail address: [email protected] (S. Nishioka). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.773
heads. However, the transport mechanism of the low RAMTJs has not been clarified yet. Tunneling spectroscopy is a useful tool to investigate detailed spin-dependent tunneling processes of electrons in MTJs. Several studies on the tunneling spectroscopy of Al–O-based MTJs [11,12] and MgO-based MTJs [13–15] have been reported. We have reported quantitative results of tunneling spectroscopy for thick MgO barrier MTJs [14]. In this study, we measured the tunneling spectra of low RA CoFeB/MgO/CoFeB MTJs, which were developed for the purpose of application to TMR heads, to investigate the transport mechanism. 2. Experimental procedure The spin valve type MTJs were prepared using a magnetron sputtering system (Canon ANELVA C-7100) and were micro-fabricated using electron beam lithography method [6,9,10]. The fundamental structure of the film was as follows (nm): Ru(7)/Ta(8)/CoFeB(3)/MgO/CoFeB(3)/
ARTICLE IN PRESS e650
S. Nishioka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e649–e651
Ru(0.85)/CoFe(2.5)/PtMn(15)/Ta(3)/oxidized Si(1 0 0) substrate. We prepared five kinds of MTJs with different MgO barrier thicknesses (1.01, 1.05, 1.08, 1.12, and 1.8 nm). The MgO barrier layer was deposited by an rf sputtering, while the other layers were deposited by a DC sputtering method. The samples were annealed in a high vacuum at 360 1C for 2 h in a magnetic field of 8 kOe. We performed tunnel spectroscopy measurement using a cryostat with superconducting solenoids (OXFORD MagLab system2000). First, MR (magnetoresistance) curves were measured by a DC two-probe method at liquid helium temperature. Then, we measured second derivative conductance with a conventional lock-in method at liquid helium temperature. An AC modulation voltage was 4 mVp–p (p–p represents peak to peak) and the frequency was 1300 Hz. The measurements were performed under parallel and anti-parallel configurations of ferromagnetic CoFeB layers. 3. Results and discussion The TMR ratios and RA of five MTJs are shown in Table 1. Around 1.8 nm MTJs (represents MTJs with 1.8 nm of MgO thickness) showed high TMR ratio of 308% at 4 K. With decreasing MgO barrier thickness, TMR ratio and RA gradually dropped down to 170% and 1.2 O mm2 for the MgO thickness of 1.05 nm. At 1.01 nm, they dropped abruptly to 37% and 0.4 O mm2, respectively [10]. To compare the tunneling spectra of MTJs with different resistance, we normalized d2I/dV2 spectra by employing (d2I/dV2)/(dI/dV) [14]. Fig. 1 shows the (d2I/dV2)/(dI/dV) spectra measured at 4.3 K. In antiparallel configuration, the spectrum of 1.01 nm MTJs is thoroughly different from that of 1.8 and 1.12 nm MTJs. This reveals that the transport mechanism of 1.01 nm MTJs is different from MTJs with thicker barriers in spite of its significant TMR ratio of 37%. Around zero-bias voltage (below 10 mV), intense peaks were observed in both configurations. These peaks are often called ‘‘zero-bias anomaly’’. The origins of these peaks have been thought to be magnetic impurities. Above 10 mV, subtle peaks appear down to 1.12 nm. Since
Table 1 Resistance area product and MR ratio measured at 300 and 4 K for various MgO barrier thickness MgO thickness (nm)
Rp (O mm2)
MR (%) 4 K
300 K
1.8 1.12 1.08 1.05 1.01
120 2.3 1.8 1.2 0.4
308 217 204 170 37
— 170 150 130 35
those peaks change its intensities and positions depending on magnetization configuration of the electrodes, those are attributed to magnon excitations. In 1.01 nm MTJ these peaks were not observed. This significant change in the conduction property can be explained by taking existence of pin-holes in the thin MgO barrier into account. If there are pin-holes, electrons undergo Ohmic conduction through them and do not excite magnons since electrons cannot get enough excess kinetic energy to excite it. Therefore, we believe that this clear spectrum change suggests a contribution of pin-hole conductance in 1.01 nm MTJ. 4. Conclusion We performed differential conductance measurements for low-resistance MTJs with thin MgO barriers. In MTJs with MgO barriers thicker than 1.05 nm, the shapes of spectra were unchanged. The spectrum of MTJs with 1.01 nm-MgO thickness was very different from those of thicker MgO barrier MTJs. Therefore, we conclude that this MTJ should have different transport mechanism from conventional tunnel conduction. Acknowledgments This work was supported by the 21st Century COE program (G18) of the Japan Society for the Promotion of Science. We acknowledge Ms. M. Yamamoto of AIST for micro-fabrication of the samples. This study was partly supported by Research and Development of Nanodevices for Practical Utilization of Nanotechnology (Nanotech Challenge Project) of New Energy and Industrial Technology Development Organization (NEDO). References
Fig. 1. d2I/dV2 spectra of CoFeB/MgO (1.8, 1.12, and 1.01 nm)/CoFeB MTJs. Arrows with ‘‘ZB’’ and ‘‘Mag’’ stand for zero-bias anomaly and contributions of magnon excitations, respectively.
[1] T. Miyazaki, N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231. [2] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273. [3] S. Yuasa, A. Fukushima, T. Nagahama, K. Ando, Y. Suzuki, Jpn. J. Appl. Phys. Part 2 43 (2004) L588. [4] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.H. Yang, Nat. Mater. 3 (2004) 862. [5] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. [6] D.D. Djayaprawira, et al., Appl. Phys. Lett. 86 (2005) 092502. [7] S. Ikeda, et al., Jpn. J. Appl. Phys. 44 (2005) L1442.
ARTICLE IN PRESS S. Nishioka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e649–e651 [8] W.H. Butler, X.-G. Zhang, T.C. Schulthess, J.M. MacLaren, Phys. Rev. B 63 (2001) 054416. [9] K. Tsunekawa, et al., Appl. Phys. Lett. 87 (2005) 072503. [10] Y. Nagamine et al., Digest of Intermag 2006, DD-08, 2006. [11] J.S. Moodera, J. Nowak, R.J.M. van de Veerdonk, Phys. Rev. Lett. 80 (1998) 2941.
e651
[12] J. Murai, Y. Ando, M. Kamijo, H. Kubota, T. Miyazaki, Jpn. J. Appl. Phys. Part 2 38 (1999) L1106. [13] Y. Ando, T. Miyakoshi, M. Oogane, T. Miyazaki, H. Kubota, K. Ando, S. Yuasa, Appl. Phys. Lett. 87 (2005) 142502. [14] R. Matsumoto, et al., Solid State Commun. 136 (2005) 611. [15] G.-X. Miao, et al., J. Appl. Phys. 99 (2006) 08T305.