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Observation of a tunneling magnetoresistance effect in magnetic tunneling junctions with a high resistance ferromagnetic oxide Fe2⋅5Mn0⋅5O4 electrode
Solid State Communications 151 (2011) 1296–1299
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Observation of a tunneling magnetoresistance effect in magnetic tunneling junctions with a high resistance ferromagnetic oxide Fe2·5 Mn0·5 O4 electrode Eiji Shikoh a,∗ , Teruo Kanki b , Hidekazu Tanaka b , Teruya Shinjo a , Masashi Shiraishi a a
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
b
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
article
info
Article history: Received 5 April 2011 Received in revised form 14 May 2011 Accepted 19 May 2011 by T. Kimura Available online 27 May 2011
abstract A high resistance ferromagnetic oxide Fe2·5 Mn0·5 O4 (FMO) property as a novel spin injector was investigated with a structure of a magnetic tunneling junction (MTJ) composed of FMO/Al–O/Ni80 Fe20 , in order to reduce an impedance mismatch problem on molecular spintronics. A tunneling magnetoresistance (TMR) effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at room temperature (RT), and the spin-polarization of FMO was estimated to be at least 0.94% at RT. © 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic films and multilayers A. Thin films D. Tunneling
1. Introduction In the field of molecular spintronics [1–9] which combines spintronics [10] and molecular electronics [11], novel physical properties of molecular materials are expected to be produced by controlling the spin degree of freedom. Molecular materials composed of light elements (carbon, hydrogen, and so on) possess a small spin–orbit interaction, indicating a low spin-scattering probability on spin transport in molecular materials. Since the spin-dependent transport properties are directly connected to device applications, magnetoresistance (MR) effects of devices with molecular materials have been intensively investigated [1–3,5,7,8]. The MR ratio in such devices with molecular materials is not yet enough large for practical use. One reason for the small MR ratio in such devices is a low spin injection efficiency from ferromagnetic electrodes into molecular materials, and one reason for the low spin injection efficiency is an impedance mismatch (IM) problem between ferromagnetic electrodes and non-magnetic materials [12,13]. Two methods to solve the IM problem are considered: one is the selection of materials based on resistivities, that is to say, the use of high resistance ferromagnetic electrodes and/or the use of low resistance molecular materials are required. Since the resistance of molecular materials is high in general, except for special materials like molecular conductors, it
∗
Corresponding author. Tel.: +81 6 6850 6331; fax: +81 6 6850 6341. E-mail address: [email protected] (E. Shikoh).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.05.032
is not easy to use the latter. The other method is the utilization of the spin injection via a tunneling process with a tunneling barrier at the interface between ferromagnetic electrodes and molecular materials [4,7,14], although it is challenging to form a homogeneous tunneling barrier on molecular materials [4,7]. Considering the above methods, the use of high resistance ferromagnetic electrodes is more accessible than any other methods. Ferromagnetic oxides show higher resistivities of 10−3 –10−2 cm for La0.7 Sr0.3 MnO3 [15] and 10−3 –10−2 cm for Fe3 O4 [16] than those of conventional ferromagnetic metals. Therefore, using ferromagnetic oxide electrodes is an approach to solve the IM problem. At present, La0.7 Sr0.3 MnO3 and Fe3 O4 have already been used on molecular spintronics [3,7,8,17], not from the viewpoint of the IM problem, but from the viewpoint of their air stability. However, the ferromagnetic transition temperature TC of La0.7 Sr0.3 MnO3 is close to room temperature (RT), which is not appropriate for practical use. Meanwhile, Fe3 O4 with the TC of 860 K has relatively small resistivity compared with resistivities of molecular materials, which is not enough to solve the IM problem. In this study, another high resistance ferromagnetic oxide Fe2.5 Mn0.5 O4 (FMO) [16,18,19] was studied. The FMO has the TC of >360 K [16] and a high spin-polarization (P ) estimated up to 200 K [18]. Furthermore, a high resistivity of 10−2 –10−1 cm at 300 K of the FMO is reported [16], which allows us to expect a reduction of the resistance difference between ferromagnetic electrodes and molecular materials. Observation of a tunneling magnetoresistance (TMR) effect [20–22] in MTJs using
E. Shikoh et al. / Solid State Communications 151 (2011) 1296–1299
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Fig. 1. Schematic illustration of a cross sectional view of an MTJ, composed of MgO(100)-substrate/Fe2.5 Mn0.5 O4 (FMO: thickness, 25–30 nm)/Al–O(1.5–2)/ Ni80 Fe20 (20)/Au(5)/Al(120).
the structure of a magnetic tunneling junction (MTJ) with an FMO electrode was implemented for investigation of the FMO property as a novel spin injector. 2. Experiments
Fig. 2. Magnetization curves at room temperature of a Ni80 Fe20 electrode (black solid squares) and of an Fe2.5 Mn0.5 O4 electrode (red solid circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
MTJs in this study are composed of MgO(100)-substrate/FMO (thickness, 25–30 nm)/Al–O(1.5–2)/Ni80 Fe20 (20)/Au(5)/Al(120) as shown in Fig. 1, and the fabrication process was as follows: first, an FMO layer was grown on a 10 × 10 mm2 × 1 mm(thickness) MgO(100)-substrate by using a pulsed laser deposition (PLD) under an oxygen gas pressure of 1.0 × 10−4 Pa, with a deposition rate of 0.01 nm/s. The substrate temperature was kept at 400 °C during the deposition. The FMO lattice constant perpendicular to the substrate plane was estimated with an XRD analysis to be 0.852 nm, which was approximately twice as long as the MgOsubstrate lattice constant of 0.421 nm. Second, for the making of an Al–O tunneling insulator, a thin Al layer with a thickness of 0.5 nm was formed by an EB deposition under high vacuum (<10−6 Pa) with the deposition rate of 0.02 nm/s, and then the Al surface was exposed to oxygen gas at 105 Pa for 12 h. This Al deposition and the oxidation process were repeatedly carried out 3 times per sample substrate. Third, a Ni80 Fe20 layer as the other ferromagnetic electrode was formed by an EB deposition under high vacuum (<10−6 Pa) with a deposition rate of 0.02 nm/s, and subsequently without breaking the vacuum, an Au layer was formed by an EB deposition under high vacuum (<10−6 Pa) with a deposition rate of 0.07 nm/s, in order to avoid oxidizing the Ni80 Fe20 surface. Next, using photolithography, Ar ion milling, and conventional RF sputtering, an edge-cover layer of SiO2 (80 nm in thick) with a 16 × 32 µm2 window for an MTJ was formed, in order to form an MTJ structure without unexpected conduction paths. Finally, an Al conduction path was formed by an EB deposition with a shadow mask under high vacuum (<10−6 Pa), with a deposition rate of 0.15 nm/s. The substrate temperature during all of the depositions except for the FMO deposition was not controlled. Using the above procedure, ten MTJs per substrate were fabricated. Magnetic properties were investigated with magnetic Kerr effect measurements. At MR measurements, a 4-terminal-method was used in a vacuum probe system equipped with a pair of electromagnets. An external magnetic field at MR measurements was swept from −2,200 Oe to 2200 Oe, and subsequently down to −2,200 Oe. The average values of 3 scans of the external magnetic field were plotted in each MR graph. All of the measurements were performed at RT. 3. Results and discussion Fig. 2 shows magnetization curves of a Ni80 Fe20 electrode (black solid squares) and of an FMO electrode (red solid circles). Both electrodes show typical ferromagnetic behavior. In Fig. 2, the diamagnetic components have not been corrected because of the difficulty at magnetic Kerr effect measurements. Therefore,
Fig. 3. MR property at room temperature for only an Fe2.5 Mn0.5 O4 electrode at an applied voltage of 0.17 mV.
it seems that the saturation magnetic field of the FMO electrode is 1200 Oe, although the precise saturation magnetic field of the FMO is more than 2000 Oe at a SQUID measurement [16]. The coercivity of FMO was approximately 600 Oe, which was in agreement with the previously reported value [16]. Fig. 3 shows an MR property for only an FMO electrode at an applied voltage of 0.17 mV (at a constant current of 0.01 µA). Arrows show the sweep directions of an external magnetic field. The external magnetic field was applied parallel to the applied current flow in the FMO electrode. The resistivity of FMO in this study was lower than the expected value [16] and comparable to that of Fe3 O4 . Although we have confirmed the crystal growth of the FMO at an XRD measurement, there might be not only the FMO, but also the products with lower resistivity, for example, Fe3 O4 and so on. There may be compositional inhomogeneity. The FMO resistance almost monotonously decreased with the increase of the external magnetic field. The resistance change ratio in this magnetic field scan range was at most 0.06%. A similar behavior was observed when the magnetic field was applied perpendicular to the applied current flow in the FMO electrode. That is to say, no anisotropic MR was shown in the FMO. It has been suggested that the origin of MR in Fig. 3 is the magnetization process of the FMO itself. Fig. 4 shows a current–voltage (I–V ) property for an MTJ with an FMO electrode. That I–V curve shows a non-linear property, indicating that a barrier to the applied current exists in the MTJ. This behavior was observed in 20% of MTJs in each substrate. The other 80% of MTJs showed linear I–V property. This yield might be due to the Al film thickness distribution at the making of the Al–O tunneling barrier. Fig. 5 shows an MR property for the MTJ with an FMO electrode at an applied voltage of 0.25 V (at a constant current
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E. Shikoh et al. / Solid State Communications 151 (2011) 1296–1299
Fig. 4. Current–voltage (I–V ) property for an MTJ with an Fe2.5 Mn0.5 O4 electrode.
process of Ni80 Fe20 was not observed. The maximum TMR ratio was approximately 0.85% as shown in Fig. 5. Using that maximum TMR ratio, the P of FMO was estimated with a Julliere’s model [20]. In the model, TMR ratio in an MTJ is defined by using P values of both ferromagnetic electrodes in the MTJ. The P of FMO at RT was estimated to be 0.94%, with the P of Ni81 Fe19 , 45% [26] (on behalf of Ni80 Fe20 ). This P of 0.94% is definitely small compared with P values of conventional ferromagnetic materials. In the Julliere’s model, P values of ferromagnetic electrodes at the interfaces between the tunneling barrier and the ferromagnetic electrodes strongly affect the TMR ratio. An improvement in the quality of the interface between the materials is a key for obtaining a larger TMR ratio. For example, if the MTJs were fabricated without any exposure to the air from the beginning of the fabrication process to the end, the TMR ratio would be larger because the cleanness of the FMO surface would be kept and the magnetic impurities at the interface between the FMO and the Al–O tunneling barrier would be reduced. When a larger TMR ratio is obtained by improving the MTJ quality, the P of FMO is estimated to be larger. 4. Conclusions A high resistance ferromagnetic oxide FMO property as a novel spin injector was investigated with a structure of a MTJ composed of FMO/Al–O/Ni80 Fe20 , in order to reduce an impedance mismatch problem on molecular spintronics. A TMR effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at RT, and the spin-polarization of FMO was estimated to be at least 0.94% at RT. Acknowledgments
Fig. 5. MR property at room temperature for an MTJ with an Fe2.5 Mn0.5 O4 electrode at an applied voltage of 0.25 V.
of 0.03 µA). Arrows show the sweep directions of an external magnetic field. The external magnetic field was applied in the sample plane. In Fig. 5, an MR effect different from that in Fig. 3 was observed. In both sweep directions of the external magnetic field in Fig. 5, the resistance increased around 0 Oe and then decreased at a higher magnetic field, corresponding to typical TMR effects [20–22], although a great amount of the noise may be due to inhomogeneity of the Al–O tunneling barrier. The magnitude of resistance change and the MR ratio in Fig. 5 were larger than those in the FMO electrode itself. The MR effect in Fig. 5 was observed in the MTJs where the non-linear IV property was shown. We conclude that a TMR effect in MTJs with an FMO electrode has been observed. It should be noted that the coercivities in the TMR curves do not agree well with those in the magnetization curves of the electrodes. Because the surface of the FMO electrode is exposed to the air before the making of Al–O tunneling barrier, the FMO surface is contaminated. It causes the change of the surface magnetic property of the FMO, which is critical on the TMR effect. There is not only FMO, but also a ferromagnet Fe3 O4 , an antiferromagnet α -Fe2 O3 and so on. An external magnetic field of 3000 Oe is needed, in order to break the antiferromagnetic coupling in the α - Fe2 O3 [23]. The antiferromagnet makes another exchange coupling with the ferromagnets at the FMO surface and the exchange coupling pins the domain wall motion [24]. Therefore, the coercivity at the FMO surface easily changes. As the result, the coercivity discrepancy of FMO between that in TMR curves and that in magnetization curves was occurred. Meanwhile, we have not annealed the Ni80 Fe20 electrode in any magnetic field for improvement of the Ni80 Fe20 quality [25] because the annealing easily causes compositional inhomogeneity in the FMO. Therefore, a steep resistance change at 0 Oe expected from the magnetization
This work was partly supported by a Grant-in-Aid for Scientific Research from the MEXT, Japan, by the Global COE program of ‘‘Core Research and Engineering of Advanced Materials Interdisciplinary Education Center for Materials Research’’, and by the Cooperative Research Program of ‘‘Network Joint Research Center for Materials and Devices’’. References [1] K. Tsukagoshi, B.W. Alphenaar, H. Ago, Nature 401 (1999) 572. [2] V. Dediu, M. Murgia, F.C. Matacotta, C. Taliani, S. Barbanera, Solid State Commun. 122 (2002) 181. [3] Z.H. Xiong, D. Wu, Z.V. Vardeny, J. Shi, Nature 427 (1999) 821. [4] E. Shikoh, A. Fujiwara, Y. Ando, T. Miyazaki, Japan. J. Appl. Phys. 45 (2006) 6897. [5] T.S. Santos, J.S. Lee, P. Migdal, I.C. Lekshmi, B. Satpati, J.S. Moodera, Phys. Rev. Lett. 98 (2007) 016601. [6] W.J.M. Naber, S. Faez, W.G. van der Wiel, J. Phys. D 40 (2007) R205. [7] V. Dediu, L.E. Hueso, I. Bergenti, A. Riminucci, F. Borgatti, P. Graziosi, C. Newby, F. Casoli, M.P. De Jong, C. Taliani, Y. Zhan, Phys. Rev. B 78 (2008) 115203. [8] C. Barraud, P. Seneor, R. Mattana, S. Fusil, K. Bouzehouane, C. Deranlot, P. Graziosi, L. Hueso, I. Bergenti, V. Dediu, F. Petroff, A. Fert, Nat. Phys. 6 (2010) 615. [9] M. Shiraishi, T. Ikoma, Physica E 43 (2011) 1295. [10] S.A. Wolf, D.D. Awschlom, R.A. Buhrman, J.M. Daughton, S. von Molnar, M.L. Roukes, A.Y. Chtchelkanova, D.M. Treger, Science 294 (2001) 1488. [11] J.R. Heath, M.A. Ratner, Phys. Today 56 (2003) 43. [12] G. Schmidt, D. Ferrand, L.W. Molenkamp, A.T. Filip, B.J. van Wees, Phys. Rev. B 62 (2000) R4790. [13] A. Fert, H. Jaffres, Phys. Rev. B 64 (2001) 184420. [14] E.I. Rashba, Phys. Rev. B 62 (2000) R16267. [15] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [16] M. Ishikawa, H. Tanaka, T. Kawai, Appl. Phys. Lett. 86 (2005) 222504. [17] E. Arisi, I. Bergenti, M. Cavallini, A. Riminucci, G. Ruani, V. Dediu, M. Ghidini, C. Pernechele, M. Solzi, Appl. Phys. Lett. 93 (2008) 113305. [18] I. Satoh, J. Takaobushi, H. Tanaka, T. Kawai, Solid State Commun. 147 (2008) 397. [19] J. Takaobushi, M. Ishikawa, S. Ueda, E. Ikenaga, J. Kim, M. Kobata, Y. Takeda, Y. Saitoh, M. Yabashi, Y. Nishino, D. Miwa, K. Tamasaku, T. Ishikawa, I. Satoh, H. Tanaka, K. Kobayashi, T. Kawai, Phys. Rev. B 76 (2007) 205108.
E. Shikoh et al. / Solid State Communications 151 (2011) 1296–1299 [20] M. Julliere, Phys. Lett. A 54 (1975) 225. [21] T. Miyazaki, N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231. [22] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273.
[23] [24] [25] [26]
R. Chevallier, J. Phys. Radium 12 (1951) 172. C.K. Kim, R.C. O’Handley, Mater. Sci. Eng. B 38 (1996) 16. S. Chikazumi, T. Oomura, J. Phys. Soc. Japan 10 (1955) 842. D.J. Monsma, S.S.P. Parkin, Appl. Phys. Lett. 77 (2000) 720.
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Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Observation of a tunneling magnetoresistance effect in magnetic tunneling junctions with a high resistance ferromagnetic oxide Fe2·5 Mn0·5 O4 electrode Eiji Shikoh a,∗ , Teruo Kanki b , Hidekazu Tanaka b , Teruya Shinjo a , Masashi Shiraishi a a
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
b
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
article
info
Article history: Received 5 April 2011 Received in revised form 14 May 2011 Accepted 19 May 2011 by T. Kimura Available online 27 May 2011
abstract A high resistance ferromagnetic oxide Fe2·5 Mn0·5 O4 (FMO) property as a novel spin injector was investigated with a structure of a magnetic tunneling junction (MTJ) composed of FMO/Al–O/Ni80 Fe20 , in order to reduce an impedance mismatch problem on molecular spintronics. A tunneling magnetoresistance (TMR) effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at room temperature (RT), and the spin-polarization of FMO was estimated to be at least 0.94% at RT. © 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Magnetic films and multilayers A. Thin films D. Tunneling
1. Introduction In the field of molecular spintronics [1–9] which combines spintronics [10] and molecular electronics [11], novel physical properties of molecular materials are expected to be produced by controlling the spin degree of freedom. Molecular materials composed of light elements (carbon, hydrogen, and so on) possess a small spin–orbit interaction, indicating a low spin-scattering probability on spin transport in molecular materials. Since the spin-dependent transport properties are directly connected to device applications, magnetoresistance (MR) effects of devices with molecular materials have been intensively investigated [1–3,5,7,8]. The MR ratio in such devices with molecular materials is not yet enough large for practical use. One reason for the small MR ratio in such devices is a low spin injection efficiency from ferromagnetic electrodes into molecular materials, and one reason for the low spin injection efficiency is an impedance mismatch (IM) problem between ferromagnetic electrodes and non-magnetic materials [12,13]. Two methods to solve the IM problem are considered: one is the selection of materials based on resistivities, that is to say, the use of high resistance ferromagnetic electrodes and/or the use of low resistance molecular materials are required. Since the resistance of molecular materials is high in general, except for special materials like molecular conductors, it
∗
Corresponding author. Tel.: +81 6 6850 6331; fax: +81 6 6850 6341. E-mail address: [email protected] (E. Shikoh).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.05.032
is not easy to use the latter. The other method is the utilization of the spin injection via a tunneling process with a tunneling barrier at the interface between ferromagnetic electrodes and molecular materials [4,7,14], although it is challenging to form a homogeneous tunneling barrier on molecular materials [4,7]. Considering the above methods, the use of high resistance ferromagnetic electrodes is more accessible than any other methods. Ferromagnetic oxides show higher resistivities of 10−3 –10−2 cm for La0.7 Sr0.3 MnO3 [15] and 10−3 –10−2 cm for Fe3 O4 [16] than those of conventional ferromagnetic metals. Therefore, using ferromagnetic oxide electrodes is an approach to solve the IM problem. At present, La0.7 Sr0.3 MnO3 and Fe3 O4 have already been used on molecular spintronics [3,7,8,17], not from the viewpoint of the IM problem, but from the viewpoint of their air stability. However, the ferromagnetic transition temperature TC of La0.7 Sr0.3 MnO3 is close to room temperature (RT), which is not appropriate for practical use. Meanwhile, Fe3 O4 with the TC of 860 K has relatively small resistivity compared with resistivities of molecular materials, which is not enough to solve the IM problem. In this study, another high resistance ferromagnetic oxide Fe2.5 Mn0.5 O4 (FMO) [16,18,19] was studied. The FMO has the TC of >360 K [16] and a high spin-polarization (P ) estimated up to 200 K [18]. Furthermore, a high resistivity of 10−2 –10−1 cm at 300 K of the FMO is reported [16], which allows us to expect a reduction of the resistance difference between ferromagnetic electrodes and molecular materials. Observation of a tunneling magnetoresistance (TMR) effect [20–22] in MTJs using
E. Shikoh et al. / Solid State Communications 151 (2011) 1296–1299
1297
Fig. 1. Schematic illustration of a cross sectional view of an MTJ, composed of MgO(100)-substrate/Fe2.5 Mn0.5 O4 (FMO: thickness, 25–30 nm)/Al–O(1.5–2)/ Ni80 Fe20 (20)/Au(5)/Al(120).
the structure of a magnetic tunneling junction (MTJ) with an FMO electrode was implemented for investigation of the FMO property as a novel spin injector. 2. Experiments
Fig. 2. Magnetization curves at room temperature of a Ni80 Fe20 electrode (black solid squares) and of an Fe2.5 Mn0.5 O4 electrode (red solid circles). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
MTJs in this study are composed of MgO(100)-substrate/FMO (thickness, 25–30 nm)/Al–O(1.5–2)/Ni80 Fe20 (20)/Au(5)/Al(120) as shown in Fig. 1, and the fabrication process was as follows: first, an FMO layer was grown on a 10 × 10 mm2 × 1 mm(thickness) MgO(100)-substrate by using a pulsed laser deposition (PLD) under an oxygen gas pressure of 1.0 × 10−4 Pa, with a deposition rate of 0.01 nm/s. The substrate temperature was kept at 400 °C during the deposition. The FMO lattice constant perpendicular to the substrate plane was estimated with an XRD analysis to be 0.852 nm, which was approximately twice as long as the MgOsubstrate lattice constant of 0.421 nm. Second, for the making of an Al–O tunneling insulator, a thin Al layer with a thickness of 0.5 nm was formed by an EB deposition under high vacuum (<10−6 Pa) with the deposition rate of 0.02 nm/s, and then the Al surface was exposed to oxygen gas at 105 Pa for 12 h. This Al deposition and the oxidation process were repeatedly carried out 3 times per sample substrate. Third, a Ni80 Fe20 layer as the other ferromagnetic electrode was formed by an EB deposition under high vacuum (<10−6 Pa) with a deposition rate of 0.02 nm/s, and subsequently without breaking the vacuum, an Au layer was formed by an EB deposition under high vacuum (<10−6 Pa) with a deposition rate of 0.07 nm/s, in order to avoid oxidizing the Ni80 Fe20 surface. Next, using photolithography, Ar ion milling, and conventional RF sputtering, an edge-cover layer of SiO2 (80 nm in thick) with a 16 × 32 µm2 window for an MTJ was formed, in order to form an MTJ structure without unexpected conduction paths. Finally, an Al conduction path was formed by an EB deposition with a shadow mask under high vacuum (<10−6 Pa), with a deposition rate of 0.15 nm/s. The substrate temperature during all of the depositions except for the FMO deposition was not controlled. Using the above procedure, ten MTJs per substrate were fabricated. Magnetic properties were investigated with magnetic Kerr effect measurements. At MR measurements, a 4-terminal-method was used in a vacuum probe system equipped with a pair of electromagnets. An external magnetic field at MR measurements was swept from −2,200 Oe to 2200 Oe, and subsequently down to −2,200 Oe. The average values of 3 scans of the external magnetic field were plotted in each MR graph. All of the measurements were performed at RT. 3. Results and discussion Fig. 2 shows magnetization curves of a Ni80 Fe20 electrode (black solid squares) and of an FMO electrode (red solid circles). Both electrodes show typical ferromagnetic behavior. In Fig. 2, the diamagnetic components have not been corrected because of the difficulty at magnetic Kerr effect measurements. Therefore,
Fig. 3. MR property at room temperature for only an Fe2.5 Mn0.5 O4 electrode at an applied voltage of 0.17 mV.
it seems that the saturation magnetic field of the FMO electrode is 1200 Oe, although the precise saturation magnetic field of the FMO is more than 2000 Oe at a SQUID measurement [16]. The coercivity of FMO was approximately 600 Oe, which was in agreement with the previously reported value [16]. Fig. 3 shows an MR property for only an FMO electrode at an applied voltage of 0.17 mV (at a constant current of 0.01 µA). Arrows show the sweep directions of an external magnetic field. The external magnetic field was applied parallel to the applied current flow in the FMO electrode. The resistivity of FMO in this study was lower than the expected value [16] and comparable to that of Fe3 O4 . Although we have confirmed the crystal growth of the FMO at an XRD measurement, there might be not only the FMO, but also the products with lower resistivity, for example, Fe3 O4 and so on. There may be compositional inhomogeneity. The FMO resistance almost monotonously decreased with the increase of the external magnetic field. The resistance change ratio in this magnetic field scan range was at most 0.06%. A similar behavior was observed when the magnetic field was applied perpendicular to the applied current flow in the FMO electrode. That is to say, no anisotropic MR was shown in the FMO. It has been suggested that the origin of MR in Fig. 3 is the magnetization process of the FMO itself. Fig. 4 shows a current–voltage (I–V ) property for an MTJ with an FMO electrode. That I–V curve shows a non-linear property, indicating that a barrier to the applied current exists in the MTJ. This behavior was observed in 20% of MTJs in each substrate. The other 80% of MTJs showed linear I–V property. This yield might be due to the Al film thickness distribution at the making of the Al–O tunneling barrier. Fig. 5 shows an MR property for the MTJ with an FMO electrode at an applied voltage of 0.25 V (at a constant current
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E. Shikoh et al. / Solid State Communications 151 (2011) 1296–1299
Fig. 4. Current–voltage (I–V ) property for an MTJ with an Fe2.5 Mn0.5 O4 electrode.
process of Ni80 Fe20 was not observed. The maximum TMR ratio was approximately 0.85% as shown in Fig. 5. Using that maximum TMR ratio, the P of FMO was estimated with a Julliere’s model [20]. In the model, TMR ratio in an MTJ is defined by using P values of both ferromagnetic electrodes in the MTJ. The P of FMO at RT was estimated to be 0.94%, with the P of Ni81 Fe19 , 45% [26] (on behalf of Ni80 Fe20 ). This P of 0.94% is definitely small compared with P values of conventional ferromagnetic materials. In the Julliere’s model, P values of ferromagnetic electrodes at the interfaces between the tunneling barrier and the ferromagnetic electrodes strongly affect the TMR ratio. An improvement in the quality of the interface between the materials is a key for obtaining a larger TMR ratio. For example, if the MTJs were fabricated without any exposure to the air from the beginning of the fabrication process to the end, the TMR ratio would be larger because the cleanness of the FMO surface would be kept and the magnetic impurities at the interface between the FMO and the Al–O tunneling barrier would be reduced. When a larger TMR ratio is obtained by improving the MTJ quality, the P of FMO is estimated to be larger. 4. Conclusions A high resistance ferromagnetic oxide FMO property as a novel spin injector was investigated with a structure of a MTJ composed of FMO/Al–O/Ni80 Fe20 , in order to reduce an impedance mismatch problem on molecular spintronics. A TMR effect in the MTJs was observed. The maximum TMR ratio observed in the MTJs was approximately 0.85% at RT, and the spin-polarization of FMO was estimated to be at least 0.94% at RT. Acknowledgments
Fig. 5. MR property at room temperature for an MTJ with an Fe2.5 Mn0.5 O4 electrode at an applied voltage of 0.25 V.
of 0.03 µA). Arrows show the sweep directions of an external magnetic field. The external magnetic field was applied in the sample plane. In Fig. 5, an MR effect different from that in Fig. 3 was observed. In both sweep directions of the external magnetic field in Fig. 5, the resistance increased around 0 Oe and then decreased at a higher magnetic field, corresponding to typical TMR effects [20–22], although a great amount of the noise may be due to inhomogeneity of the Al–O tunneling barrier. The magnitude of resistance change and the MR ratio in Fig. 5 were larger than those in the FMO electrode itself. The MR effect in Fig. 5 was observed in the MTJs where the non-linear IV property was shown. We conclude that a TMR effect in MTJs with an FMO electrode has been observed. It should be noted that the coercivities in the TMR curves do not agree well with those in the magnetization curves of the electrodes. Because the surface of the FMO electrode is exposed to the air before the making of Al–O tunneling barrier, the FMO surface is contaminated. It causes the change of the surface magnetic property of the FMO, which is critical on the TMR effect. There is not only FMO, but also a ferromagnet Fe3 O4 , an antiferromagnet α -Fe2 O3 and so on. An external magnetic field of 3000 Oe is needed, in order to break the antiferromagnetic coupling in the α - Fe2 O3 [23]. The antiferromagnet makes another exchange coupling with the ferromagnets at the FMO surface and the exchange coupling pins the domain wall motion [24]. Therefore, the coercivity at the FMO surface easily changes. As the result, the coercivity discrepancy of FMO between that in TMR curves and that in magnetization curves was occurred. Meanwhile, we have not annealed the Ni80 Fe20 electrode in any magnetic field for improvement of the Ni80 Fe20 quality [25] because the annealing easily causes compositional inhomogeneity in the FMO. Therefore, a steep resistance change at 0 Oe expected from the magnetization
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