- Email: [email protected]
Fe tunnel junctions patterned by in situ shadow-masks
Journal of Alloys and Compounds 662 (2016) 79e83
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Inverse tunnel magnetoresistance in epitaxial FeCo/MgO/Fe tunnel junctions patterned by in situ shadow-masks Xiaoyang Gao a, Qiang Li a, b, c, *, Shandong Li a, Jie Xu a, Youzhi Qin a, Xingjun Shi a, Shishen Yan b, Guoxing Miao c a b c
College of Physics Science, Qingdao University, Qingdao 266071, China School of Physics, National Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Department of Electrical and Computer Engineering, University of Waterloo, Ontario N2L 3G1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2015 Received in revised form 7 November 2015 Accepted 9 December 2015 Available online 12 December 2015
Fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates were fabricated using in situ shadow-masks in an electron-beam deposition system. An inverse tunneling magnetoresistance (TMR) of 39% was observed at 77 K after annealing, which was not obtained in MTJs grown in better vacuum with the same device structure. This inverse TMR is attributed to the oxidation of the FeCo/MgO interface, which provides a negative spin polarization. Our work highlights the importance of interfacial properties on tunneling magnetoresistance and points to a simple processing route to achieve inverse TMR by carefully controlling the oxidation condition of the bottom layer. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic tunneling junction Inverse magnetoresistance Oxide/metal interface Annealing effect
1. Introduction The tunnel magnetoresistance (TMR) effect has attracted significant attention in recent years not only due to the extensive applications of the magnetic tunnel junctions (MTJs) but also for its scientific interest [1]. Ten years after the successful demonstration of room temperature TMR with Al2O3 tunnel barriers [2], giant TMR around 200% in epitaxial and textured MgO based MTJs are achieved [3,4]. The fully epitaxial MTJs on MgO substrates are of great advantage in studying the physics of spin-dependent tunneling [3,5,6], because the experiments can be compared with firstprinciple calculations [7]. As to the in situ shadow-mask fabrication method, it is widely used in probing the tunneling spinpolarization of textured MTJs by superconducting tunneling spectroscopy [4,8,9], because the pollution from optical/e-beam lithography resist can be avoided. Thus, the superconductive characteristics of the detection electrodes cannot be effected during the fabrication process. Therefore, fabricating epitaxial MTJs by in situ shadow-masks is of great benefit in spin-dependent
tunneling understanding. However, up to now, there are no available reports of epitaxial MTJs patterned by in situ shadow-masks due to fabrication difficulties. It is challenging to avoid pinholes in single crystal barrier with large junction size and oxidation of bottom layers at high temperature for epitaxial growth. In our experiment, fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates were fabricated using in situ shadow-masks in an electron-beam deposition system. A large inverse TMR of 39% was observed at 77 K after annealing. However, if we fabricate the same magnetic tunnel junction in a better vacuum, the inverse TMR cannot be obtained. This is because a FeCo-oxide layer at the bottom FeCo/MgO interface provided a negative spin polarization and resulted in the inverse TMR effect. Our work demonstrates once more the important influence of interface on the spin tunneling process and points to a simple processing route to achieving inverse TMR, which is useful for magnetic logic circuits to compose complementary switching elements with conventional normal TMR elements [10]. 2. Experiments
* Corresponding author. College of Physics Science, Qingdao University, Qingdao 266071, China. E-mail address: [email protected] (Q. Li). http://dx.doi.org/10.1016/j.jallcom.2015.12.073 0925-8388/© 2015 Elsevier B.V. All rights reserved.
We fabricated our epitaxial layers in an e-beam evaporation system, with the base pressure better than 5 109 torr. Fig. 1a displays the cross-sectional view of FeCo(2.5 nm)/MgO(2.7 nm)/
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Fig. 1. (a) Schematic structure of MTJs. (b) Optical micrograph of the whole device. (c) Optical micrograph of the junction area.
Fe(10 nm) junctions. The MTJs were patterned by in situ metal shadow masks into a cross configuration as shown in Fig. 1b. The junction area was defined by MgO isolation layers with an area of 50 50 mm2 as shown in Fig. 1c. Four separate shadow masks were used to form sequentially the bottom layer, bottom isolation pads, top isolation pads, and finally the top electrode. Si wafers were cleaned in isopropyl alcohol, dilute H2SO4, and HF before loading into the chamber, which helped establish a hydrogen-terminated surface. A 10 nm MgO (100) buffer layer was firstly grown across the wafer at 300 C, which also prevents the diffusion of Fe into Si. The high deposition temperature helps overcome the amorphous surface oxides and the large lattice mismatch between MgO and Si. After the deposition of the buffer layer, the bottom electrode Fe/ FeCo, and the barrier MgO were deposited at 180 C using the first mask in the form of a rectangle of 10 1 mm2 in area. Then bottom and top electrical isolation pads were formed from 10 nm MgO layers through shadow masks, defining a junction area of 50 50 mm2. In the end, the top electrode was patterned through the fourth mask. In order to distinguish the coercivities of the bottom and top magnetic layers, we deposited 20 nm Co to magnetically harden the top electrode. The crystalline quality of the MTJ stacks were characterized by high-resolution x-ray diffraction (HRXRD). Magneto-transport characterizations were measured by a computer controlled four-point-probe test station. 3. Results and discussion Fig. 2a shows q-2q XRD patterns of an MTJ stack deposited on (100) Si. Intense peaks diffracted from the (200) planes of the planes of MgO, Fe and FeCo were observed, suggesting a very strong
preferred orientation along the [100] axis. We also performed 4 scan on the Fe (110) reflections to verify the epitaxy. As shown in Fig. 2b, four-fold symmetry is clear in the scan, which confirms the heteroepotaxial growth of the MTJs. It should be noted that the growth temperature of MgO buffer layer was important for the epitaxial growth. The MTJ stacks grown at temperatures lower than 200 C did not show any epitaxial MgO growth and the subsequently deposited Fe layer turned out to be polycrystalline with predominantly (110) out-of-plane orientation. Fig. 3 shows the magnetotransport properties of MTJs. The tunneling magnetoresistance is defined as TMR¼(RAP/RP)/RP, where RAP and RP are the resistances corresponding to the antiparallel and parallel magnetization states of the two magnetic electrodes, respectively. Comparing with our previous study on MTJs patterned by standard optical lithography in higher vacuum [5], in the present work the TMR was much lower at room temperature (being only 34%), and increased to 65% when lowering the measurement temperature to 77 K [Fig. 3a and b]. Moreover, the resistance area product (RA) and the temperature dependence of junction resistance were much larger than those grown in higher vacuum [5]. These phenomenon are related to the oxidation of bottom FeCo/MgO interface. As it is mentioned in Ref [3], during the growth of isolation pads, some of the evaporated MgO decomposed to O atoms and formed O2 molecules in the vacuum chamber, increasing the chamber pressure to 1 107 torr. Then the bottom FeCo layer was oxidized, which is similar to the oxidation of the CoFeB electrode [11]. Previous reports show that anneal can incorporate oxygen atoms from the electrode into the MgO barrier and reduce the oxidization at the interface [12e14]. In order to optimize the TMR, we annealed the MTJs at 350 C for 1 h.
X. Gao et al. / Journal of Alloys and Compounds 662 (2016) 79e83
Fig. 2. (a) X-ray diffraction qe2q scans of an MTJ stack. To avoid the intense Si (400) diffraction and its satellite peaks, q was offset by 2 in the scan. (b) In-plane 4 scan of the heterostructure taken on (110) reflection of Fe.
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Interestingly, the TMR reversed its sign to 39% at 77 K [Fig. 3c]. As shown in Fig. 3d, the TMR ratio decreased symmetrically with increasing bias voltage at 77 K. The magnitude of TMR also decreased quickly with increasing temperature, but it was still up to 3% at room temperature [as shown in the inset of Fig. 3d]. Inverse TMR can be caused by either the intrinsic spin polarization of the ferromagnetic electrodes [15,16] or the resonant spin dependent tunneling through the impurity states in the barrier re's model [19], if a junction is formed [17,18]. According to the Jullie with one negative polarization electrode and one positive polarization electrode, the TMR will be negative. On the other hand, tunneling via localized states in the barrier under resonant conditions leads to a change in sign of the effective spin polarization of conducting electrons [17]. In resonantly tunneling MTJs, the voltage dependence of TMR was usually non-symmetric due to the distribution of the localized states in barriers [17,18,20], which is not consistent with our symmetric dependence. Without considering the resonant tunneling effect, the negative TMR can arise from the opposite sign of the spin polarization in two electrodes, which is obviously not the case for pure FeCo and Fe electrodes with MgO barrier. The most likely explanation is that the FeCo-oxide formed at the bottom interface provides a negative polarization [16,21]. There has been an increasing amount of evidence that electric properties of MTJs was strongly influenced by the interface between magnetic electrodes and insulator barrier [22e25]. By thermal annealing or inserting interlayer to engineer the interface states, the TMR varied vastly or even reversed its magnetoresistance sign [26e29]. In MTJs with Al2O3 barriers, Shi et al. and Yang et al. reported a reverse TMR around 7% induced by interfacial oxide Fe3O4 [30,31]. They confirmed the origin of reverse TMR by xray photoelectron spectroscopy (XPS) and STEM respectively. In MgO based MTJs, Greullet et al. fabricated epitaxial Fe/Fe3O4/MgO/ Co structure with a large negative TMR of 22% at 80 K [16] and Yang et al. grew FeCo/NiO/MgO/FeCo junctions with a negative TMR -16% at 2.8 K [27]. Recently, Galceran et al. observed a reverse of TMR in La0.7Sr0.3MnO3/MgO/Fe MTJs after annealing treatments and attributed it to the appearance of an FeOX layer by the interface [32]. For our MTJs, one possibility is the formation of half-metal
Fig. 3. Magnetotransport properties of MTJs grown under a base pressure of 5 109 torr: (a) The magnetoresistance for MTJs as grown at room temperature. (b) The magnetoresistance for MTJs as grown at 77 K. (c) The magnetoresistance for MTJs at 77 K after annealing. (d) The voltage dependence of TMR in MTJs at 77 K after annealing. The inset shows its magnetoresistance at room temperature.
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Fe3O4 with negative polarization and another possibility is the formation of insulating CoFe2O4 interface layer with negative spin filter efficiency [33]. To confirm the influence of the interfacial oxidation on the reverse TMR, we fabricated CoFe/MgO/Fe MTJs in an improved vacuum by adding a liquid nitrogen cooled shroud for effectively absorbing the floating oxygen molecules. The base pressure was reduced to 5 1010 torr and the pressure during growth of MgO isolation pads was about 1 108 torr. As a result of the higher vacuum, the as-grown MTJs showed a higher TMR of about 50% at room temperature and 92% at 77 K as shown in Fig. 4a. However, after annealing at 350 C, we did not observe the reverse TMR. Moreover, the normal positive TMR increased to 127% at 77 K and 70% at room temperature as shown in Fig. 4b and c. Based on the above analysis, we believe that the inverse TMR is attributed to the oxide layer with a negative spin polarization formed in the FeCo/MgO interface. The formation of oxide was further confirmed by XPS measurements. As shown in Fig. 5, the peaks of Fe2p3/2 locating at 711 eV, 709 eV and Co 2p3/2 locating at 780 eV confirmed the oxidation of the FeCo/MgO interface. These results also indicates that the growth pressure of isolation pads is crucial in the fabrication of epitaxial MTJs by in situ shadow-masks,
Fig. 5. (a) The XPS spectrum of Fe 2p3/2 at the FeCo/MgO interface. (b) The XPS spectrum of Co 2p3/2 at the FeCo/MgO interface.
because bottom layers are much easier to be oxidized at high temperature that required for epitaxial growth. To get higher TMR, we need higher chamber vacuum to prevent the oxidation of bottom electrode. 4. Conclusions Fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates patterned by in situ shadow-masks were firstly fabricated. A large inverse TMR of 39% was observed at 77 K after annealing due to a FeCo-oxide layer formed by the bottom FeCo/ MgO interface. In higher vacuum, positive TMR of 127% at 77 K and 70% at room temperature were observed. Our work highlights interface effects in spin-dependent tunneling and opens simple ways to achieve inverse TMR using well-established materials, which is useful for magnetic logic circuits combining MTJs with normal and inverse magnetoresistive effects. Acknowledgments This work was supported partly by the National Sciences Foundation of China No. 11504192, the National Science Foundation of Shandong Province BSB2014010, ZR2012FZ006 and 2015M570570. References
Fig. 4. Magneto-transport properties of MTJs grown under a base pressure of 5 1010 torr: (a) The magnetoresistance for MTJs at 77 K, as grown. (b) The magnetoresistance for MTJs at 77 K, after annealing. (c) The magnetoresistance for MTJs at room temperature, after annealing.
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X. Gao et al. / Journal of Alloys and Compounds 662 (2016) 79e83 tunnel barriers, Nat. Mater. 3 (2004) 862e867. [5] G.X. Miao, Y.J. Park, J.S. Moodera, Disturbance of tunneling coherence by oxygen vacancy in epitaxial Fe/MgO/Fe magnetic tunnel junctions, Phys. Rev. Lett. 100 (2008) 246803. vre, L. Calmels, A. Tejeda, [6] F. Bonell, T. Hauet, S. Andrieu, F. Bertran, P. Le Fe F. Montaigne, B. Warot-Fonrose, B. Belhadji, A. Nicolaou, A. Taleb-Ibrahimi, Spin-polarized electron tunneling in bcc FeCo/MgO/FeCo(001) magnetic tunnel junctions, Phys. Rev. Lett. 108 (2012) 176602. [7] W.H. Butler, X.G. Zhang, T.C. Schulthess, J.M. Maclaren, Spin-dependent tunneling conductance of Fe/MgO/Fe sandwiches, Phys. Rev. B 63 (2001) 054416. [8] H. Yang, S.-H. Yang, S.S.P. Parkin, Optimized thickness of superconducting aluminum electrodes for measurement of spin polarization with MgO tunnel barriers, Appl. Phys. Lett. 90 (2007) 202502. [9] O. Schebaum, S. Fabretti, J.S. Moodera, A. Thomas, Spin-polarized tunneling in MgO-based tunnel junctions with superconducting electrodes, New J. Phys. 14 (2012) 033023. [10] S. Isogami, T. Owada, Electronic circuit using magnetic tunnel junctions with normal and inverse magnetoresistive effects, IEEJ Trans. Electr. Electron. Eng. 9 (2014) S73eS75. [11] J.-G. Song, J. Park, J. Yoon, K. Kim, Y. Jang, K. Kim, H. Kim, Investigation of atomic layer deposition of magnesium oxide on a CoFeB layer for threedimensional magnetic tunneling junctions, J. Alloys Compd. 588 (2014) 716e719. [12] J. Read, P. Mather, R. Buhrman, X-ray photoemission study of CoFeB/MgO thin film bilayers, Appl. Phys. Lett. 90 (2007) 132503. [13] A.T. Hindmarch, K.J. Dempsey, D. Ciudad, E. Negusse, D.A. Arena, C.H. Marrows, Fe diffusion, oxidation, and reduction at the CoFeB/MgO interface studied by soft X-ray absorption spectroscopy and magnetic circular dichroism, Appl. Phys. Lett. 96 (2010) 092501. [14] Y. Liu, A.N. Chiaramonti, D.K. Schreiber, H. Yang, S.S.P. Parkin, O.G. Heinonen, A.K. Petford-Long, Effect of annealing and applied bias on barrier shape in CoFe/MgO/CoFe tunnel junctions, Phys. Rev. B 83 (2011) 165413. [15] K. Sunaga, M. Tsunoda, K. Komagaki, Y. Uehara, M. Takahashi, Inverse tunnel magnetoresistance in magnetic tunnel junctions with an Fe4N electrode, J. Appl. Phys. 102 (2007) 013917. [16] F. Greullet, E. Snoeck, C. Tiusan, M. Hehn, D. Lacour, O. Lenoble, C. Magen, L. Calmels, Large inverse magnetoresistance in fully epitaxial Fe/Fe3O4/MgO/ Co magnetic tunnel junctions, Appl. Phys. Lett. 92 (2008) 053508. [17] E.Y. Tsymbal, A. Sokolov, I.F. Sabirianov, B. Doudin, Resonant inversion of tunneling magnetoresistance, Phys. Rev. Lett. 90 (2003) 186602. [18] C. Tiusan, J. Faure-Vincent, C. Bellouard, M. Hehn, E. Jouguelet, A. Schuhl, Interfacial resonance state probed by spin-polarized tunneling in epitaxial Fe/ MgO/Fe tunnel junctions, Phys. Rev. Lett. 93 (2004) 106602. [19] M. Julliere, Tunneling between ferromagnetic films, Phys. Lett. A 54 (1975)
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225. [20] G.X. Miao, G. Xiao, A. Gupta, Anomalous bias dependence in magnetic tunnel junctions based on half-metallic CrO2 with hetero epitaxial SnO2 tunnel barrier, EPL 87 (2009) 47006. [21] J.A. Moyer, D.P. Kumah, C.A.F. Vaz, D.A. Arena, V.E. Henrich, Nanomagnetism of cobalt ferrite-based spin filters probed by spin-polarized tunneling, Appl. Phys. Lett. 101 (2012) 042409. le my, A. Fert, J. Contour, Role of metal-oxide interface in [22] J. Teresa, A. Barthe determining the spin polarization of magnetic tunnel junctions, Science 286 (1999) 507e509. [23] E.Y. Tsymbal, K.D. Belashchenko, J.P. Velev, S.S. Jaswal, M. van Schilfgaarde, I.I. Oleynik, D.A. Stewart, Interface effects in spin-dependent tunneling, Prog. Mater. Sci. 52 (2007) 401e420. [24] G.H. Yu, X.L. Peng, Inter-diffusion study in MgO tunneling magneto-resistive (TMR) system by XPS, Appl. Surf. Sci. 256 (2010) 6592e6595. [25] P. Vlaic, E. Burzo, K. Carva, Impact of Fe/NaCl (001) interface structure on electronic, magnetic and spin-polarized transport properties of Fe/NaCl/Fe (001) heterojunctions: an ab initio study, J. Alloys Compd. 598 (2014) 41e53. [26] J. Zhang, Y. Wang, X.G. Zhang, X.F. Han, Inverse and oscillatory magnetoresistance in Fe(001)/MgO/Cr/Fe magnetic tunnel junctions, Phys. Rev. B 82 (2010) 134449. [27] H. Yang, S.-H. Yang, D.C. Qi, A. Rusydi, H. Kawai, M. Saeys, T. Leo, D.J. Smith, S.S.P. Parkin, Negative tunneling magnetoresistance by canted magnetization in MgO/NiO tunnel barriers, Phys. Rev. Lett. 106 (2011) 167201. [28] Y.T. Chen, J.Y. Tseng, C.C. Chang, W.C. Liu, J.S.C. Jang, Effect of field-annealing on magnetostriction and tunneling magnetoresistance of Co/AlOx/Co/IrMn junctions, J. Alloys Compd. 489 (2010) 242e245. [29] F. Godel, M.V. Kamalakar, B. Doudin, Y. Henry, D. Halley, J.-F. Dayen, Voltagecontrolled inversion of tunnel magnetoresistance in epitaxial nickel/graphene/MgO/cobalt junctions, Appl. Phys. Lett. 105 (2014) 152407. [30] F.Y. Shi, H. Xiang, J.J. Yang, M. Srzchowski, Y.A. Chang, P.M. Voyles, Inverse TMR in a nominally symmetric CoFe/AlOx/CoFe junction induced by interfacial Fe3O4 investigated by STEM-EEL, J. M. M. M. 324 (2012) 1837e1844. [31] J.J. Yang, H. Xiang, C.X. Ji, W.F. Stickle, D.R. Stewart, D.A.A. Ohlberg, R.S. Williams, Y.A. Chang, Origin of inverse tunneling magnetoresistance in a symmetric junction revealed by delaminating the buried electronic interface, Appl. Phys. Lett. 95 (2009) 233117. andez, [32] R. Galceran, L. Balcells, C. Martinez-Boubeta, B. Bozzo, J. Cisneros-Fern M. de la Mata, C. Magen, J. Arbiol, J. Tornos, F.A. Cuellar, Z. Sefrioui, A. Cebollada, F. Golmar, L.E. Hueso, F. Casanova, J. Santamaria, B. Martinez, Interfacial effects on the tunneling magnetoresistance in La0.7Sr0.3MnO3/MgO/ Fe tunneling junctions, Phys. Rev. B 92 (2015) 094428. [33] Z. Szotek, W.M. Temmerman, D. Koedderitzsch, A. Svane, L. Petit, H. Winter, Electronic structure of normal and inverse spinel ferrites from first principles, Phys. Rev. B 74 (2006) 174431.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Inverse tunnel magnetoresistance in epitaxial FeCo/MgO/Fe tunnel junctions patterned by in situ shadow-masks Xiaoyang Gao a, Qiang Li a, b, c, *, Shandong Li a, Jie Xu a, Youzhi Qin a, Xingjun Shi a, Shishen Yan b, Guoxing Miao c a b c
College of Physics Science, Qingdao University, Qingdao 266071, China School of Physics, National Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China Department of Electrical and Computer Engineering, University of Waterloo, Ontario N2L 3G1, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 September 2015 Received in revised form 7 November 2015 Accepted 9 December 2015 Available online 12 December 2015
Fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates were fabricated using in situ shadow-masks in an electron-beam deposition system. An inverse tunneling magnetoresistance (TMR) of 39% was observed at 77 K after annealing, which was not obtained in MTJs grown in better vacuum with the same device structure. This inverse TMR is attributed to the oxidation of the FeCo/MgO interface, which provides a negative spin polarization. Our work highlights the importance of interfacial properties on tunneling magnetoresistance and points to a simple processing route to achieve inverse TMR by carefully controlling the oxidation condition of the bottom layer. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic tunneling junction Inverse magnetoresistance Oxide/metal interface Annealing effect
1. Introduction The tunnel magnetoresistance (TMR) effect has attracted significant attention in recent years not only due to the extensive applications of the magnetic tunnel junctions (MTJs) but also for its scientific interest [1]. Ten years after the successful demonstration of room temperature TMR with Al2O3 tunnel barriers [2], giant TMR around 200% in epitaxial and textured MgO based MTJs are achieved [3,4]. The fully epitaxial MTJs on MgO substrates are of great advantage in studying the physics of spin-dependent tunneling [3,5,6], because the experiments can be compared with firstprinciple calculations [7]. As to the in situ shadow-mask fabrication method, it is widely used in probing the tunneling spinpolarization of textured MTJs by superconducting tunneling spectroscopy [4,8,9], because the pollution from optical/e-beam lithography resist can be avoided. Thus, the superconductive characteristics of the detection electrodes cannot be effected during the fabrication process. Therefore, fabricating epitaxial MTJs by in situ shadow-masks is of great benefit in spin-dependent
tunneling understanding. However, up to now, there are no available reports of epitaxial MTJs patterned by in situ shadow-masks due to fabrication difficulties. It is challenging to avoid pinholes in single crystal barrier with large junction size and oxidation of bottom layers at high temperature for epitaxial growth. In our experiment, fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates were fabricated using in situ shadow-masks in an electron-beam deposition system. A large inverse TMR of 39% was observed at 77 K after annealing. However, if we fabricate the same magnetic tunnel junction in a better vacuum, the inverse TMR cannot be obtained. This is because a FeCo-oxide layer at the bottom FeCo/MgO interface provided a negative spin polarization and resulted in the inverse TMR effect. Our work demonstrates once more the important influence of interface on the spin tunneling process and points to a simple processing route to achieving inverse TMR, which is useful for magnetic logic circuits to compose complementary switching elements with conventional normal TMR elements [10]. 2. Experiments
* Corresponding author. College of Physics Science, Qingdao University, Qingdao 266071, China. E-mail address: [email protected] (Q. Li). http://dx.doi.org/10.1016/j.jallcom.2015.12.073 0925-8388/© 2015 Elsevier B.V. All rights reserved.
We fabricated our epitaxial layers in an e-beam evaporation system, with the base pressure better than 5 109 torr. Fig. 1a displays the cross-sectional view of FeCo(2.5 nm)/MgO(2.7 nm)/
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Fig. 1. (a) Schematic structure of MTJs. (b) Optical micrograph of the whole device. (c) Optical micrograph of the junction area.
Fe(10 nm) junctions. The MTJs were patterned by in situ metal shadow masks into a cross configuration as shown in Fig. 1b. The junction area was defined by MgO isolation layers with an area of 50 50 mm2 as shown in Fig. 1c. Four separate shadow masks were used to form sequentially the bottom layer, bottom isolation pads, top isolation pads, and finally the top electrode. Si wafers were cleaned in isopropyl alcohol, dilute H2SO4, and HF before loading into the chamber, which helped establish a hydrogen-terminated surface. A 10 nm MgO (100) buffer layer was firstly grown across the wafer at 300 C, which also prevents the diffusion of Fe into Si. The high deposition temperature helps overcome the amorphous surface oxides and the large lattice mismatch between MgO and Si. After the deposition of the buffer layer, the bottom electrode Fe/ FeCo, and the barrier MgO were deposited at 180 C using the first mask in the form of a rectangle of 10 1 mm2 in area. Then bottom and top electrical isolation pads were formed from 10 nm MgO layers through shadow masks, defining a junction area of 50 50 mm2. In the end, the top electrode was patterned through the fourth mask. In order to distinguish the coercivities of the bottom and top magnetic layers, we deposited 20 nm Co to magnetically harden the top electrode. The crystalline quality of the MTJ stacks were characterized by high-resolution x-ray diffraction (HRXRD). Magneto-transport characterizations were measured by a computer controlled four-point-probe test station. 3. Results and discussion Fig. 2a shows q-2q XRD patterns of an MTJ stack deposited on (100) Si. Intense peaks diffracted from the (200) planes of the planes of MgO, Fe and FeCo were observed, suggesting a very strong
preferred orientation along the [100] axis. We also performed 4 scan on the Fe (110) reflections to verify the epitaxy. As shown in Fig. 2b, four-fold symmetry is clear in the scan, which confirms the heteroepotaxial growth of the MTJs. It should be noted that the growth temperature of MgO buffer layer was important for the epitaxial growth. The MTJ stacks grown at temperatures lower than 200 C did not show any epitaxial MgO growth and the subsequently deposited Fe layer turned out to be polycrystalline with predominantly (110) out-of-plane orientation. Fig. 3 shows the magnetotransport properties of MTJs. The tunneling magnetoresistance is defined as TMR¼(RAP/RP)/RP, where RAP and RP are the resistances corresponding to the antiparallel and parallel magnetization states of the two magnetic electrodes, respectively. Comparing with our previous study on MTJs patterned by standard optical lithography in higher vacuum [5], in the present work the TMR was much lower at room temperature (being only 34%), and increased to 65% when lowering the measurement temperature to 77 K [Fig. 3a and b]. Moreover, the resistance area product (RA) and the temperature dependence of junction resistance were much larger than those grown in higher vacuum [5]. These phenomenon are related to the oxidation of bottom FeCo/MgO interface. As it is mentioned in Ref [3], during the growth of isolation pads, some of the evaporated MgO decomposed to O atoms and formed O2 molecules in the vacuum chamber, increasing the chamber pressure to 1 107 torr. Then the bottom FeCo layer was oxidized, which is similar to the oxidation of the CoFeB electrode [11]. Previous reports show that anneal can incorporate oxygen atoms from the electrode into the MgO barrier and reduce the oxidization at the interface [12e14]. In order to optimize the TMR, we annealed the MTJs at 350 C for 1 h.
X. Gao et al. / Journal of Alloys and Compounds 662 (2016) 79e83
Fig. 2. (a) X-ray diffraction qe2q scans of an MTJ stack. To avoid the intense Si (400) diffraction and its satellite peaks, q was offset by 2 in the scan. (b) In-plane 4 scan of the heterostructure taken on (110) reflection of Fe.
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Interestingly, the TMR reversed its sign to 39% at 77 K [Fig. 3c]. As shown in Fig. 3d, the TMR ratio decreased symmetrically with increasing bias voltage at 77 K. The magnitude of TMR also decreased quickly with increasing temperature, but it was still up to 3% at room temperature [as shown in the inset of Fig. 3d]. Inverse TMR can be caused by either the intrinsic spin polarization of the ferromagnetic electrodes [15,16] or the resonant spin dependent tunneling through the impurity states in the barrier re's model [19], if a junction is formed [17,18]. According to the Jullie with one negative polarization electrode and one positive polarization electrode, the TMR will be negative. On the other hand, tunneling via localized states in the barrier under resonant conditions leads to a change in sign of the effective spin polarization of conducting electrons [17]. In resonantly tunneling MTJs, the voltage dependence of TMR was usually non-symmetric due to the distribution of the localized states in barriers [17,18,20], which is not consistent with our symmetric dependence. Without considering the resonant tunneling effect, the negative TMR can arise from the opposite sign of the spin polarization in two electrodes, which is obviously not the case for pure FeCo and Fe electrodes with MgO barrier. The most likely explanation is that the FeCo-oxide formed at the bottom interface provides a negative polarization [16,21]. There has been an increasing amount of evidence that electric properties of MTJs was strongly influenced by the interface between magnetic electrodes and insulator barrier [22e25]. By thermal annealing or inserting interlayer to engineer the interface states, the TMR varied vastly or even reversed its magnetoresistance sign [26e29]. In MTJs with Al2O3 barriers, Shi et al. and Yang et al. reported a reverse TMR around 7% induced by interfacial oxide Fe3O4 [30,31]. They confirmed the origin of reverse TMR by xray photoelectron spectroscopy (XPS) and STEM respectively. In MgO based MTJs, Greullet et al. fabricated epitaxial Fe/Fe3O4/MgO/ Co structure with a large negative TMR of 22% at 80 K [16] and Yang et al. grew FeCo/NiO/MgO/FeCo junctions with a negative TMR -16% at 2.8 K [27]. Recently, Galceran et al. observed a reverse of TMR in La0.7Sr0.3MnO3/MgO/Fe MTJs after annealing treatments and attributed it to the appearance of an FeOX layer by the interface [32]. For our MTJs, one possibility is the formation of half-metal
Fig. 3. Magnetotransport properties of MTJs grown under a base pressure of 5 109 torr: (a) The magnetoresistance for MTJs as grown at room temperature. (b) The magnetoresistance for MTJs as grown at 77 K. (c) The magnetoresistance for MTJs at 77 K after annealing. (d) The voltage dependence of TMR in MTJs at 77 K after annealing. The inset shows its magnetoresistance at room temperature.
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Fe3O4 with negative polarization and another possibility is the formation of insulating CoFe2O4 interface layer with negative spin filter efficiency [33]. To confirm the influence of the interfacial oxidation on the reverse TMR, we fabricated CoFe/MgO/Fe MTJs in an improved vacuum by adding a liquid nitrogen cooled shroud for effectively absorbing the floating oxygen molecules. The base pressure was reduced to 5 1010 torr and the pressure during growth of MgO isolation pads was about 1 108 torr. As a result of the higher vacuum, the as-grown MTJs showed a higher TMR of about 50% at room temperature and 92% at 77 K as shown in Fig. 4a. However, after annealing at 350 C, we did not observe the reverse TMR. Moreover, the normal positive TMR increased to 127% at 77 K and 70% at room temperature as shown in Fig. 4b and c. Based on the above analysis, we believe that the inverse TMR is attributed to the oxide layer with a negative spin polarization formed in the FeCo/MgO interface. The formation of oxide was further confirmed by XPS measurements. As shown in Fig. 5, the peaks of Fe2p3/2 locating at 711 eV, 709 eV and Co 2p3/2 locating at 780 eV confirmed the oxidation of the FeCo/MgO interface. These results also indicates that the growth pressure of isolation pads is crucial in the fabrication of epitaxial MTJs by in situ shadow-masks,
Fig. 5. (a) The XPS spectrum of Fe 2p3/2 at the FeCo/MgO interface. (b) The XPS spectrum of Co 2p3/2 at the FeCo/MgO interface.
because bottom layers are much easier to be oxidized at high temperature that required for epitaxial growth. To get higher TMR, we need higher chamber vacuum to prevent the oxidation of bottom electrode. 4. Conclusions Fully epitaxial FeCo/MgO/Fe magnetic tunnel junctions on silicon substrates patterned by in situ shadow-masks were firstly fabricated. A large inverse TMR of 39% was observed at 77 K after annealing due to a FeCo-oxide layer formed by the bottom FeCo/ MgO interface. In higher vacuum, positive TMR of 127% at 77 K and 70% at room temperature were observed. Our work highlights interface effects in spin-dependent tunneling and opens simple ways to achieve inverse TMR using well-established materials, which is useful for magnetic logic circuits combining MTJs with normal and inverse magnetoresistive effects. Acknowledgments This work was supported partly by the National Sciences Foundation of China No. 11504192, the National Science Foundation of Shandong Province BSB2014010, ZR2012FZ006 and 2015M570570. References
Fig. 4. Magneto-transport properties of MTJs grown under a base pressure of 5 1010 torr: (a) The magnetoresistance for MTJs at 77 K, as grown. (b) The magnetoresistance for MTJs at 77 K, after annealing. (c) The magnetoresistance for MTJs at room temperature, after annealing.
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