Spin-based electronics and its activities and progress

Current Applied Physics 6S1 (2006) e141–e148 www.elsevier.com/locate/cap www.kps.or.kr

Spin-based electronics and its activities and progress S.H. Lim a

a,b

, S.H. Han a, K.H. Shin

a,*

Nano Device Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea b Division of Materials Science and Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea Received 1 August 2005 Available online 6 March 2006

Abstract Fundamental limitation of current charge-based electronics technology is imminent as the technology progresses ever faster than before. Spin-based electronics or spintronics, which is a new field of electronics utilizing additional spin degree of freedom, is one of many technology options to fundamentally solve the limitation of current electronics. In this paper, current programs and accomplishments of spintronics research in Korea are briefly reviewed. In particular, research accomplishments achieved at KIST are described.  2006 Elsevier B.V. All rights reserved. PACS: 85.75. d; 85.75.Hh; 75.50.Pp Keywords: Spin-based electronics; Spintronics; Spin field effect transistors; Diluted magnetic semiconductors; Spin transfer devices

1. Introduction Spintronics is an acronym for ‘spin transport electronics’ or ‘spin-based electronics’ and refers to a research field of intense current interest, which aims to develop novel sensor, memory and logic devices by manipulating the spin states of electrons or holes. Adding the spin degree of freedom to conventional charge-based electronics or using the spin alone will improve capability and performance of present electronic products. The potential advantages of these new devices over conventional charge-based devices would be non-volatility, higher speed in data processing, lower consumption of electric power, and higher densities but, for some applications, we have not even conceived of today. The Korean government has supported spintronics research programs since 1999 as they believe this field would become one of the growth engines for the Korea economy because it represents new types of electronic devices. Spintronics research programs in Korea can be classified into either R&D programs on devices or basic science programs. *

Corresponding author. Tel./fax: +82 2 958 5418. E-mail address: [email protected] (K.H. Shin).

1567-1739/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2006.01.027

The largest research project on spintronics in Korea has been carried out by the Korea Institute of Science and Technology (KIST) since 2001, while basic researches have been conducted at several universities since 1999. The major spintronics research groups at Korean universities were designated and supported financially by the Korea Science and Engineering Foundation (KOSEF). The three primary sites are ‘The Electron Spin Science Center (eSSC)’ at the Pohang University of Science and Technology (POSTECH), ‘Center for Nanospinics of Spintronic Materials (CNSM)’ at the Korea Advanced Institute of Science and Technology (KAIST), and ‘Quantum Functional Semiconductor Research Center (QSRC) at Dongguk University. This article reviews the current programs and accomplishments of spintronics research in Korea, and, in particular, at KIST. 2. The program at KIST 2.1. Overview As was mentioned in Section 1, it was year 2001 when KIST started working on spintronics research. Even before

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this, there were spintronics-related research programs on magnetic random access memory (MRAM) and giant magnetoresistance (GMR). There was even a program on diluted magnetic semiconductors (DMSs), though the activity was very weak. In this sense, the spintronics project in 2001 can be considered as the first concerted efforts on spintronics by the KIST community. The project was funded internally and also KIST members were mainly involved in the project. There were two main research fields under the project; one is on the spin-dependent transport mainly dealing with tunneling magnetoresistance (TMR) and GMR, and the other is about DMSs. The research subject of the first program was similar to that carried out before under scattered and small-sized programs, in a sense being the continuation of the previous activities. Then, the turning point came in year 2002 when KIST was able to receive a big grant directly from the government. The level of funding, roughly USD 4 millions per year for 10 years (total USD 40 millions) is really huge, at least on a Korean standard. With the big boost from the government, the spintronics program at KIST was much upgraded from institutional to national level. Naturally, several prominent outside groups from national research institutes and universities joined the KIST team for the combined efforts. The program consists of three phases. The first phase of 3 years ended successfully several months ago and we are now in an early stage of the second phase for another 3 years from 2005 to 2007 (inclusive). The last 4 years from 2008 to 2011 (inclusive) constitute the third phase. The main objective of the first phase was to buildup infrastructure for spintronics research at the site of KIST. Spintronics itself is a new research field with a history of only several years. However, our research level is behind the level of the USA, Japan and the EU. It is therefore very important to have a suitable research strategy to narrow the gap as early as possible. So, the priority was placed to buildup infrastructure. In the past, KIST had most of the facility for micron-based technology, as we have been doing this kind of research for the last two decades. The establishment of the infrastructure is essential for KIST to be a hub of spintronics research. Many essentially required experimental apparatuses were established during the first phase. Some of them are: a UHV (ultra-high vacuum) sputter and an MOCVD for thin film fabrication; an e-beam writer, an ICP (inductively coupled plasma) etcher and an ion beam etcher for nanofabrication; and finally an AGM (alternating gradient magnetometer) and a PPMS (physical properties measurement system) for characterization down to LHe temperature. Currently under establishment is an MBE cluster system, which may enable to fabricate metal/semiconductor hybrid spin devices within a single chamber without breaking vacuum. It is assessed that the main goal of the first phase has been achieved successfully. Of course, during the first phase, much research work was also carried out and some highlights will be discussed in the next subsection.

2.2. Main achievements One of the important goals of the KIST spintronics program is to develop a fully operated spin-FET at room temperature. Obviously, this goal is challenging. The success can only be realized by solving a series of many daunting problems related with spin injection, transport and detection. This is the reason why no one has ‘‘really’’ realized this goal in spite of intensive work by numerous research groups around the world, after the first proposal in 1990 by Datta and Das [1]. An extensive review on the field was done and then our assessment was that the work by the Johnson group at NRL [2–4] was very close to a workable spin-FET. The device was based on an InAs 2DEG (two-dimensional electron gas) structure. The group was able to observe a spin signal from the device, but the detected signal was very weak. So, we decided to work on a similar device but with some modifications in order to improve the signal. The main modification was to reduce the dimensions of the channel width and channel length. In the original work, these dimensions were all micron-sized. It is well accepted that the number of spin-polarized carriers decays exponentially with increasing channel length. So, a short channel length is essential for a large spin signal. Also, there are many negative effects related to a wide channel width and some of them include a local Hall and a fringe field effect as well as the effect due to the DP mechanism [5,6]. Several fabrication processes were developed to fabricate an InAs based spin-FET with nanochannels and one typical structure is shown in Fig. 1. The channel length of the fabricated devices is in the range of 150–1200 nm, while the channel width in the range of 200–800 nm. A large spin signal of DV = 0.17 mV was observed at 5 K and 77 K in a potenti-

Fig. 1. Scanning electron microscopy image of the InAs based spin-FET with nanochannels (upper figure), together with the schematic diagram showing the channel dimensions (lower figure).

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ometric geometry as shown in Fig. 2 and this level of signal is much higher than that reported previously in a similar device. This result is encouraging and this improvement, we believe, is mainly due to the size reduction of the spin channel. There are still many problems to be solved on the road to fully workable spin-FETs, which may include further improvement of spin signal and the signal observation at room temperature. Nonetheless, our result is surely a good starting point. In addition to the InAs based spinFETs, we are currently working on other types of spinFETs which are based on SOI (Si on insulator) or GaAs heterostructures. In this case also, the main emphasis is the use of nanochannels. Many problems related with nanofabrication have been identified and solved and we expect some promising results from these devices in the near future. One of the most serious problems in the realization of spin-FETs is a low spin signal. There are many possible reasons and, among these, spin scattering at the interfaces can be a dominant factor. It was demonstrated experimentally that spin injection efficiency is reduced significantly by interface defect spin scattering in a spin-LED (light emitting diode) [7]. In our spin-FETs with nanochannels described earlier, there are several interfaces involving FM (ferromagnetic metal)/SC (semiconductor) and SC/SC interfaces. That is the reason why the spin injection and detection across various interfaces are currently real hot issues. A

Fig. 2. Resistance versus applied curves obtained in a potentiometric geometry at (a) 5 K and (b) 77 K. The filled and unfilled circles indicate the field sweep up and down, respectively.

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novel idea was proposed to overcome this problem. The main idea is to use the well-known compound semiconductor, HgCdTe (MCT), with a large Zeeman effect (5.7 meV at 1 T) [8] and a large Rashba coupling (11.5 meV) [9]. Also the compound is known to have a long spin life time (356 ps at T = 150 K) [10]. The device structure is simple, as can be expected from no interfaces along the spin transport. An MCT channel is fabricated by using conventional lithography and source and drain contacts are formed at both ends. Then, in the middle of the channel, a gate electrode with a length of 5 lm is positioned. MCT is non-magnetic, but, due to a large Zeeman effect, spin imbalance can be generated with an applied magnetic field. Surprisingly, a clear resistance modulation was observed at low temperatures by modulating the gate voltage. In Fig. 3 are shown some of the results for the magnetoconductance (r) versus applied field curves at various gate voltages. The variation of the Rashba coefficient (a) and spin–orbit scattering time (sSO) with the gate voltage, extracted from the experimental data, is summarized in the inset. It is clear from the figure that the magnetoconductance varies appreciably with the gate voltage. The obtained Rashba coefficients are comparable to those reported in the literature [11], confirming that the observed resistance modulation is due to the gate effect through the Rashba coupling. We believe this is the first realization of resistance modulation due to spin effect. Then, it is of interest to consider the reason for the large resistance modulation. Quite likely, the key to the success is the absence of any interface along the spin transport that has prevented spin injection from ferromagnetic material to semiconductor. This new device has several disadvantages

Fig. 3. The results for the magnetoconductance versus applied magnetic field curve. The results were obtained at three different gate voltages of 2.5 V (circles), 0 V (asterisks), and +2.5 V (inverse triangles). The minimum points are indicated by the arrows. The variation of the Rashba coefficient (a) spin–orbit scattering time (sSO) with the gate voltage, obtained from the results, is summarized in the inset.

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some of which include the application of a large magnetic field during the device operation and low temperature operation. In addition to the ‘‘full’’ spin devices described above, unit devices were also investigated mainly to demonstrate the spin injection, a key ingredient of workable spin-FETs. One example is an FM/Bi/FM device. The element Bi was selected because Bi has a long mean free path [12], although it has a rough surface in a thin film state. So, one key issue is to make the surface smooth as much as possible. The structure of the FM/Bi/FM devices is shown in Fig. 4. FM electrodes (CoFe or FeNi) were first deposited and then Bi was coated on the electrodes. The shape of Bi was controlled either with a photomask (upper figure) or a shadow mask (lower figure). The gap between the two FM electrodes, which corresponds to the channel length, was varied widely from 200 nm to several microns. Nonlocal geometry measurements showed that the output voltage depends on the magnetization configuration (parallel or antiparallel), a sign of spin injection and detection. This spin signal was further confirmed from experiments based on the Hanle effect.

Similar unit devices were constructed by using a carbon nanotude (CNT) instead of Bi. CNT was selected because it has a long spin coherence length [13]. Another advantage with CNT is the absence of anomalous Hall effect, often a notorious noise source during the measurement. The fabricated device is shown in Fig. 5 where a multi-walled CNT is located across many permalloy electrodes. An MR ratio of 16% was observed at 2.2 K. MR decreases with increasing temperature and finally vanishes well below room temperature. The results on unit devices are quite promising, indicating a clear spin injection and detection. The main question is to combine these unit devices into a ‘‘full’’ device without deteriorating the performance. This is not a trivial matter though. Thus far, an FM metal such as Fe, FeCo or FeNi was considered as a spin aligner in spintronic devices including spin-FETs. This is a natural choice because ordinary FM metals are good spin aligners and possess high Curie temperatures. Furthermore, thin film deposition of these metals can be done with ease. However, it was observed experimentally and also predicted theoretically that the spin injection from FM metals into SCs occurs very ineffi-

Fig. 4. Scanning electron microscopy images of FM/Bi/FM spin injection devices. FM electrodes (CoFe or FeNi) were first deposited, followed by Bi. The shape of Bi was controlled either by a photomask (upper devices) or a shadow mask (lower devices).

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Fig. 5. A scanning electron microscopy image of an FM/CNT/FM spin injection device.

ciently, if it does not occur at all, when there is an Ohmic contact at the FM/SC interface [14–16]. The reason behind this is known as the conductivity or energy band mismatch. One way to overcome this problem is to form a Schottky barrier at the interface [17] or to insert a tunneling barrier between them [18]. Indeed, a Schottky or a tunneling barrier has been found to be successful in injecting spins from FM metals to SCs. It is worth noting here that the successful spin injection was realized in a spin-LED (light emitting diode) geometry [17,18], not in a spin-FET structure. The reason for this is not clear, but a large impedance related with the Schottky or tunneling barrier can be a factor, at least in part. Another plausible explanation for the less electrical spin injection compared to large optical injection is as following. GaAs is a direct band gap material and injected electrons tend to experience transition to the conduction band edge where most electron–hole recombination occurs. However, these electrons at the conduction band edge have small k values (or very small velocity). Therefore, these electrons do not contribute transport, resulting in less electrical spin injection. If the conductivity or energy band mismatch is a source of the problem of spin injection, then the natural extension is to use a magnetic material with a SC energy band and resistivity, known as magnetic SCs or DMSs. Great interest in magnetic semiconductors were revived with the discovery of ferromagnetic III–V based materials such as GaMnAs in 1996 [19], although Eu-based chalcogenides were reported in the 1960s and early 1970s [20] and II–VI based alloys in the 1980s [21]. In both II–VI and III– V based systems, the semiconductors become ferromagnetic with the incorporation of magnetic transition metals (TM) such as Mn. The incorporation of TM into II–VI semiconductors is rather easy, because TM typically exhibits the valence state of +2, being the same as one of the constituents in II–V. However, this is not the case for III–V systems; actually, TM is thermodynamically not stable in III–V, resulting in TM segregation. So, low temper-

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ature molecular beam epitaxy (LTMBE) was usually used to fabricate III–V based magnetic semiconductors. Even in this case, the amount of TM incorporated in III–V is limited up to 10 at.%. So far, GaMnAs, with a rather high ferromagnetic Curie temperature over 170 K [22], is the most extensively studied system and the origin of its ferromagnetism is reasonably well established [23,24]. Recently, group IV based magnetic semiconductors received much attention, due to the theoretical prediction of a high Curie temperature based on the Zener model [25]. Among many group IV semiconductors, Ge has been studied most extensively [26–28]. Ge has an important advantage in that it is lattice-matched to the AlGaAs/GaAs family, thus facilitating incorporation into III–V heterostructures. Furthermore, Ge has higher intrinsic hole mobility than GaAs and Si. Results reported so far on group IV based magnetic semiconductors are encouraging and the most significant results were observed by Park et al. in epitaxial Ge–Mn thin films by MBE [26]. Ferromagnetic ordering with reasonably high Curie temperatures was reported. Also, voltage controlled ferromagnetic order was demonstrated with a low gate voltage of 0.5 V, opening up the possibility of new spintronic devices. However, the highest Curie temperature achieved so far is 116 K, being still far lower than room temperature, causing the biggest hurdle to wide-spread applications in spintronics. The main reason for the low Curie temperature may be the limited Mn content incorporated into Ge. Recent first principles calculations predict that the Mn–Mn exchange interactions and hence Curie temperature can be increased by increasing Mn content [29]. However, the introduction of Mn into Ge was found to be very much limited; the highest amount of Mn incorporated without segregation was reported to be 3.3 at.% even with a very low temperature (70 C) MBE process [26]. It is well known that the terminal solubility increases as the metastability of the system increases. This is precisely the reason why, in earlier works [21,26,28], LTMBE process was used to increase the metastability and hence Mn solubility. An ultimate increase of the metastability can be obtained by forming an amorphous phase, and this is the main idea of amorphous magnetic semiconductors. With the formation of an amorphous phase, a large amount of a magnetic transition metal is expected to be incorporated into Ge without forming second phases, possibly leading to a high Curie temperature. Amorphous semiconductor thin films of Ge100 xCrx, Ge100 xMnx and Ge100 xFex (x in at.%) were prepared by thermal co-evaporation onto oxidized Si or glass substrates held either at room temperature or quench condensed onto LN2 cooled surface. The composition was varied widely up to x = 50. Systematic microstructural characterization by X-ray and high resolution transmission electron microscopy was carried out and it was found that the samples are a single amorphous phase without any second phases. Ferromagnetic ordering at room temperature was observed in Ge–Cr and Ge–Mn thin films and some

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of the evidence showing this is shown in Fig. 6 where M–H loops measured at room temperature for thin films of (a) Ge85Cr15 and (b) Ge71Cr29 are presented. The M–H loops are typical of ferromagnetic materials, a complete saturation occurring within the applied field range. These results are promising, but it is necessary to conduct further experiments to check, for example, whether the spin polarization actually occurs at room temperature, in order to apply the materials to a spin device as a spin aligner. Spin devices involving semiconductors were considered so far. Very recently, a new type of spin device without any semiconductors began to attract much attention. It has been shown, both theoretically [30,31] and experimentally [32,33], that the magnetization of a thin film can be reversed by a spin effect called spin transfer or spin accumulation, not by the magnetic field (Ampere field) from the currents. This type of magnetization reversal driven only by currents is called current-induced magnetization reversal (CIMR). This novel phenomenon can only be observed in nanostructures usually below 100 nm, and, in this sense, it is a product of a marriage between magnetism and nanotechnology. A general procedure to fabricate a spin transfer device is as follows. A multilayer stack showing GMR or TMR is first fabricated. Next, a very delicate nanofabrica-

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tion is carried out to form a nanopillar of the multilayer with characteristic lateral dimensions of 100 nm or less. A typical nanopillar fabricated at KIST is shown in Fig. 7. The left figure shows a top view of the device seen by scanning electron microscopy, while the figure on the right displays a schematic diagram showing the cross-section of the multilayer stack. It is clear from the figure on the left that the short axis of the nanopillar is smaller than 100 nm. For spin transfer (and hence magnetization reversal), current is applied along the pillar, which is indicated in the right figure. This spin transfer device is very suitable for high density memory mainly due to its ultra-small size. One biggest problem, however, is a very high current density, of the order of 108 A/cm2, which is required to generate enough spin torque leading to the magnetization reversal. This level of current density is too high for real practical applications. So, one of main research directions is to reduce this current density. Work toward this direction has been performed intensively at KIST and some of the recent results are displayed in Fig. 8 where the junction resistance – DC current curves are shown for two spin transfer devices with different stack structures. One has an exchange-biased spin valve structure designed at KIST, while the other a normal spin valve structure. It is clear

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Fig. 6. M–H loops measured at room temperature for thin films of (a) Ge85Cr15 and (b) Ge71Cr29.

Fig. 7. A spin transfer device fabricated at KIST. (a) A scanning electron microscopy image showing the top view. (b) A schematic diagram showing the cross-section of the multilayer stack.

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3.1. The Electron Spin Science Center (eSSC)

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Main efforts are dedicated to understanding and exploitation of electronic spin degrees of freedom in matter on the scales of nanometer and picosecond. The activities of eSSC are designed to supply the basic knowledge to the realization of spintronics and quantum computation, which are considered as the core technology of the 21st century. The eSSC was established in 2000 as a national science research center at the Pohang University of Science and Technology. About 20 physicists from several universities and national laboratories belong to the center. The annual research budget is about USD 800,000. The role of eSSC as a national center is defined as follows: (1) advancement of the basic knowledge on electron spin science (2) formation of national and international network for effective collaboration, and (3) transfer of knowledge to the industry sector.

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Fig. 8. The junction resistance – DC current curves for two different spin transfer devices. The results shown in (a) are based on an exchange-biased spin valve structure, while those in (b) are based on a conventional spin valve structure. The junction resistance – applied magnetic field curves (no applied currents) of the respective devices are shown in the insets.

from the results that indeed magnetization is reversed only with the application of currents. Note that no magnetic field was applied during the measurement. Resistance change of the junctions was also measured by only applying magnetic field and these results are shown in the insets. Another important point is that our exchange-biased device has a much lower current density for magnetization switching. The measured critical current density is 7.5 · 106 A/cm2. This magnitude can be compared with the value of 3.5 · 107 A/cm2 observed for the conventional device. The reduction of current density is really significant. The current density level achieved at KIST is one of the lowest reported so far. Intensive work is continuously under way to further reduce the current density. 3. Activities at other institutes in Korea The major research groups on spintronics other than KIST are located at universities and they are financially supported by Korea Science and Engineering Foundation (KOSEF). It is impossible, in a limited space, to describe the activities and also achievements done by the groups. Here, only an overall introduction will be given.

The center was established in 1999 at the Korea Advanced Institute of Science and Technology (KAIST) and the annual budget is about USD 600,000. The ultimate goal of this center is to establish nanomagnetism as well as to develop noble spintronic materials through studying the domain configurations and spin dynamics in nanomagnetic materials. To achieve the goal, various nanomagnetic materials including nanoscale thin films, nanodots and arrays, and molecular magnets will be artificially synthesized, and ultra-sensitive structural, magnetic, magnetooptical, and domain dynamic measurement techniques will be developed. Their research will be carried out with focus on three fundamental issues such as (1) domain configurations and spin orientations in the ground state, (2) domain reversal mechanism under external conditions, and (3) possibility of macroscopic quantum effects. These issues will not only provide the essential knowledge for establishment of nanomagnetism but also answer for the ultimate limitations of storage density, switching speed, and archival time of spintronic devices. 3.3. Quantum Functional Semiconductor Research Center (QSRC) The center was established in 1999 at the Dongguk University and the annual budget is about USD 800,000. The ultimate goal of this center is to establish the new quantum structure semiconductor combined by the spin state. To achieve the goal, various researches are carried out with focus on four issues such as (1) growth of the homogeneous and reproducible quantum wires and dots, (2) growth of new materials to control the spin or charge easily, (3) combination of the new materials and the semiconductor quantum structure, and (4) realization of new multinary-bit structure.

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4. Summary and outlook A brief review on the current programs and accomplishments of spintronics research in Korea has been given in this article. Research accomplishments achieved at KIST are only described due to space limitation. In the past several years, a lot of efforts were given to buildup infrastructure for spintronics research which is critical for a speedy progress in the field. Some results on spin-FETs (both conventional and new types), unit spin devices, DMSs, and spin transfer devices have been described and many of the results are quite encouraging. However, our level of research is still behind advanced countries such as the US, Japan, and the EU. Fortunately, the Korean government has committed considerable resources to develop spintronics expertise. This commitment, coupled with Korean initiative and determination, are likely to bring about significant catch-up over the next few years. It is believed that that the international cooperation is also very important to speed up the progress. In this sense, the cooperation work with the group led by Prof. Jagadeesh Moodera at MIT, started only several months ago, should jumpstart this exciting field. A good thing is that KIST and MIT play a complementary role. KIST strengths are in process technology, while MIT provides underlying science and design technology. Acknowledgements This work was supported by the Korea Institute of Science and Technology Vision 21 Program, by the TND Frontier Project funded by KISTEP, and by the Ministry of Science and Technology of Korea through the Cavendish-KAIST Research Cooperation Center. References [1] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [2] P.R. Hammar, B.R. Bennett, M.J. Yang, M. Johnson, J. Appl. Phys. 87 (2000) 4665. [3] P.R. Hammar, M. Johnson, Appl. Phys. Lett. 79 (2001) 2591. [4] P.R. Hammar, M. Johnson, Phys. Rev. Lett. 88 (2002) 066806.

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