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High magnetoresistance tunnel junctions with Mg–B–O barriers
and Ni–Fe–B free electrodes
J. C. Read,1,a͒
Judy J. Cha,1
Torr͒ that is connected to a UHV ͑Pbaseϳ5
Torr͒ scanning tunneling microscope.
The CIPT data shown in Fig. ...
ments ͓Fig. 3͑d͒, and inset͔ reveal that it has a highly tex-
tured ͑001͒ bcc crystal structure, which is optimal accordin...
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Read et al. - 2009 - High magnetoresistance tunnel junctions with Mg–B–O barriers and Ni–Fe–B free electrodes(3)


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Read et al. - 2009 - High magnetoresistance tunnel junctions with Mg–B–O barriers and Ni–Fe–B free electrodes(3)

  1. 1. High magnetoresistance tunnel junctions with Mg–B–O barriers and Ni–Fe–B free electrodes J. C. Read,1,a͒ Judy J. Cha,1 William F. Egelhoff, Jr.,2 H. W. Tseng,1 P. Y. Huang,1 Y. Li,1 David A. Muller,1 and R. A. Buhrman1 1 School of Applied and Engineering Physics and Center for Materials Research, Cornell University, Ithaca, New York 14853, USA 2 Magnetic Materials Group, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA ͑Received 7 January 2009; accepted 16 February 2009; published online 18 March 2009͒ The use of boron-alloyed electrodes with the radio frequency ͑rf͒ sputter deposition of MgO yields magnetic tunnel junctions ͑MTJs͒ with Mg–B–O tunnel barriers. After annealing, such MTJs can exhibit very high tunneling magnetoresistance ͑TMR͒ in the thin ͑ϳ1.0 nm͒ barrier regime. Scanning tunneling spectroscopy of Mg–B–O layers reveals a better defined, but smaller band gap in comparison to that of thin MgO. We produced Fe60Co20B20/Mg–B–O/Ni65Fe15B20 MTJs where after a 350 °C annealing the Ni–Fe–B free electrode crystallizes into a highly textured ͑001͒-normal body centered cubic ͑bcc͒ crystal structure and the MTJs achieve 155% TMR. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3095595͔ The development of high tunneling magnetoresistance ͑TMR͒ MgO magnetic tunnel junctions1–3 ͑MTJs͒ is key to the realization of next generation magnetic random access memory ͑MRAM͒ and high-performance sensors for high- density data storage, biomedical, and security applications. The understanding of high TMR in MgO MTJs is based on coherent spin-filtered tunneling due to properly oriented crystalline electrodes and an epitaxial barrier layer.4,5 However, sputter deposition of MgO between amorphous Co–Fe–B electrodes that crystallize during annealing6 achieves the best results in the thin barrier ͑Ͻ1.5 nm͒, low resistance-area ͑RA͒ regime required for many important ap- plications. We find this deposition process partially oxidizes the surface of the base electrode, generally resulting in the formation of a Mg–B–O tunnel barrier where B trigonally coordinated with O ͑BO3͒ composes ϳ12% of the oxide cations.7–9 In high TMR MgO MTJs, CoFe-based alloys are often used as the “free” electrode, but the magnetic properties of this material are not optimal in comparison to Permalloy ͑Py͒, Ni81Fe19.10 Py has essentially no magnetostriction in comparison to larger magnetostriction ͑ϳ10−5 ͒ in Co80Fe20 ͑Ref. 11͒ and low microcrystalline anisotropy ͑ϳ102 J/m3 ͒ relative to Co70Fe30 ͑ϳ104 J/m3 ͒.11 The saturation magneti- zation ͑MS͒ of Py is about half of that of Co80Fe20,10 which is beneficial12 for spin-torque ͑ST͒ MRAM,13 where the spin polarized tunnel current reverses the free layer magnetiza- tion. ST-MRAM promises to substantially enhance circuit density and speed, in comparison to magnetic field switch- ing, and could become a universal memory solution. Until now the TMR level realized in MTJs incorporating Py free electrodes is considerably less ͑ϳ85%,10 ϳ100%,14 in the higher RA regime͒ than the 150% level that essentially maxi- mizes the efficiency of the ST process15 that we report here in low RA MTJs. In this letter we confirm that Mg–B–O barriers have spin filtering properties in the low RA junction regime similar to those of pure MgO layers, and we examine the electronic structure of the Mg–B–O material using ultrahigh-vacuum ͑UHV͒ scanning tunneling spectroscopy ͑STS͒. We also demonstrate that annealed Co–Fe–B/Mg–B–O/Ni–Fe–B MTJs have body centered cubic ͑bcc͒ textured Py free elec- trodes, and that these MTJs achieve TMR values in the low RA, Ͻ20 ⍀ ␮m2 , regime that are comparable to those ob- tained with less desirable CoFe free electrodes, provided the PyB alloy is sufficiently B rich. This approach simulta- neously achieves high TMR in low RA MTJs with in-plane magnetized free electrode layers with low MS, magnetostric- tion, and microcrystalline anisotropy, that should lead to im- proved ST-MRAM structures. We grew the thin film stacks on 3 in. thermally oxidized Si wafers in a vacuum system ͑Pbaseϳ3ϫ10−9 Torr͒ containing multiple magnetron sputtering sources. The film structure is 5 nm Ta/͓20 nm Cu͑N͒/3 nm Ta͔x4/ 15 nm IrMn/4 nm base electrode/MgO ͑1.1–1.7 nm͒/ 2.5 nm top electrode/8 nm Ta/7 nm Ru. Cu͑N͒ indicates Cu reactively sputtered in an Ar/N2 atmosphere and the Cu͑N͒/Ta multilayer stack produces a smooth but highly con- ducting base layer, necessary for current-in-plane tunneling16 ͑CIPT͒ measurement of TMR and RA. Atomic force micros- copy measurements of the base electrodes indicate a rms roughness of 0.3–0.4 nm over a 9 ␮m2 area. We use Co60Fe20B20 ͑CFB͒ and Fe60Co20B20 ͑FCB͒ alloys for the reference or pinned electrode. We compare the results ob- tained with two different Permalloy-boron ͑PyB͒ alloy free layer electrodes, Ni77Fe18B5 ͑Py95B5͒ and Ni65Fe15B20 ͑Py80B20͒, with those obtained with symmetric junctions us- ing either CFB or FCB as both the reference and free layer electrodes. The tunnel barrier material is formed from rf sputtering of a sintered MgO target. After deposition, we cut the centers of the wafers into small ϳ1 cm2 chips and an- nealed some in modest vacuum ͑ϳ3ϫ10−6 Torr͒ to 250, 300, or 350 °C, for up to 2 h, prior to study with CIPT and scanning transmission electron microscopy utilizing electron energy-loss spectroscopy ͑EELS͒. For the in situ STS stud- ies, we grew CFB/MgO samples with sputtering and electron beam deposition ͑EBD͒ in a separate vacuum system ͑Pbase a͒ Electronic mail: APPLIED PHYSICS LETTERS 94, 112504 ͑2009͒ 0003-6951/2009/94͑11͒/112504/3/$25.00 © 2009 American Institute of Physics94, 112504-1 Downloaded 08 Dec 2010 to Redistribution subject to AIP license or copyright; see
  2. 2. ϳ2ϫ10−9 Torr͒ that is connected to a UHV ͑Pbaseϳ5 ϫ10−10 Torr͒ scanning tunneling microscope. The CIPT data shown in Fig. 1 confirm that Mg–B–O barrier layers can yield high values of TMR. Upon anneal- ing, the TMR of both symmetric CFB ͓Fig. 1͑a͔͒ and FCB ͓Fig. 1͑b͔͒ junctions increases dramatically, reaching the 160%–190% range after a 350 °C annealing. There is only slight variation in TMR with thickness over the 1.1–1.7 nm range. This behavior is not predicted by the theory of ideal MgO MTJs, but the TMR of our thicker junctions is also below these predictions.4,5 In considering the effect of an- nealing, it is useful to note that in the context of the Julliere model,17 increasing TMR from 25% to 160% indicates a 100% increase in the tunnel current polarization ͑Pt͒ from ϳ33% to ϳ67%. An additional increase in TMR to 600%, the current 300 K record18 for high RA MgO MJTs, repre- sents another 28% improvement in polarization ͑Ptϳ86%͒. Thus, the improvement of the spin filtering properties of these Mg–B–O barriers that occurs with annealing is already very good at the ϳ160% TMR level. Comparative in situ STS measurements of the electronic structure of thin MgO layers grown on CFB electrodes pro- vide insight into the beneficial effect of mixing BO3 into the MgO barrier. Figure 2 compares STS results obtained from a 2 nm MgO layer formed by EBD with those taken from a rf sputter deposited 2 nm Mg–B–O layer, before and after in situ annealing. Each trace represents averaged STS data taken at multiple spots on the sample. In the EBD MgO case ͓Fig. 2͑a͔͒, wherein oxidation of the base electrode is mini- mal, the STS data indicate a band gap of ϳ3 eV with no band offset, but significant band tailing to nearly the Fermi level ͑0 V͒, and there is only a small change, primarily in the band offset, upon annealing. Similar STS results were previ- ously reported for EBD MgO and dc-reactively sputtered Mg/MgO bilayers deposited on ͑001͒ Fe, and likewise we attribute the small band gap and low energy band-tail states of EBD MgO on CFB to lattice distortion and atomic defects in the crystalline MgO arising from strain at the oxide- electrode interface.19 The STS data of the as-sputtered layer ͓Fig. 2͑b͔͒ show a relatively wide band gap ͑ϳ4 eV͒ and greatly reduced band tailing in comparison to EBD MgO, which we attribute to the strain reducing effect of forming a mixed oxide with a sub- stantial BO3 component at the electrode-MgO interface. Note that the conduction band edge of the mixed oxide is shifted significantly toward the Fermi level, consistent with our previous x-ray photoemission spectroscopy ͑XPS͒ measurements,7 indicating that Mg–B–O has a higher work function than MgO. After annealing, the Mg–B–O band gap shrinks substantially to ϳ2.5 eV, smaller than EBD MgO, and the conduction band edge becomes more abrupt, sug- gesting a more ideal barrier material forms as the mixed oxide becomes more uniform and ordered, as also indicated by XPS ͑Ref. 7͒ and EELS ͑Ref. 9͒ studies. The beneficial effect of incorporating BO3 into the MgO barrier promotes consideration of PyB alloys as the free elec- trode in order to enhance the formation of Mg–B–O at the top of the barrier layer through reaction with surface oxygen on the previously deposited MgO and to possibly achieve preferred crystallization of the Py after the annealing pro- cess. As indicated by Fig. 1͑c͒, the use of a Py95B5 free electrode results in a roughly thickness independent TMR of ϳ40% ͑Ptϳ40%͒ upon moderate ͑300 °C͒ annealing, but higher temperature annealing deteriorates TMR. Junctions with a Py80B20 free electrode ͓Fig. 1͑d͔͒ show a steadily increasing TMR with annealing and little dependence of TMR on barrier thickness. FCB/1.1 nm Mg–B–O/Py80B20 MTJs annealed to 350 °C achieve ϳ155% TMR ͑Pt ϳ66%͒ and low RA ͑ϳ15 ⍀ ␮m2 ͒. TEM images of the two types of PyB junctions are shown in Fig. 3. The ϳ1.1 nm Mg–B–O barriers are poly- crystalline in both cases, but the Py95B5 and Py80B20 elec- trodes exhibit quite different crystal structures. The polycrys- talline Py95B5 electrode has some texturing in the as-grown case ͓Fig. 3͑a͔͒ but after annealing to 350 °C becomes less textured ͓Fig. 3͑c͔͒ as is clearly indicated by the nanometer spot size convergent beam electron diffraction ͑CBED͒ pat- tern ͓Fig. 3͑c͒, inset͔. In contrast, the as-deposited Py80B20 electrode is amorphous ͓Fig. 3͑b͔͒ as expected for such a high concentration of the glass-forming B component,20 but after annealing to 350 °C, the TEM and CBED measure- FIG. 1. ͑Color online͒ CIPT measurements of MTJ structures showing dependence of TMR vs RA on annealing for ͑a͒ all-Co60Fe20B20 electrodes, ͑b͒ all-Fe60Co20B20 electrodes, ͑c͒ Co60Fe20B20/Mg–B–O/ Py95B5 structures, and ͑d͒ Fe60Co20B20/Mg–B–O/Py80B20 structures. Each sample has been measured before ͑squares͒ and after annealing to 250 ͑circles͒, 300 ͑up triangles͒, and 350 °C ͑down triangles͒ as labeled. FIG. 2. ͑Color online͒ STS measurements of MgO and Mg–B–O layers deposited on Co60Fe20B20 films, before ͑black solid lines͒ and after ͑red dotted lines͒ annealing to 375 °C. The EBD MgO layer ͑a͒ shows the pres- ence of low energy states down to the Fermi level ͑0 V͒ and little change in the band gap after annealing. The rf-sputtered Mg–B–O layer ͑b͒ shows few low energy states before annealing with a conduction band offset of ϳ0.8 eV, and a dramatic decrease in the band gap after annealing, with a conduction band offset of ϳ0.6 eV. 112504-2 Read et al. Appl. Phys. Lett. 94, 112504 ͑2009͒ Downloaded 08 Dec 2010 to Redistribution subject to AIP license or copyright; see
  3. 3. ments ͓Fig. 3͑d͒, and inset͔ reveal that it has a highly tex- tured ͑001͒ bcc crystal structure, which is optimal according to the coherent spin-dependent tunneling model.4,5 EELS shows that this crystallization is accomplished by any glass- forming B that is not incorporated into the oxide barrier dif- fusing out of the Py80B20 into the capping layer.9 Magnetization measurements of the various free layers indicate that, as expected from the literature,20 the MS of CFB and FCB increases as B diffuses out of the electrodes during annealing. However, the MS of the Py80B20 layer is, to within measurement accuracy, the same as Py ͑4␲MS ϳ9600 G͒ and does not change significantly with annealing. The magnetic coupling between the fixed and free electrodes, measured by the shift ͑Hd͒ from zero of the centroid of the free layer magnetization loops of unpatterned samples, is an important parameter in MTJs.21 This coupling is typically ferromagnetic and attributable to thin film roughness.22 We find that as-deposited MTJs with FCB base electrodes have Hd values that are roughly twice those of MTJs with CFB base electrodes. This is consistent with the former being magnetically rougher due to greater oxidation of the Fe rich electrode. Upon 350 °C annealing, Hd decreases in all samples, with the most pronounced decrease apparent in FCB/Mg–B–O/Py80B20 MTJs. After annealing, these MTJs with barrier thicknesses Ͼ1.1 nm exhibit HdՅ2.5 Oe, in- dicative of very magnetically smooth junctions. However, Hd is higher ͑ϳ10 Oe͒ for the 1.1 nm barrier case, which we tentatively attribute to the onset of significant ferromagnetic interlayer exchange coupling.23 In summary, we obtain high TMR MTJs with a rf plasma MgO deposition process that naturally utilizes the reactivity of the B component of the base electrode with adventitious O in the process chamber to incorporate B into the Mg–B–O tunnel barrier. Consistent with the TMR results, the elec- tronic properties of Mg–B–O appear quite favorable for yielding low leakage junctions, which indicates why the rf sputter deposition of MgO onto B-alloyed base electrodes is effective in the thin barrier, low RA regime. Using PyB alloy electrodes, this technique yields high TMR ͑155%, Pt ϳ66%͒, low RA ͑ϳ15 ⍀ ␮m2 ͒ MTJs with Py-like free electrodes whose magnetic properties could to be advanta- geous for magnetic sensing and ST-MRAM applications. The authors thank P. J. Chen and Audie Castillo of NIST for assistance with CIPT measurements, Derek Stewart of the Cornell Nanoscale Facility for helpful discussions, and Daniel Worledge and Eileen Galligan of IBM, Yorktown Heights and Phil Mather and Jon Slaughter of Everspin for helpful discussions regarding CIPT measurements and for measurements of initial MTJ structures. This research was supported by the Cornell Center for Materials Research, a National Science Foundation ͑NSF͒ Materials Research Sci- ence and Engineering Center. Support was also provided by the Center for Nanoscale Systems, which is a NSF Nanoscale Science and Engineering Center, the Semiconductor Re- search Corporation, and the Office of Naval Research. 1 T. Linn and D. Mauri, US Patent No. 6841395 ͑January 11, 2005͒. 2 S. S. P. Parkin, C. Kaiser, A. Panchula, P. M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Mater. 3, 862 ͑2004͒. 3 S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Mater. 3, 868 ͑2004͒. 4 W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, Phys. Rev. B 63, 054416 ͑2001͒. 5 J. Mathon and A. Umerski, Phys. Rev. B 63, 220403͑R͒ ͑2001͒. 6 S. Yuasa, Y. Suzuki, T. Katayama, and K. Ando, Appl. Phys. Lett. 87, 242503 ͑2005͒. 7 J. C. Read, P. G. Mather, and R. A. Buhrman, Appl. Phys. Lett. 90, 132503 ͑2007͒. 8 J. J. Cha, J. C. Read, R. A. Buhrman, and D. A. Muller, Appl. Phys. Lett. 91, 062516 ͑2007͒. 9 The barrier composition is Mg0.38B0.05O0.57 and the cation ratio is B/͑MgϩB͒ϭ0.05/͑0.05ϩ0.38͒ϳ0.12, J. J. Cha, J. C. Read, R. A. Buhrman, and D. A. Muller ͑unpublished͒. 10 R. S. Dave, G. Steiner, J. M. Slaughter, J. J. Sun, B. Craigo, S. Pietam- baram, K. Smith, G. Grynkewich, M. DeHerrera, J. Akerman and S. Te- hrani, IEEE Trans. Magn. 42, 1935 ͑2006͒. 11 R. M. Bozorth, Ferromagnetism ͑IEEE, Piscataway, NJ, 1951͒. 12 P. M. Braganca, I. N. Krivorotov, O. Ozatay, A. G. F. Garcia, N. C. Emley, J. C. Sankey, D. C. Ralph, and R. A. Buhrman, Appl. Phys. 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Lett. 89, 112503 ͑2006͒. FIG. 3. Cross-sectional TEM images of as-grown CFB/Mg–B–O/Py95B5 ͑a͒ and FCB/Mg–B–O/Py80B20 MTJs ͑b͒, and after annealing to 350 °C ͓͑c͒ and ͑d͔͒. Insets: CBED patterns from the Py95B5 electrode ͓inset in ͑c͔͒ and from the Py80B20 electrode ͓inset in ͑d͔͒ after annealing. 112504-3 Read et al. Appl. Phys. Lett. 94, 112504 ͑2009͒ Downloaded 08 Dec 2010 to Redistribution subject to AIP license or copyright; see