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CoFeB magnetic tunnel junctions
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 1009– 1011
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Room-temperature magnetoresistance in CoFeB/STO/CoFeB magnetic tunnel junctions Kaan Oguz , J.M.D. Coey CRANN and School of Physics, Trinity College, Dublin 2, Ireland
a r t i c l e in fo
abstract
Available online 2 May 2008
A series of Co40Fe40B20/SrTiO3/Co40Fe40B20 magnetic tunnel junctions with a bottom-pinned synthetic antiferromagnet have been prepared by sputtering. Devices optimally annealed at 325 1C exhibit an exchange bias of about 65 mT, and a tunnel magnetoresistance of 2%. The smaller than predicted effect is attributed to the lack of epitaxy between the crystallized CoFeB electrodes and the SrTiO3 (STO) barrier, due to poor crystal quality of the barrier layer. Unlike MgO, well-crystallized, oriented STO does not grow on amorphous Co40Fe40B20. & 2008 Elsevier B.V. All rights reserved.
Keywords: Magnetic tunnel junction Sputtering STO
High tunneling magnetoresistance values for magnetic tunnel junctions (MTJs) are critical in order to have high-performance devices in spintronics. Recently, very high TMR (4250% at room temperature) MTJs have been fabricated with MgO barriers in several laboratories worldwide [1–3], including our own. However, investigation of different barriers is still interesting for researchers. SrTiO3 (STO) is another barrier material, which has been quite widely used in MTJs [4–6] with different electrodes such as LSMO, Co, and CoFe. Both positive and negative magnetoresistances have been observed in the junctions depending on the electrode materials. Recently, there was a prediction that large TMR should be observable in epitaxial Co/STO/Co MTJs with bcc (0 0 1) cobalt electrodes [7]. The spin-filtering effect due to the different symmetry of majority and minority electrons is the main reason behind this prediction. A similar prediction had been made for Fe/MgO/Fe MTJ [8] where giant TMR was subsequently observed using recrystallized amorphous CoFeB electodes [1–3]. In this study, we investigated the properties of STO-based tunnel junctions with CoFeB electrodes. A room-temperature magnetoresistance of just 2% is found in thermally annealed Co40Fe40B20/STO/Co40Fe40B20 structures. The reasons for the difference between STO and MgO structures are discussed.
base pressure of 3.8 10 8 Torr. The STO MTJ structure, bottom pinned with IrMn and with a synthetic antiferromagnet as the pinning layer, consisted of Si (0 0 1)/SiO2(substrate)/Ta (5)/Ru (35)/Ta (5)/Ni80Fe20 (5)/Ir22Mn78 (10)/Co90Fe10 (2)/Ru (0.8)/ Co40Fe40B20 (3)/STO (t)/Co40Fe40B20 (3)/Ta (5)/Ru (5), where the numbers in parentheses are the layer thicknesses in nm. The thickness of the STO barrier was varied from 1.0 to 2.5 nm. The barrier was grown by low rate RF sputtering in an interconnected chamber of our sputtering system using a targetfacing-target gun with ceramic STO targets. Pure Ar gas was used in STO growth to prevent oxidation of bottom CoFeB electrode. DC sputtering was used for the other layers in the stack at around 3 mTorr Ar pressure. All MTJs were fabricated at room temperature. After deposition of the stack, the MTJs were microfabricated into square-shaped junctions of different areas from 16 16 to 50 200 mm2 by a combination of conventional UV lithography and Ar ion milling. The patterned junctions were annealed at 300–375 1C under high vacuum for 1 h in an applied field of 0.8 T. Magnetoresistance measurements were carried out at room temperature by using a conventional four-point probe technique with applied magnetic fields up to 180 mT. X-ray diffraction (XRD) experiments were done to determine the crystal quality of STO films using Phillips X’pert diffractometer with 101o2yo1001. X-ray reflectivity (XRR) measurements were utilized for the thickness calibration of the films in the stack.
2. Experimental methods
3. Results and discussion
The MTJs discussed in this study were prepared on thermally oxidized Si (0 0 1) substrates in a Shamrock sputtering tool with a
The XRR measurement of an STO film on an Si/SiO2 substrate is illustrated in Fig. 1. The fitting of the reflection oscillations resulted in 43.8 nm film thickness and 1.6 nm average surface roughness. The surface roughness is reasonable for an oxide film of such thickness. The growth rate of the STO film was deduced to
1. Introduction
Corresponding author. Tel.: +353 18964640; fax: +353 18963037.
E-mail address: [email protected] (K. Oguz). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.153
ARTICLE IN PRESS 1010
K. Oguz, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 321 (2009) 1009–1011
be 0.045 nm/s in these growth conditions (50 W and 1 mTorr). The crystal structure of STO layer was investigated by growing a very thick (100 nm) STO layer on the bottom stack of the MTJ, which is illustrated in Fig. 2b. The XRD pattern reveals that the STO barrier is poorly crystalline, with only a very broad and low-intensity (11 0) peak at 32.11. This is consistent with the literature, where STO is reported to be amorphous when it is grown at room temperature [9]. Achieving good-quality STO film requires hightemperature growth at around 700 1C or high-temperature annealing treatment at around 800 1C. Unfortunately, these options are not available in our case due to increasing diffusion of atoms such as Ru and Mn from underlying layers in the MTJ stack which results in the loss of exchange bias and the degradation of TMR. In the XRD pattern, no CoFe related peaks are present, indicating that the CoFeB layer is amorphous. However, good (111) texture for IrMn and NiFe has been observed, which is essential to obtain high-exchange bias fields in the stack. Room-temperature magnetoresistance curves of the as-grown and annealed samples with 2.5 nm STO barriers are shown in Fig. 1. X-ray reflectivity data of a 43.8 nm SrTiO3 film on a SiO2 substrate.
Fig. 2. Schematic of the MTJ stack (a) and XRD pattern of 100 nm STO on full bottom stack of the MTJ (b).
Fig. 3. Room-temperature magnetoresistance curve of CoFeB/STO/CoFeB MTJ for the asgrown (a) and annealed at 325 1C (b) samples which have 2.5 nm STO barrier.
ARTICLE IN PRESS K. Oguz, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 321 (2009) 1009–1011
Fig. 3(a) and (b), respectively. The TMR value of the as-grown sample is found to be 1.6% without having proper antiparallel magnetic alignment of the magnetization of the electrodes. However, annealing at 325 1C improved the magnetic structure and the TMR value to 2.0%. Further annealing at 375 1C led to the degradation of TMR and loss of exchange bias due to the diffusion of Ru and Mn into STO barrier. The same effect has been observed for similar stacks with MgO barriers. A contributing factor to the low TMR is the crystallization process of the CoFeB electrode. Due to the absence of the desired (0 0 1) orientation in the STO barrier, CoFeB electrodes crystallize in a random fashion which does not result in the correct band structure matching between Co and STO layers. This is the main difference between the present devices and CoFeB/MgO/CoFeB MTJs, where MgO grows in a (0 0 1) orientation on the amorphous CoFeB electrodes and postannealing at above 300 1C then induces (0 0 1) texture in the electrodes. The exchange field for STO MTJs is found to be around 65 mT in the sample annealed at 325 1C. No TMR has been observed for the samples which have STO barriers thinner than 2.5 nm, mainly due to the high conductivity of the STO. The 24 48 mm2 junction with an STO barrier has a resistance of 4 O, whereas a similar junction with an MgO barrier giving 200% TMR has a resistance of 4 kO. The low resistance–area (RA) values (104 O mm2 for the junctions with 2.5 nm STO barriers) are attributed to the lack of oxygen stoichiometry in the STO barrier due to the growth conditions. Usually, STO is grown in oxygen–argon mixed plasma to provide sufficient oxygen during growth whereas we have had to use only argon during sputtering of STO to prevent oxidizing the bottom CoFeB electrode. STO barriers suffer from oxygen deficiency and become less resistive when sputtered in pure argon. Despite the low RA values, the I–V curves of the 2.5 nm junctions show the typical tunneling behavior.
1011
4. Conclusion The relatively small room-temperature TMR of 2.0% can be understood in terms of the lack of (0 0 1) crystal orientation of the STO barrier, which is needed to produce the spin-filtering effect. It seems that low-temperature growth of STO on a ferromagnetic metal electrode cannot produce the required quality and orientation of the barrier layer required for large TMR. An alternative approach, such as using a ferromagnetic perovskite as the bottom electrode, might prove more successful.
Acknowledgement This work is funded by Science Foundation Ireland as a part of the MANSE project. References [1] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yang, Nat. Mater. 3 (2004) 862. [2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. [3] Y.M. Lee, J. Hayakawa, S. Ikeda, F. Matsukura, H. Ohno, Appl. Phys. Lett. 90 (2007) 212507. [4] J.M. de Teresa, A. Barthelemy, A. Fert, J.P. Contour, F. Montaigne, P. Seneor, Science 286 (1999) 507. [5] J. Hayakawa, K. Ito, S. Kokado, M. Ichimura, A. Sakuma, M. Sugiyama, H. Asano, M. Matsui, J. Appl. Phys. 91 (2002) 8792. [6] A. Thomas, J.S. Moodera, B. Satpati, J. Appl. Phys. 97 (2005) 10C908. [7] J.P. Velev, K.D. Belashchenko, D.A. Stewart, M. van Schilfgaarde, S.S. Jaswal, E.Y. Tsymbal, Phys. Rev. B 95 (2005) 216601. [8] W.H. Butler, X.-G. Zhang, T.C. Schulthess, J.M. MacLaren, Phys. Rev. B 63 (2001) 054416. [9] H.-Y. Chou, T.-M. Chen, T.-Y. Tseng, J. Phys. D: Appl. Phys. 38 (2005) 2446.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Room-temperature magnetoresistance in CoFeB/STO/CoFeB magnetic tunnel junctions Kaan Oguz , J.M.D. Coey CRANN and School of Physics, Trinity College, Dublin 2, Ireland
a r t i c l e in fo
abstract
Available online 2 May 2008
A series of Co40Fe40B20/SrTiO3/Co40Fe40B20 magnetic tunnel junctions with a bottom-pinned synthetic antiferromagnet have been prepared by sputtering. Devices optimally annealed at 325 1C exhibit an exchange bias of about 65 mT, and a tunnel magnetoresistance of 2%. The smaller than predicted effect is attributed to the lack of epitaxy between the crystallized CoFeB electrodes and the SrTiO3 (STO) barrier, due to poor crystal quality of the barrier layer. Unlike MgO, well-crystallized, oriented STO does not grow on amorphous Co40Fe40B20. & 2008 Elsevier B.V. All rights reserved.
Keywords: Magnetic tunnel junction Sputtering STO
High tunneling magnetoresistance values for magnetic tunnel junctions (MTJs) are critical in order to have high-performance devices in spintronics. Recently, very high TMR (4250% at room temperature) MTJs have been fabricated with MgO barriers in several laboratories worldwide [1–3], including our own. However, investigation of different barriers is still interesting for researchers. SrTiO3 (STO) is another barrier material, which has been quite widely used in MTJs [4–6] with different electrodes such as LSMO, Co, and CoFe. Both positive and negative magnetoresistances have been observed in the junctions depending on the electrode materials. Recently, there was a prediction that large TMR should be observable in epitaxial Co/STO/Co MTJs with bcc (0 0 1) cobalt electrodes [7]. The spin-filtering effect due to the different symmetry of majority and minority electrons is the main reason behind this prediction. A similar prediction had been made for Fe/MgO/Fe MTJ [8] where giant TMR was subsequently observed using recrystallized amorphous CoFeB electodes [1–3]. In this study, we investigated the properties of STO-based tunnel junctions with CoFeB electrodes. A room-temperature magnetoresistance of just 2% is found in thermally annealed Co40Fe40B20/STO/Co40Fe40B20 structures. The reasons for the difference between STO and MgO structures are discussed.
base pressure of 3.8 10 8 Torr. The STO MTJ structure, bottom pinned with IrMn and with a synthetic antiferromagnet as the pinning layer, consisted of Si (0 0 1)/SiO2(substrate)/Ta (5)/Ru (35)/Ta (5)/Ni80Fe20 (5)/Ir22Mn78 (10)/Co90Fe10 (2)/Ru (0.8)/ Co40Fe40B20 (3)/STO (t)/Co40Fe40B20 (3)/Ta (5)/Ru (5), where the numbers in parentheses are the layer thicknesses in nm. The thickness of the STO barrier was varied from 1.0 to 2.5 nm. The barrier was grown by low rate RF sputtering in an interconnected chamber of our sputtering system using a targetfacing-target gun with ceramic STO targets. Pure Ar gas was used in STO growth to prevent oxidation of bottom CoFeB electrode. DC sputtering was used for the other layers in the stack at around 3 mTorr Ar pressure. All MTJs were fabricated at room temperature. After deposition of the stack, the MTJs were microfabricated into square-shaped junctions of different areas from 16 16 to 50 200 mm2 by a combination of conventional UV lithography and Ar ion milling. The patterned junctions were annealed at 300–375 1C under high vacuum for 1 h in an applied field of 0.8 T. Magnetoresistance measurements were carried out at room temperature by using a conventional four-point probe technique with applied magnetic fields up to 180 mT. X-ray diffraction (XRD) experiments were done to determine the crystal quality of STO films using Phillips X’pert diffractometer with 101o2yo1001. X-ray reflectivity (XRR) measurements were utilized for the thickness calibration of the films in the stack.
2. Experimental methods
3. Results and discussion
The MTJs discussed in this study were prepared on thermally oxidized Si (0 0 1) substrates in a Shamrock sputtering tool with a
The XRR measurement of an STO film on an Si/SiO2 substrate is illustrated in Fig. 1. The fitting of the reflection oscillations resulted in 43.8 nm film thickness and 1.6 nm average surface roughness. The surface roughness is reasonable for an oxide film of such thickness. The growth rate of the STO film was deduced to
1. Introduction
Corresponding author. Tel.: +353 18964640; fax: +353 18963037.
E-mail address: [email protected] (K. Oguz). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.153
ARTICLE IN PRESS 1010
K. Oguz, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 321 (2009) 1009–1011
be 0.045 nm/s in these growth conditions (50 W and 1 mTorr). The crystal structure of STO layer was investigated by growing a very thick (100 nm) STO layer on the bottom stack of the MTJ, which is illustrated in Fig. 2b. The XRD pattern reveals that the STO barrier is poorly crystalline, with only a very broad and low-intensity (11 0) peak at 32.11. This is consistent with the literature, where STO is reported to be amorphous when it is grown at room temperature [9]. Achieving good-quality STO film requires hightemperature growth at around 700 1C or high-temperature annealing treatment at around 800 1C. Unfortunately, these options are not available in our case due to increasing diffusion of atoms such as Ru and Mn from underlying layers in the MTJ stack which results in the loss of exchange bias and the degradation of TMR. In the XRD pattern, no CoFe related peaks are present, indicating that the CoFeB layer is amorphous. However, good (111) texture for IrMn and NiFe has been observed, which is essential to obtain high-exchange bias fields in the stack. Room-temperature magnetoresistance curves of the as-grown and annealed samples with 2.5 nm STO barriers are shown in Fig. 1. X-ray reflectivity data of a 43.8 nm SrTiO3 film on a SiO2 substrate.
Fig. 2. Schematic of the MTJ stack (a) and XRD pattern of 100 nm STO on full bottom stack of the MTJ (b).
Fig. 3. Room-temperature magnetoresistance curve of CoFeB/STO/CoFeB MTJ for the asgrown (a) and annealed at 325 1C (b) samples which have 2.5 nm STO barrier.
ARTICLE IN PRESS K. Oguz, J.M.D. Coey / Journal of Magnetism and Magnetic Materials 321 (2009) 1009–1011
Fig. 3(a) and (b), respectively. The TMR value of the as-grown sample is found to be 1.6% without having proper antiparallel magnetic alignment of the magnetization of the electrodes. However, annealing at 325 1C improved the magnetic structure and the TMR value to 2.0%. Further annealing at 375 1C led to the degradation of TMR and loss of exchange bias due to the diffusion of Ru and Mn into STO barrier. The same effect has been observed for similar stacks with MgO barriers. A contributing factor to the low TMR is the crystallization process of the CoFeB electrode. Due to the absence of the desired (0 0 1) orientation in the STO barrier, CoFeB electrodes crystallize in a random fashion which does not result in the correct band structure matching between Co and STO layers. This is the main difference between the present devices and CoFeB/MgO/CoFeB MTJs, where MgO grows in a (0 0 1) orientation on the amorphous CoFeB electrodes and postannealing at above 300 1C then induces (0 0 1) texture in the electrodes. The exchange field for STO MTJs is found to be around 65 mT in the sample annealed at 325 1C. No TMR has been observed for the samples which have STO barriers thinner than 2.5 nm, mainly due to the high conductivity of the STO. The 24 48 mm2 junction with an STO barrier has a resistance of 4 O, whereas a similar junction with an MgO barrier giving 200% TMR has a resistance of 4 kO. The low resistance–area (RA) values (104 O mm2 for the junctions with 2.5 nm STO barriers) are attributed to the lack of oxygen stoichiometry in the STO barrier due to the growth conditions. Usually, STO is grown in oxygen–argon mixed plasma to provide sufficient oxygen during growth whereas we have had to use only argon during sputtering of STO to prevent oxidizing the bottom CoFeB electrode. STO barriers suffer from oxygen deficiency and become less resistive when sputtered in pure argon. Despite the low RA values, the I–V curves of the 2.5 nm junctions show the typical tunneling behavior.
1011
4. Conclusion The relatively small room-temperature TMR of 2.0% can be understood in terms of the lack of (0 0 1) crystal orientation of the STO barrier, which is needed to produce the spin-filtering effect. It seems that low-temperature growth of STO on a ferromagnetic metal electrode cannot produce the required quality and orientation of the barrier layer required for large TMR. An alternative approach, such as using a ferromagnetic perovskite as the bottom electrode, might prove more successful.
Acknowledgement This work is funded by Science Foundation Ireland as a part of the MANSE project. References [1] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yang, Nat. Mater. 3 (2004) 862. [2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, K. Ando, Nat. Mater. 3 (2004) 868. [3] Y.M. Lee, J. Hayakawa, S. Ikeda, F. Matsukura, H. Ohno, Appl. Phys. Lett. 90 (2007) 212507. [4] J.M. de Teresa, A. Barthelemy, A. Fert, J.P. Contour, F. Montaigne, P. Seneor, Science 286 (1999) 507. [5] J. Hayakawa, K. Ito, S. Kokado, M. Ichimura, A. Sakuma, M. Sugiyama, H. Asano, M. Matsui, J. Appl. Phys. 91 (2002) 8792. [6] A. Thomas, J.S. Moodera, B. Satpati, J. Appl. Phys. 97 (2005) 10C908. [7] J.P. Velev, K.D. Belashchenko, D.A. Stewart, M. van Schilfgaarde, S.S. Jaswal, E.Y. Tsymbal, Phys. Rev. B 95 (2005) 216601. [8] W.H. Butler, X.-G. Zhang, T.C. Schulthess, J.M. MacLaren, Phys. Rev. B 63 (2001) 054416. [9] H.-Y. Chou, T.-M. Chen, T.-Y. Tseng, J. Phys. D: Appl. Phys. 38 (2005) 2446.