1130
IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 3, MARCH 2012
Perpendicular Magnetic Tunnel Junctions Having CoFeB/CoPt Alloy Layers Gyung-Min Choi, Byoung-Chul Min, and Kyung-Ho Shin Korea Institute of Science and Technology, Seoul 136-791, South Korea We have fabricated perpendicular magnetic tunnel junctions consisting of hcp Ru underlayer/hcp CoPt alloy/CoFeB/MgO/CoFeB/ CoPt alloy (or Co)/Pt capping layer. By inserting the CoPt alloy (or Co) between the CoFeB and Pt capping layers, it is possible to increase a perpendicular magnetic anisotropy of the top electrode. Using a top electrode of Co40 Fe40 B20 (0 4 nm) Co72 Pt28 (1 6 nm) Pt, we have obtained an effective magnetic anisotropy of 3 3 106 erg/cm3 and a tunnel magnetoresistance (TMR) of 9.4%. The analysis of the crystal structure reveals that the low TMR is a consequence of the fcc (111) texture of CoFeB layers. Index Terms—Magnetic tunnel junctions (MTJs), perpendicular magnetic anisotropy (PMA), tunnel magnetoresistance (TMR).
I. INTRODUCTION
M
AGNETIC tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) materials have been intensively studied for the realization of high-density magnetoresistive random access memories (MRAMs) [1]–[13]. Scaling down the cell size of perpendicular MTJs (p-MTJs) is limited by the thermal stability of magnetic elements. The magnetic layer of p-MTJs should have a large enough thermal stability to avoid the unwanted switching of the magnetic elements. of a thin magThe effective magnetic anisotropy netic layer is a key factor determining the thermal stability: , where is the uniaxial perpendicular anisotropy, is the saturation magnetization, and is the shape anisotropy of the magnetic layer. The thermal stability factor is defined as , where is the volume of the magnetic layer, is the Boltzmann constant, and is the temperature. With a larger , a smaller is sufficient to provide a required thermal stability. Usually the thermal stability issue is more critical in the free layer, since the thickness of the free layer is thinner than that of the reference layer. PMA materials directly deposited on the CoFeB layer, resulting in a high TMR in the MTJ with an MgO tunnel barrier, are rare-earth transition-metal alloys [1]–[3], and [Co-based alloy/Pd] multilayers [4]. Unfortunately, the PMA of these materials can be deteriorated at an annealing temperature higher than 300 C during the MRAM fabrication process. It has been reported that the AlO /Co/Pt and MgO/CoFeB/Pt structures show a PMA after annealing at a high temperature over 300 [14]–[16]. Very recently, Ikeda et al. reported a remarkable achievement that the p-MTJs consisting of the MgO/ CoFeB/Ta with the interfacial perpendicular anisotropy show a high TMR over 120% and a low switching current [17]. The of the MgO/CoFeB/Ta is about erg/cm with the CoFeB thickness of 1.3 nm. For the realization of a cell
Manuscript received November 02, 2010; revised May 09, 2011; accepted November 07, 2011. Date of publication November 18, 2011; date of current version March 02, 2012. Corresponding author: G.-M. Choi (e-mail:
[email protected]). 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.2011.2176744
size smaller than 40 nm, a further increase of the magnetic anisotropy will be beneficial. of the MgO/ In this paper, we have demonstrated that the CoFeB/Pt free layer can be enhanced by inserting a Co Pt (or Co) layer in between the CoFeB and Pt layers. Unfortunately, the TMR of p-MTJs having a Ru/CoPt/CoFeB bottom electrode (reference layer) and a CoFeB/CoPt (or Co)/Pt top electrode (free layer) is very low. The analysis of the crystal structure explains the origin of the small TMR in this structure. II. EXPERIMENTAL PROCEDURES The typical p-MTJs structure consists of substrate/ (or Co) (2). The thickness of each layer (in nanometer) is indicated in parentheses, and its composition is in atomic percent. To enhance the PMA of bottom electrodes, a thick hcp Ru underlayer is used; it is over erg/cm is obtained known that a large 80 in hcp CoPt alloys, with a Co composition of 70 atomic percent, deposited on a thick hcp Ru underlayer [18]. The hcp CoPt alloy maintains a strong PMA at an annealing temperature up to 400 C. The film stacks are deposited on thermally oxidized Si substrates (SiO 300 nm) using a multi-target high-vacuum magnetron sputtering system with a base pressure of better Torr. The MgO layer is sputtered from a single than crystalline MgO target using an RF power supply, whereas the metal layers are sputtered using a DC power supply. All layers are deposited at room temperature with an Ar pressure of 1 4 mTorr. The Co Fe B layer is sputtered from an alloy target with the same composition. The Co Pt layer is co-sputtered from Co and Pt targets, and its composition is measured by an Auger electron spectroscopy after the deposition. The crystal structure of the film is observed by a cross-sectional high-resolution transmission electron microscopy (HRTEM) and X-ray diffractometer (XRD). The magnetization-field hysteresis loop is measured with an alternating gradient magnetometer and vibrating sample magnetometer. For TMR measurements, cross junction geometries with a junction size of 10 m square are fabricated by a standard optical lithography and Ar ion milling. Thereafter, the samples are annealed at 400 C for 30 minutes. During the annealing the base Torr and no magnetic field pressure is kept to below is applied. The TMR of p-MTJs is measured by a four probe method at room temperature.
0018-9464/$26.00 © 2011 IEEE
CHOI et al.: PERPENDICULAR MAGNETIC TUNNEL JUNCTIONS HAVING CoFeB/CoPt ALLOY LAYERS
1131
Employing a simple model and assuming no interlayer diffusions, we can express the effective anisotropy energy of the or in terms of the surface and volume anisotropy contributions: anisotropy (1) (2)
K
Fig. 1. (a) The effective anisotropy ( ) as a function of the CoFeB thickness ( ) for the MgO (2)/CoFeB (t)/CoPt (2-t)/Pt (2) ( ), and MgO (2)/CoFeB (t)/Co (2-t)/Pt (2) ( ) after 400 C annealing. A solid circle () represents the e of MgO (2)/CoFeB (0.4)/CoPt (1.6)/ Pt (2) after 300 C annealing. (b) The saturation magnetization ( ) of the same structures in (a) after 400 C annealing.
K
t
M
III. RESULTS AND DISCUSSION Fig. 1(a) shows the of the substrate/ (or Co) after annealing at 400 C, where is the thickness of the CoFeB layer. Here we fix the total thickness of the magnetic layer (CoFeB CoPt (or Co)) to 2 nm, so that we can exclude the effect of superparamagas well as netism, caused by a very thin magnetic layer, on is calculated from the area enclosed between on TMR. The the perpendicular and in-plane magnetization curves. The of the top electrode with a 2-nm-thick CoFeB layer, in which erg/cm . a CoPt (or Co) layer is not inserted, is of the CoFeB layer calculated The shape anisotropy of CoFeB (1100 emu/cm ) is erg/cm , from the is erg/cm . Because the perpendicular and the anisotropy originated from the MgO/CoFeB and CoFeB/Pt is not interfaces is smaller than the shape anisotropy, the enough to align the magnetization of the 2-nm-thick CoFeB layer into the out-of-plane direction. further, we inserted a CoPt (or Co) layer To increase the between the CoFeB and Pt layers. As we increase the thickness of the CoPt (or Co) layer (or decrease the thickness of of top electrodes has been increased the CoFeB layer), the is correspondingly. The CoFeB thickness dependence of notably different depending on which material is used for the insertion layer. Inserting a CoPt layer is more effective to inthan inserting a Co layer, and the highest , crease erg/cm , is obtained with a CoFeB thickness of 0.4 value of the sample (CoFeB 0.4 nm/CoPt 1.6 nm) nm. The prepared with an annealing at 300 C is very small [Fig. 1(a)], indicating a high temperature annealing is required to obtain a high PMA. The results in Fig. 1(b) show that the of the free layer is basically determined by the volume-weighted average of magnetic moments of the constituting layers; the of , , and Co are 1100, 1000, and 1400 emu/cm , respectively. It should be noted that inserting a Co layer results in an increase of , whereas inserting a CoPt . layer leads to a slight decrease of
where is the total thickness of magnetic layers, is the CoFeB thickness, is the surface anisotropy at the MgO/CoFeB is the surface anisotropy at the CoPt (or Co)/Pt interface, interface, is the volume anisotropy of the insertion layer is the volume anisotropy of the (CoPt or Co), and of each material has contributions from CoFeB layer. The and the shape the perpendicular crystalline anisotropy . Although there anisotropy of a thin film: would be a significant intermixing between CoFeB, CoPt (or Co), and Pt layers, these simple relations provide us useful clues to understand the effect of the insertion layer. When a CoPt layer is inserted in the structure, the perpendicular crystalline anisotropy of the CoPt film enhances the of the entire strucversus the CoFeB thickture. This can be deduced from the data in Fig. 1(a). The large negative slope in Fig. 1(a) ness is shows that the volume anisotropy of the CoPt layer . The significantly larger than that of the CoFeB layer is determined by the crystalline anisotropy and shape . anisotropy of the CoPt layer: of the CoPt The shape anisotropy contribution layer is almost compensated by the shape anisotropy contriof the CoPt layer is bution of the CoFeB layer, since the slightly different from that of the CoFeB layer. This means that mainly determines the slope observed in Fig. 1(a), and the large negative slope indicates that the CoPt layer has a substantial perpendicular crystalline anisotropy. The of the CoPt film deposited on the CoFeB layer might come from the crystal texture of the CoPt alloy film; there has been a report that CoPt alloy films deposited directly on the glass substrate shows a PMA [19]. and volume contribuTo compare the surface in a more direct way, we have measured the tions on the of the substrate (or with varying the total thickness and Co) maintaining a fixed thickness ratio (1:4) of the CoFeB and CoPt (or Co) layers. As shown in Fig. 2, it is possible to obtain the volume anisotropy of the composite layers from the slope, and the surface anisotropy from the -intercept: (3) The CoFeB/CoPt (1:4)/Pt structure has a larger volume anisotropy (smaller negative slope) and a smaller surface anisotropy (smaller -intercept) than those of the CoFeB/Co (1:4)/Pt structure. Even though the CoPt layer has a smaller surface anisotropy than that of the Co layer, its higher volume anisotropy makes it more effective in enhancing the when the total thickness of the magnetic layer is thicker than 1 nm. This higher volume anisotropy of CoPt comes from both the and larger in comparison with those of Co. smaller Using the knowledge on the of various top electrodes, we have fabricated p-MTJs with three different top electrodes.
1132
IEEE TRANSACTIONS ON MAGNETICS, VOL. 48, NO. 3, MARCH 2012
TABLE I. LAYER STACKS OF P-MTJS
K t = t 2 = = t 2 = = t2= = t2= =
Fig. 2. The product of by the total thickness ( ) of magnetic layers (CoFeB + CoPt (or Co)) as a function of the total thickness ( ) 1 5) CoPt( 4 5) Pt(2)( ), and for the MgO(2) CoFeB( 1 5) Co( 4 5) Pt(2)( ) after 400 C annealing. MgO(2) CoFeB(
=
t
The layer stacks of the p-MTJs are summarized in Table I. Fig. 3 shows the - hysteresis loops and TMR curves of p-MTJs with three different top electrodes measured in perpendicular magnetic fields. The large difference in the switching field between the top and bottom electrodes enables us to distinguish the parallel and antiparallel magnetic configurations of the top and bottom electrodes. The switching of the free layer strongly . In sample A, after annealing at 400 C, we depends on the have obtained the TMR of 9.4% with a steep magnetic switching [see Fig. 3(e)]. A gradual switching of the free layer is observed in sample B [Fig. 3(f)] where a thicker CoFeB layer is used, in sample A with a lower annealing temperature [Fig. 3(g)], and in sample C [Fig. 3(h)] where the Co insertion layer is used. In these cases, the gradual switching indicates the lower PMA of free layers compared to that of the CoFeB (0.4)/CoPt (1.6)/Pt (2) structure annealed at 400 . Taking into account an incomplete antiparallel alignment of the reference and free layers in Fig. 3(g) and (h), the TMR values of four MTJs are almost the same, and are very small. The TMR value and Resistance-Area (RA) product with in-plane MTJs consisting of CoFeB (4)/MgO m , respectively, after (2)/CoFeB (3) are 180% and annealing at 400 C. In order to understand the origin of the low TMR, we have investigated the crystal structure of the p-MTJs using HRTEM. A high TMR in MgO MTJs is associated with the crystal structure of a ferromagnetic layer on the MgO tunnel barrier; it is important to obtain a ferromagnetic layer with a bcc (001) texture epitaxially grown on the MgO (001) tunnel barrier [20]. Fig. 4(a) and (b) shows cross-sectional TEM images of the sample A after annealing at 400 . The CoFeB layers in both top and bottom electrodes are not clearly distinguished from the adjacent CoPt layers, and the CoFeB layers have a crystal structure different from the bcc (001) texture. The lattice spacings of the MgO and CoFeB layers are 0.22 nm
Fig. 3. (a), (b), and (d), respectively, are the full loops ( ) and minor loops ; of hysteresis of the sample A, B, and C after 400 C annealing. (e), (f), and (h), respectively, are the TMR curves of sample A, B, and C after 400 C annealing. (c) and (g), respectively, are the - hysteresis and TMR curve of sample A after 300 C annealing. The RA products (parallel states) of (e), (f), (g), and (h) are 3 5 10 m , 2 3 10 m , 4 0 10 m , and 0 5 10 m , respectively. The RA products of four MTJs show a sample-tosample variation mainly due to a subtle MgO thickness variation, since the MTJ stacks are made from different deposition runs.
M 0H
:2
MH :2 :2
:2
and 0.21 nm respectively [Fig. 4(a)], and the lattice fringes of the top CoFeB layer have an intersection angle of [Fig. 4(b)]. This indicates that, even though the MgO layer has a B1 (001) texture, both CoFeB layers adjacent to MgO has an fcc (111) texture following the crystalline structure of the CoPt layer, not following that of the MgO layer [4], [21]. To support this TEM analysis, we have also investigated the crystal structure of the magnetic layers by XRD [Fig. 4(c) and (d)]. A 10-nm-thick CoFeB layer, deposited in between 2-nm-thick MgO under layer and 4-nm-thick CoPt/Pt capping layer, shows annealing [Fig. 4(c)]. This an fcc (111) texture after 400 is the case of the top CoFeB layer. Similarly, a 10-nm-thick CoFeB layer, deposited in between 10-nm-thick CoPt under layer and 2-nm-thick MgO capping layer, shows an fcc (111) annealing [Fig. 4(d)]. This is the case of texture after 400 the bottom CoFeB layer. In contrast, the CoFeB layer with a Ta capping layer [Fig. 4(c)] or a Ta underlayer [Fig. 4(d)] does not show the fcc (111) texture. These results explain why the p-MTJs show a very low TMR; the low TMR is related with the propagation of the crystal structure during the annealing process. It has been reported that the MTJs with a NiFe, Cu, Al, or Pd capping layer, where the capping layer is deposited directly on top of a CoFeB layer, shows a reduced TMR,
CHOI et al.: PERPENDICULAR MAGNETIC TUNNEL JUNCTIONS HAVING CoFeB/CoPt ALLOY LAYERS
Fig. 4. (a) and (b) are the cross-sectional TEM images of sample A after annealing at 400 C. (c) and (d), respectively, are the XRD results for the top and bottom CoFeB layers. The XRD sample structures are given in the labels where the layer thicknesses are given in nm. All the XRD samples are deposited on thermally oxidized Si substrates, and annealed at 400 C.
because the capping layer influences the crystallization of the CoFeB layer [20]-[22]. IV. CONCLUSION We have fabricated MgO-based p-MTJs using top electrodes of CoFeB (t)/CoPt (or Co) (2-t)/Pt and bottom electrodes of hcp Ru/hcp CoPt (8)/CoFeB (0.5). To increase the PMA of the top electrodes, we have inserted the CoPt (or Co) layer between the CoFeB and Pt capping layers. Using the insertion layers, it is of erg/cm , which is sufpossible to achieve the ficient to align the magnetization of the 2-nm-thick CoFeB/CoPt layer to the perpendicular direction. The p-MTJ with the CoFeB (t)/CoPt (2-t)/Pt top electrode shows a steep magnetic switching in perpendicular magnetic fields, but a low TMR less than 10%. From the analysis of the crystal structure, it has been found that the low TMR is mainly attributed to the fcc (111) texture of the CoFeB layer. The propagation of the crystal structure of the adjacent layers to the CoFeB layer should be avoided to obtain a high TMR in p-MTJs having CoFeB/CoPt alloy layers. ACKNOWLEDGMENT This work was supported by the KIST institutional program, by the IT R&D program of MKE/KEIT (2009-F-004-01), and by the Pioneer Research Center Program (2011-0027905) and the Grant (No.20110019108) from the NRF funded by the MEST. REFERENCES [1] N. Nishimura, T. Hirai, A. Koganei, T. Ikeda, K. Okano, Y. Sekiguchi, and Y. Osada, “Magnetic tunnel junction device with perpendicular magnetization films for high-density magnetic random access memory,” J. Appl. Phys., vol. 91, p. 5246, 2002. [2] M. Nakayama, T. Kai, N. Shimomura, M. Amano, E. Kitagawa, T. Nagase, M. Yoshikawa, T. Kishi, S. Ikegawa, and H. Yoda, “Spin transfer switching in TbCoFe/CoFeB/MgO/CoFeB/TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy,” J. Appl. Phys., vol. 103, p. 07A710, 2008.
1133
[3] H. Ohmori, T. Hatori, and S. Nakagawa, “Perpendicular magnetic tunnel junction with tunneling magnetoresistance ratio of 64% using MgO (100) barrier layer prepared at room temperature,” J. Appl. Phys., vol. 103, p. 07A911, 2008. [4] K. Mizunuma, S. Ikeda, J. H. Park, H. Yamamoto, H. Gan, K. Miura, H. Hasegawa, J. Hayakawa, F. Matsukura, and H. Ohno, “MgO barrierperpendicular magnetic tunnel junctions with CoFe/Pd multilayers and ferromagnetic insertion layers,” Appl. Phys. Lett., vol. 95, p. 232516, 2009. [5] M. Yoshikawa, E. Kitagawa, T. Nagase, T. Daibou, M. Nagamine, K. Nishiyama, T. Kishi, and H. Yoda, “Tunnel magnetoresistance over 100% in MgO-based magnetic tunnel junction films with perpendicular magnetic L1 -FePt electrodes,” IEEE Trans. Magn., vol. 44, p. 2573, 2008. [6] B. Carvello, C. Ducruet, B. Rodmacq, S. Auffret, E. Gautier, G. Gaudin, and B. Dieny, “Sizable room-temperature magnetoresistance in cobalt based magnetic tunnel junctions with out-of-plane anisotropy,” Appl. Phys. Lett., vol. 92, p. 102508, 2008. [7] J. H. Park, C. Park, T. Jeong, M. T. Moneck, N. T. Nufer, and J. G. Zhu, “Co/Pt multilayer based magnetic tunnel junctions using perpendicular magnetic anisotropy,” J. Appl. Phys., vol. 103, p. 07A917, 2008. [8] C. Ducruet, B. Carvello, B. Rodmacq, S. Auffret, G. Gaudin, and B. Dieny, “Magnetoresistance in Co/Pt based magnetic tunnel junctions with out-of-plane magnetization,” J. Appl. Phys., vol. 103, p. 07A918, 2008. [9] D. Watanabe, S. Mizukami, M. Oogane, H. Naganuma, Y. Ando, and T. Miyazaki, “Fabrication of MgO-based magnetic tunnel junctions with CoCrPt perpendicularly magnetized electrodes,” J. Appl. Phys., vol. 105, p. 07C911, 2009. [10] Z. R. Tadisina, A. Natarajarathinam, B. D. Clark, A. L. Highsmith, and T. Mewes, “Perpendicular magnetic tunnel junctions using Co-based multilayers,” J. Appl. Phys., vol. 107, p. 09C703, 2010. [11] Yi. Wang, W. X. Wang, H. X. Wei, B. S. Zhang, W. S. Zhan, and X. F. Han, “Effect of annealing on the magnetic tunnel junction with Co/Pt perpendicular anisotropy ferromagnetic multilayers,” J. Appl. Phys., vol. 107, p. 09C711, 2010. [12] K. Yakushiji, K. Noma, T. Saruya, H. Kubota, A. Fukushima, T. Nagahama, S. Yuasa, and K. Ando, “High magnetoresistance ratio and low resistance-area product in magnetic tunnel junctions with perpendicularly magnetized electrodes,” Appl. Phys. Exp., vol. 3, p. 053003, 2010. [13] K. Machida, K. Furukawa, N. Funabashi, K.-I. Aoshima, K. Kuga, T. Ishibashi, and N. Shimidzu, “Spin transfer switching and MR properties of Co/Pt multilayered free layers for submicron sized magneto-optical light modulation device,” IEEE Trans. Magn., vol. 46, p. 2171, 2010. [14] Y. Dahmane, S. Auffret, U. Ebels, B. Rodmacq, and B. Dieny, “Perpendicular magnetic anisotropy at Co/AlOx interface,” IEEE Trans. Magn., vol. 44, p. 2865, 2008. [15] B. Rodmacq, A. Manchon, C. Ducruet, S. Auffret, and B. Dieny, “Influence of thermal annealing on the perpendicular magnetic anisotropy of Pt/Co/AlOx trilayers,” Phys. Rev. B, vol. 79, p. 024423, 2009. [16] L. E. Nistor, B. Rodmacq, S. Auffret, and B. Dieny, “Pt/Co/oxide and oxide/Co/Pt electrodes for perpendicular magnetic tunnel junctions,” Appl. Phys. Lett., vol. 94, p. 012512, 2009. [17] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno, “A perpendicularanisotropy CoFeB-MgO magnetic tunnel junction,” Nat. Mater., vol. 9, p. 721, 2010. [18] T. Shimatsu, H. Sato, Y. Okazaki, H. Aoi, H. Muraoka, Y. Nakamura, S. Okamoto, and O. Kitakami, “Large uniaxial magnetic anisotropy by lattice deformation in CoPt/Ru perpendicular films,” J. Appl. Phys., vol. 99, p. 08G908, 2006. [19] J. P. Hu and P. Lin, “High-coercivity CoPt alloy films grown by sputtering,” IEEE Trans. Magn., vol. 32, p. 4096, 1996. [20] S. Yuasa and D. D. Djayaprawira, “Giant tunnel magnetoresistance in magnetic tunnel junctions with a crystalline MgO(001) barrier,” J. Phys. D: Appl. Phys., vol. 40, p. R337, 2007. [21] T. Miyajima, T. Ibusuki, S. Umehara, M. Sato, S. Eguchi, M. Tsukada, and Y. Kataoka, “Transmission electron microscopy study on the crystallization and boron distribution of CoFeB/MgO/CoFeB magnetic tunnel junctions with various capping layers,” Appl. Phys. Lett., vol. 94, p. 122501, 2009. [22] J.-H. Park, S. Ikeda, H. Yamamoto, H. Gan, K. Mizunuma, K. Miura, H. Hasegawa, J. Hayakawa, K. Ito, F. Matsukura, and H. Ohno, “Perpendicular magnetic tunnel junctions with CoFe/Pd multilayer electrodes and an MgO barrier,” IEEE Trans. Magn., vol. 45, p. 3476, 2009.
