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All-electric spin transistor using perpendicular spins
Author’s Accepted Manuscript All-electric spin transistor using perpendicular spins Ji Hoon Kim, Joohyung Bae, Byoung-Chul Min, Hyung-jun Kim, Joonyeon Chang, Hyun Cheol Koo www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(15)30827-1 http://dx.doi.org/10.1016/j.jmmm.2015.11.056 MAGMA60887
To appear in: Journal of Magnetism and Magnetic Materials Received date: 27 August 2015 Revised date: 19 November 2015 Accepted date: 20 November 2015 Cite this article as: Ji Hoon Kim, Joohyung Bae, Byoung-Chul Min, Hyung-jun Kim, Joonyeon Chang and Hyun Cheol Koo, All-electric spin transistor using perpendicular spins, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.11.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
All-electric spin transistor using perpendicular spins
Ji Hoon Kima,b, Joohyung Baea,c, Byoung-Chul Mina, Hyung-jun Kima, Joonyeon Changa, and Hyun Cheol Kooa,b,*
a
Center for Spintronics, Korea Institute of Science and Technology, Seoul 136-791, Korea
b
KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, 136-
701, Korea c
Department of Materials Science and Engineering, Korea University, Seoul, 136-701, Korea
Abstract All-electric spin transistor is demonstrated using perpendicular spins in an InAs quantum well channel. For the injection and detection of perpendicular spins in the quantum well channel, we use Tb20Fe62Co18/Co40Fe40B20 electrodes, where the Tb20Fe62Co18 layer produces the perpendicular magnetization and the Co40Fe40B20 layer enhances the spin polarization. In this spin transistor device, a gate-controlled spin signal as large as 80 mΩ is observed at 10 K without an external magnetic field. In order to confirm the spin injection and relaxation independently, we measure the three-terminal Hanle effect with an in-plane magnetic field, and obtain a spin signal of 1.7 mΩ at 10 K. These results clearly present that the electric field is an efficient way to modulate spin orientation in a strong spin-orbit interaction system. 1
Keywords: spin transistor; perpendicular spin; interface resistance; Schottky tunnel barrier
*
Corresponding author. Tel.: +8-22-958-5423; fax: +82-2-958-6851. E-mail address: [email protected] (H. C. Koo).
2
1. Introduction Among various proposals for spin-based devices, the spin field effect transistor [1-4] is one of the most attractive schemes because the gate control of spin modulation can be actively utilized in the switching and logic applications. The spin field effect transistor proposed by Datta and Das [1] involves the spin injection, detection, and gate modulation using a two-dimensional electron gas system. The gate electric field controls the strength of spin-orbit interaction, the amount of spin precession, and, consequently, channel conductance depending on the spin orientation. In the previous reports [2, 3], we demonstrated the gate control of spin precession in an InAs quantum well with ferromagnetic source and drain. One disadvantage in those works was that external magnetic fields should be applied to align the magnetization of ferromagnets transverse to the Rashba field for spin precession. Recently all-electric spin transistor was presented using the gatedefined quantum point contact [5] which requires multiple gates and allows very small process margin. In order to realize fully electrical spin transistor, we employ a perpendicular magnetization system which enables us to operate the spin transistor without an external magnetic field. Because perpendicularly magnetized electrodes provide very attractive advantages in terms of the reduced interference between the neighboring cells, many research groups have actively studied the spin transport using a perpendicular magnetization system via optical [6-9] and electrical [10] methods. In our previous research [10], the transport of perpendicular spins in a GaAs bulk channel was investigated via a fully electrical method. A technical shortcoming in the previous research is that 3
the electric field control of spin orientation is difficult to achieve, since the GaAs bulk channel possesses a weak spin-orbit interaction. In this paper, we demonstrate the spin-injected transistor, combining
an
InAs
quantum
well
with
strong
spin-orbit
interaction
and
the
Tb20Fe62Co18/Co40Fe40B20 layers with a perpendicular magnetization. We present the electric field control of spin precession in the InAs quantum well with the ferromagnetic source and drain. Using three-terminal Hanle measurement [10, 11], the electrical spin injection is also independently observed. 2. Device structure and magnetic property For the spin precession of injected carriers, the vector alignment between the injected spins and the Rashba field should be regarded as shown in Fig. 1(a). In our channel system, the quantum well asymmetry occurs in the growth direction (z-axis), so that the Rashba field is along the y-axis for carriers flowing along the x-axis. Spin precession would not be observed for the electrons whose spins are polarized along the y-axis (top of Fig. 1(a)), so the source and drain magnetizations should be along the x or z-axis. In the real application, the ferromagnetic source and drain with a magnetization along the x-axis (middle of Fig. 1(a)) is not very attractive in terms of high density, because this configuration requires a relatively long aspect ratio of ferromagnetic electrodes due to the shape anisotropy. Thus, the magnetization along the z-axis (bottom of Fig. 1(a)) is the best choice for the device application.
4
Fig. 1. Spin transistor using perpendicular spins. (a) Relationship between the magnetization direction of ferromagnetic electrodes (source and drain) and the spin precession. S, D and G represent source, drain and gate electrodes, respectively. The arrows inside the channel indicate the spin orientation and the rounded arrows depict the direction of spin precession. BR is the Rashba field.
(b)
Magnetization
behavior
of
the
ferromagnetic
electrodes
consisting
of
Tb20Fe62Co18/Co40Fe40B20 layers. The schematic shows the layer stack of ferromagnetic electrodes with perpendicular magnetization.
For
the
electrodes
with
a
perpendicular
magnetization,
we
deposited
Ru/Ta/Tb20Fe62Co18/Co40Fe40B20 layers (from top to bottom) using a magnetron sputtering system. A 20-40 nm thick Tb20Fe62Co18 layer produces the perpendicular magnetization and a 0.8 nm thick
5
Co40Fe40B20 layer, which directly contacts the semiconductor channel, enhances the spin polarization of the ferromagnetic source. The Ru/Ta layers are deposited to protect ferromagnetic films. The magnetic switching behavior of Tb20Fe62Co18/Co40Fe40B20 double layers was measured by a vibrating sample magnetometer. As shown in Fig. 1(b), the magnetization curve clearly demonstrates the perpendicular magnetic anisotropy of the Tb20Fe62Co18/Co40Fe40B20 structure. The coercivity of the Tb20Fe62Co18/Co40Fe40B20 layers is approximately 1.28 T, which is identical to the switching field of a single Tb20Fe62Co18 layer. It turns out that the magnetization of the Co40Fe40B20 layer coherently rotates with the top Tb20Fe62Co18 layer due to the strong ferromagnetic exchange between the two layers. Although the spin polarization of Tb20Fe62Co18 is relatively low, the directly contacting Co40Fe40B20 layer delivers a highly polarized current into the semiconductor channel. 3. Results and discussion To observe a gate control of spin precession, the mesoscopic lateral spin valve devices were fabricated as shown in Fig. 2(a). The device consists of two Tb20Fe62Co18/Co40Fe40B20 electrodes (FMs) on the top of an InAs based two-dimensional electron gas (2DEG) layer and a gate electrode. The InAs quantum well structure was grown by molecular beam epitaxy on a semi-insulating InP(100) substrate. The heterostructure consists of an In0.52Al0.48As n-doped (4×1018 cm3) carrier supply layer, In0.52Al0.48As/In0.53Ga0.47As double cladding layers, and an InAs active layer [2]. The carrier density and mobility of the quantum well layer are n = 2 × 1012 cm2 and = 60,000 cm2V1s1 at 1.8 K. The representative size of the two FMs is 0.8 m × 20 m. The channel was defined by a mesa etch and had a width of w = 8 μm. The spacing between the two FM electrodes is 6
L = 1.35 μm, measured center to center. To control the interface resistance, before depositing ferromagnetic layers, we etched upper part of the In0.52Al0.48As/In0.53Ga0.47As cladding layers. The part of In0.53Ga0.47As layer remains after etching and the interfacial resistance depends on the remaining thickness of In0.53Ga0.47As. In a typical device, the distance between the Tb20Fe62Co18/Co40Fe40B20 layers and the InAs quantum well was 3-7 nm. For efficient spin injection, the proper interfacial resistance is required [2].To control the electric field along the vertical axis, the gate electrode was formed after depositing a 100 nm thick layer of SiO 2.
Fig. 2. Gate modulation of perpendicular spins. (a) Measurement geometry and scanning electron micrograph of device. For clarity, the image was taken before depositing a gate electrode. (b) Gate control of spin transistor signal. (c) Spin precession angle as a function of the gate electric field. (d) Temperature dependence of gate controlled spin signals.
7
We have investigated a transport of perpendicular spins in this spin transistor device and a gate-controlled modulation of spin signal. In this experiment, no external magnetic field is applied to the device. When the bias current (I = 0.1 mA) is applied, the non-local voltage [12, 13] is measured as a function of gate electric field as illustrated in Fig. 2(a). The charge current does not flow into the detection site, so the side effect such as the Hall effect can be excluded. The orientation of injected spins, along the z-axis, is perpendicular to the Rashba field. The spin precession varies as a function of gate electric field which also controls the Rashba field strength and consequent precession angle at detector. The detected voltage is high (low) when a detected spin has orientation parallel (anti-parallel) with that of the detector. Thus, the oscillation of detected voltage is observed as shown in Fig. 2(b). A further control experiment was performed to confirm the gate controlled spin orientation. When the current direction is reversed, the orientation of accumulated spins at the interface would be reversed and the spin orientation of arriving carriers at detector should be also reversed. As presented in the bottom of Fig. 2(b), the phase of the conductance oscillation shifts by 180° as expected. The solid lines in Fig. 2(b) are obtained from Datta-Das equation [1, 2], VS/I = Acos(2m*αL/ħ2), where the amplitude A is a phenomenological parameter. The Rashba parameter (α) at VG = 0 is 8.93 × 10-12 eV-m and it is a function of gate voltage [2]. From the Rashba parameter, we can calculate the spin precession angle with respect to the gate electric field as shown in Fig. 2(c), and the calculated spin precession angle is nicely matched with the oscillation signals in Fig. 2(b). Although the accurate expression of the amplitude A in ballistic regime is not verified very well, an amplitude of 0.08 Ω in this experiment is very 8
large compared to that with an in-plane spin for the similar channel structure [2]. The representative interfacial resistance of FM/2DEG is ~500 Ω μm2 and this value is larger than the threshold resistance, r1 = RS(λsf)2 =139 Ω μm2, where RS is the sheet resistance and λsf is the spin diffusion length [11, 14], so the efficient spin injection is well explained. In the previous research [2, 3], a constant external field of 0.5 T was applied to overcome the shape anisotropy of ferromagnetic electrodes and fix the magnetization vector along the x-axis (middle of Fig. 1(a)). The constant magnetic field interferes with the Rashba field and the injected spin vector which may lead to the reduction of gate control signal.
Fig. 3. Magnetic field driven spin transport. (a) Geometry of three-terminal Hanle measurement. (b) Hanle measurement curve. The spin lifetime from the Hanle curve is 7.48 ps. (c) Interaction between Rashba (BR) and Hanle (Ba) fields. 9
The temperature dependence of gate control signal is shown in Fig. 2(d). The amplitude of signal is roughly constant with increasing temperature up to T = 70 K. Above this temperature, the gate control of oscillation was not observed. The reason is that the spin transport mechanism is changed from ballistic to diffusive behaviours at T = 70 K. It should be noted here that the oscillation frequency decreases with increasing temperature. The gate controllability is deteriorated at higher temperature due to the decrease of the Rashba field strength. The observation of the gate controllability of Rashba parameter using Shubnikov-de Haas oscillations [15, 16] is not possible at higher temperature. Instead, we can estimate the trend of gate controllability using the solid line fits of which oscillation period depends on the gate controllability of the Rashba parameter. The gate controllability is lessened by 37.5% as the temperature increases from 10 K to 70 K. An additional confirmation of the spin injection into the InAs quantum well layer is to measure a non-local spin accumulation and corresponding Hanle curves in the four-terminal spin valve geometry. In the InAs quantum well, however, the Rashba field is very large (~8.5 T) and the spin scattering time is very short. Thus, it is very difficult to observe the Hanle effect using a fourterminal spin valve structure in the sample with a strong Rashba effect. It is also unattainable to measure the non-local magnetoresistance, since the switching field of the injector and detector is almost the same. An alternative approach is to measure a three-terminal Hanle effect [10, 11] where one contact is used for both spin injection and detection and a magnetic field, which induces the relaxation of injected spin, is externally applied perpendicular to the spin orientation of electrons. The Hanle signal for an in-plane spin injection is usually detected using a perpendicular magnetic 10
field which drives suppression of in-plane spins. In our case, however, the injected spins are perpendicularly polarized, so the magnetic field along the y-axis was utilized to observe spin relaxation as shown in Fig. 3(a). The lateral dimension of the FM is 3 µm × 50 µm. In this measurement, the bias current was applied from the FM to the quantum well channel, and the voltage was measured between the FM and the other side of the channel. Figure 3(b) shows the three-terminal Hanle curve measured at 10 K obtained with the geometry illustrated in Fig. 3(a). The bias current is 1 mA. The applied magnetic field, which is perpendicular to the injected spins, drives spin precession and dephasing which make the spin polarization lost. This spin relaxation causes the voltage change and the magnitude of Hanle signal is defined as ΔVHanle. The Hanle signal (ΔVHanle) is much smaller than the gate control signal shown in Fig. 2. The reason is that, during the Hanle measurement, the driving force for spin relaxation is not only the external magnetic field but also the Rashba effective field in this channel system. As shown in Fig. 3(c), the Rashba field is constant during the Hanle measurement, so the injected spins precess around the Rashba field even without an applied magnetic field. While the gate field control of spin injection signal corresponds to the full swing of spin precession (Fig. 2(b)), the Hanle signal cannot reflect the total contribution of spin precession. In the four-terminal Hanle measurement, we do not detect any signal because the injected spins fully feel the Rashba field when the carriers move along the x-axis. On the contrary, the carrier path includes both x and z terms, so the non-zero Hanle signal can be detected in the three-terminal Hanle measurement. It is very difficult to estimate how much the Rashba field exerted on the injected spins because the Rashba field depends 11
on the direction of the electron movement. 4. Conclusion We demonstrated the operation of a spin transistor using an InAs quantum well channel with perpendicularly magnetized electrodes by means of an all-electric method. By using the gate electric field, the spin injection and modulation of perpendicularly polarized spins are observed. From the three-terminal Hanle measurement, we also confirmed the spin injection into semiconductor channel with a strong spin-orbit interaction which interacts with the Hanle driving field. This work clearly shows the possibility of all-electric spin transistor which can be applied for the spin logic devices.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0017457, 2011-0027905, 2015-004870) and the KIST & KU-KIST Institutional Programs.
References [1] S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665. [2] H. C. Koo, J. H. Kwon, J. Eom, J. Chang, S. H. Han, and M. Johnson, Science, 325, (2009) 1515. [3] H. C. Koo, J. H. Kwon, J. Eom, J. Chang, S. H. Han, and M. Johnson, J. Phys. D: Appl. Phys. 44, 12
(2011) 064006. [4] W. Y. Choi, H.-j. Kim, J. Chang, S. H. Han, H. C. Koo, and M. Johnson, Nat. Nanotechnol. 10, (2015) 666. [5] P. Chuang, S.-C. Ho, L.W. Smith, F. Sfigakis, M. Pepper, C.-H. Chen, J.-C. Fan, J.P. Griffiths, I.Farrer, H.E. Beere, G.A.C. Jones, D.A. Ritchie and T.-M. Chen, Nature Nanotechnol. 10, (2015) 35. [6] S. Ikeda, K. Miura, H. Yamamoto, K. Mizunuma, H. D. Gan, M. Endo, S. Kanai, J. Hayakawa, F. Matsukura, and H. Ohno, Nat. Mater. 9, (2010) 721. [7] C. Adelmann, J. L. Hilton, B. D. Schultz, S. McKernan, and C. J. Palmstrøm, Appl. Phys. Lett. 89, (2006) 112511. [8] N. C. Gerhardt, S. Hövel, C. Brenner, M. R. Hofmann, F.-Y. Lo, D. Reuter, A. D. Wieck, E. Schuster, W. Keune, and K. Westerholt, Appl. Phys. Lett. 87, (2005) 032502. [9] A. Sinsarp, T. Manago, F. Takano, and H. Akinaga, J. J. Appl. Phys., Part 2 46, (2007) L4. [10] J. Bae, K.-H. Kim, J.-M. Han, H. C. Koo, B-C. Min, H-j. Kim, J. Chang, S. H. Han, and S. H. Lim, Appl. Phys. Lett. 102, (2013) 062412. [11] M. Tran, H. Jaffre`s, C. Deranlot, J.-M. George, A. Fert, A. Mirad, and Lemaître, Phys. Rev. Lett. 102, (2009) 036601. [12] M. Johnson and R. H. Silsbee, Phys. Rev. Lett 55, (1985) 1790. [13] H. C. Koo, H. Yi, J.-B. Ko, J. Chang, S.-H. Han, D. Jung, S.-G. Huh, and J. Eom Appl. Phys. Lett. 90, (2007) 022101. 13
[14] M. Tran, H. Jaffrès, C. Deranlot, J.-M. George, A. Fert, A. Mirad, and A. Lemaître, Phys. Rev. Lett. 107, (2011) 249901. [15] J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki, Phys. Rev. Lett. 78, (1997) 1335. [16] I. Lo et al., Phys. Rev. B 65, (2002) 161306(R).
14
Figure captions
Fig. 1. Spin transistor using perpendicular spins. (a) Relationship between the magnetization direction of ferromagnetic electrodes (source and drain) and the spin precession. S, D and G represent source, drain and gate electrodes, respectively. The arrows inside the channel indicate the spin orientation and the rounded arrows depict the direction of spin precession. BR is the Rashba field.
(b)
Magnetization
behavior
of
the
ferromagnetic
electrodes
consisting
of
Tb20Fe62Co18/Co40Fe40B20 layers. The schematic shows the layer stack of ferromagnetic electrodes with perpendicular magnetization.
Fig. 2. Gate modulation of perpendicular spins. (a) Measurement geometry and scanning electron micrograph of device. For clarity, the image was taken before depositing a gate electrode. (b) Gate control of spin transistor signal. (c) Spin precession angle as a function of the gate electric field. (d) Temperature dependence of gate controlled spin signals.
Fig. 3. Magnetic field driven spin transport. (a) Geometry of three-terminal Hanle measurement. (b) Hanle measurement curve. The spin lifetime from the Hanle curve is 7.48 ps. (c) Interaction between Rashba (BR) and Hanle (Ba) fields.
We realize all-electric spin-FET using perpendicular spins. Gate modulation of spin orientation is detected in a quantum well layer. 15
A gate-controlled spin signal as large as 80 mΩ is obtained at 10 K.
We compare spin injection signals driven by a magnetic field and an electric field.
16
PII: DOI: Reference:
S0304-8853(15)30827-1 http://dx.doi.org/10.1016/j.jmmm.2015.11.056 MAGMA60887
To appear in: Journal of Magnetism and Magnetic Materials Received date: 27 August 2015 Revised date: 19 November 2015 Accepted date: 20 November 2015 Cite this article as: Ji Hoon Kim, Joohyung Bae, Byoung-Chul Min, Hyung-jun Kim, Joonyeon Chang and Hyun Cheol Koo, All-electric spin transistor using perpendicular spins, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.11.056 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
All-electric spin transistor using perpendicular spins
Ji Hoon Kima,b, Joohyung Baea,c, Byoung-Chul Mina, Hyung-jun Kima, Joonyeon Changa, and Hyun Cheol Kooa,b,*
a
Center for Spintronics, Korea Institute of Science and Technology, Seoul 136-791, Korea
b
KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, 136-
701, Korea c
Department of Materials Science and Engineering, Korea University, Seoul, 136-701, Korea
Abstract All-electric spin transistor is demonstrated using perpendicular spins in an InAs quantum well channel. For the injection and detection of perpendicular spins in the quantum well channel, we use Tb20Fe62Co18/Co40Fe40B20 electrodes, where the Tb20Fe62Co18 layer produces the perpendicular magnetization and the Co40Fe40B20 layer enhances the spin polarization. In this spin transistor device, a gate-controlled spin signal as large as 80 mΩ is observed at 10 K without an external magnetic field. In order to confirm the spin injection and relaxation independently, we measure the three-terminal Hanle effect with an in-plane magnetic field, and obtain a spin signal of 1.7 mΩ at 10 K. These results clearly present that the electric field is an efficient way to modulate spin orientation in a strong spin-orbit interaction system. 1
Keywords: spin transistor; perpendicular spin; interface resistance; Schottky tunnel barrier
*
Corresponding author. Tel.: +8-22-958-5423; fax: +82-2-958-6851. E-mail address: [email protected] (H. C. Koo).
2
1. Introduction Among various proposals for spin-based devices, the spin field effect transistor [1-4] is one of the most attractive schemes because the gate control of spin modulation can be actively utilized in the switching and logic applications. The spin field effect transistor proposed by Datta and Das [1] involves the spin injection, detection, and gate modulation using a two-dimensional electron gas system. The gate electric field controls the strength of spin-orbit interaction, the amount of spin precession, and, consequently, channel conductance depending on the spin orientation. In the previous reports [2, 3], we demonstrated the gate control of spin precession in an InAs quantum well with ferromagnetic source and drain. One disadvantage in those works was that external magnetic fields should be applied to align the magnetization of ferromagnets transverse to the Rashba field for spin precession. Recently all-electric spin transistor was presented using the gatedefined quantum point contact [5] which requires multiple gates and allows very small process margin. In order to realize fully electrical spin transistor, we employ a perpendicular magnetization system which enables us to operate the spin transistor without an external magnetic field. Because perpendicularly magnetized electrodes provide very attractive advantages in terms of the reduced interference between the neighboring cells, many research groups have actively studied the spin transport using a perpendicular magnetization system via optical [6-9] and electrical [10] methods. In our previous research [10], the transport of perpendicular spins in a GaAs bulk channel was investigated via a fully electrical method. A technical shortcoming in the previous research is that 3
the electric field control of spin orientation is difficult to achieve, since the GaAs bulk channel possesses a weak spin-orbit interaction. In this paper, we demonstrate the spin-injected transistor, combining
an
InAs
quantum
well
with
strong
spin-orbit
interaction
and
the
Tb20Fe62Co18/Co40Fe40B20 layers with a perpendicular magnetization. We present the electric field control of spin precession in the InAs quantum well with the ferromagnetic source and drain. Using three-terminal Hanle measurement [10, 11], the electrical spin injection is also independently observed. 2. Device structure and magnetic property For the spin precession of injected carriers, the vector alignment between the injected spins and the Rashba field should be regarded as shown in Fig. 1(a). In our channel system, the quantum well asymmetry occurs in the growth direction (z-axis), so that the Rashba field is along the y-axis for carriers flowing along the x-axis. Spin precession would not be observed for the electrons whose spins are polarized along the y-axis (top of Fig. 1(a)), so the source and drain magnetizations should be along the x or z-axis. In the real application, the ferromagnetic source and drain with a magnetization along the x-axis (middle of Fig. 1(a)) is not very attractive in terms of high density, because this configuration requires a relatively long aspect ratio of ferromagnetic electrodes due to the shape anisotropy. Thus, the magnetization along the z-axis (bottom of Fig. 1(a)) is the best choice for the device application.
4
Fig. 1. Spin transistor using perpendicular spins. (a) Relationship between the magnetization direction of ferromagnetic electrodes (source and drain) and the spin precession. S, D and G represent source, drain and gate electrodes, respectively. The arrows inside the channel indicate the spin orientation and the rounded arrows depict the direction of spin precession. BR is the Rashba field.
(b)
Magnetization
behavior
of
the
ferromagnetic
electrodes
consisting
of
Tb20Fe62Co18/Co40Fe40B20 layers. The schematic shows the layer stack of ferromagnetic electrodes with perpendicular magnetization.
For
the
electrodes
with
a
perpendicular
magnetization,
we
deposited
Ru/Ta/Tb20Fe62Co18/Co40Fe40B20 layers (from top to bottom) using a magnetron sputtering system. A 20-40 nm thick Tb20Fe62Co18 layer produces the perpendicular magnetization and a 0.8 nm thick
5
Co40Fe40B20 layer, which directly contacts the semiconductor channel, enhances the spin polarization of the ferromagnetic source. The Ru/Ta layers are deposited to protect ferromagnetic films. The magnetic switching behavior of Tb20Fe62Co18/Co40Fe40B20 double layers was measured by a vibrating sample magnetometer. As shown in Fig. 1(b), the magnetization curve clearly demonstrates the perpendicular magnetic anisotropy of the Tb20Fe62Co18/Co40Fe40B20 structure. The coercivity of the Tb20Fe62Co18/Co40Fe40B20 layers is approximately 1.28 T, which is identical to the switching field of a single Tb20Fe62Co18 layer. It turns out that the magnetization of the Co40Fe40B20 layer coherently rotates with the top Tb20Fe62Co18 layer due to the strong ferromagnetic exchange between the two layers. Although the spin polarization of Tb20Fe62Co18 is relatively low, the directly contacting Co40Fe40B20 layer delivers a highly polarized current into the semiconductor channel. 3. Results and discussion To observe a gate control of spin precession, the mesoscopic lateral spin valve devices were fabricated as shown in Fig. 2(a). The device consists of two Tb20Fe62Co18/Co40Fe40B20 electrodes (FMs) on the top of an InAs based two-dimensional electron gas (2DEG) layer and a gate electrode. The InAs quantum well structure was grown by molecular beam epitaxy on a semi-insulating InP(100) substrate. The heterostructure consists of an In0.52Al0.48As n-doped (4×1018 cm3) carrier supply layer, In0.52Al0.48As/In0.53Ga0.47As double cladding layers, and an InAs active layer [2]. The carrier density and mobility of the quantum well layer are n = 2 × 1012 cm2 and = 60,000 cm2V1s1 at 1.8 K. The representative size of the two FMs is 0.8 m × 20 m. The channel was defined by a mesa etch and had a width of w = 8 μm. The spacing between the two FM electrodes is 6
L = 1.35 μm, measured center to center. To control the interface resistance, before depositing ferromagnetic layers, we etched upper part of the In0.52Al0.48As/In0.53Ga0.47As cladding layers. The part of In0.53Ga0.47As layer remains after etching and the interfacial resistance depends on the remaining thickness of In0.53Ga0.47As. In a typical device, the distance between the Tb20Fe62Co18/Co40Fe40B20 layers and the InAs quantum well was 3-7 nm. For efficient spin injection, the proper interfacial resistance is required [2].To control the electric field along the vertical axis, the gate electrode was formed after depositing a 100 nm thick layer of SiO 2.
Fig. 2. Gate modulation of perpendicular spins. (a) Measurement geometry and scanning electron micrograph of device. For clarity, the image was taken before depositing a gate electrode. (b) Gate control of spin transistor signal. (c) Spin precession angle as a function of the gate electric field. (d) Temperature dependence of gate controlled spin signals.
7
We have investigated a transport of perpendicular spins in this spin transistor device and a gate-controlled modulation of spin signal. In this experiment, no external magnetic field is applied to the device. When the bias current (I = 0.1 mA) is applied, the non-local voltage [12, 13] is measured as a function of gate electric field as illustrated in Fig. 2(a). The charge current does not flow into the detection site, so the side effect such as the Hall effect can be excluded. The orientation of injected spins, along the z-axis, is perpendicular to the Rashba field. The spin precession varies as a function of gate electric field which also controls the Rashba field strength and consequent precession angle at detector. The detected voltage is high (low) when a detected spin has orientation parallel (anti-parallel) with that of the detector. Thus, the oscillation of detected voltage is observed as shown in Fig. 2(b). A further control experiment was performed to confirm the gate controlled spin orientation. When the current direction is reversed, the orientation of accumulated spins at the interface would be reversed and the spin orientation of arriving carriers at detector should be also reversed. As presented in the bottom of Fig. 2(b), the phase of the conductance oscillation shifts by 180° as expected. The solid lines in Fig. 2(b) are obtained from Datta-Das equation [1, 2], VS/I = Acos(2m*αL/ħ2), where the amplitude A is a phenomenological parameter. The Rashba parameter (α) at VG = 0 is 8.93 × 10-12 eV-m and it is a function of gate voltage [2]. From the Rashba parameter, we can calculate the spin precession angle with respect to the gate electric field as shown in Fig. 2(c), and the calculated spin precession angle is nicely matched with the oscillation signals in Fig. 2(b). Although the accurate expression of the amplitude A in ballistic regime is not verified very well, an amplitude of 0.08 Ω in this experiment is very 8
large compared to that with an in-plane spin for the similar channel structure [2]. The representative interfacial resistance of FM/2DEG is ~500 Ω μm2 and this value is larger than the threshold resistance, r1 = RS(λsf)2 =139 Ω μm2, where RS is the sheet resistance and λsf is the spin diffusion length [11, 14], so the efficient spin injection is well explained. In the previous research [2, 3], a constant external field of 0.5 T was applied to overcome the shape anisotropy of ferromagnetic electrodes and fix the magnetization vector along the x-axis (middle of Fig. 1(a)). The constant magnetic field interferes with the Rashba field and the injected spin vector which may lead to the reduction of gate control signal.
Fig. 3. Magnetic field driven spin transport. (a) Geometry of three-terminal Hanle measurement. (b) Hanle measurement curve. The spin lifetime from the Hanle curve is 7.48 ps. (c) Interaction between Rashba (BR) and Hanle (Ba) fields. 9
The temperature dependence of gate control signal is shown in Fig. 2(d). The amplitude of signal is roughly constant with increasing temperature up to T = 70 K. Above this temperature, the gate control of oscillation was not observed. The reason is that the spin transport mechanism is changed from ballistic to diffusive behaviours at T = 70 K. It should be noted here that the oscillation frequency decreases with increasing temperature. The gate controllability is deteriorated at higher temperature due to the decrease of the Rashba field strength. The observation of the gate controllability of Rashba parameter using Shubnikov-de Haas oscillations [15, 16] is not possible at higher temperature. Instead, we can estimate the trend of gate controllability using the solid line fits of which oscillation period depends on the gate controllability of the Rashba parameter. The gate controllability is lessened by 37.5% as the temperature increases from 10 K to 70 K. An additional confirmation of the spin injection into the InAs quantum well layer is to measure a non-local spin accumulation and corresponding Hanle curves in the four-terminal spin valve geometry. In the InAs quantum well, however, the Rashba field is very large (~8.5 T) and the spin scattering time is very short. Thus, it is very difficult to observe the Hanle effect using a fourterminal spin valve structure in the sample with a strong Rashba effect. It is also unattainable to measure the non-local magnetoresistance, since the switching field of the injector and detector is almost the same. An alternative approach is to measure a three-terminal Hanle effect [10, 11] where one contact is used for both spin injection and detection and a magnetic field, which induces the relaxation of injected spin, is externally applied perpendicular to the spin orientation of electrons. The Hanle signal for an in-plane spin injection is usually detected using a perpendicular magnetic 10
field which drives suppression of in-plane spins. In our case, however, the injected spins are perpendicularly polarized, so the magnetic field along the y-axis was utilized to observe spin relaxation as shown in Fig. 3(a). The lateral dimension of the FM is 3 µm × 50 µm. In this measurement, the bias current was applied from the FM to the quantum well channel, and the voltage was measured between the FM and the other side of the channel. Figure 3(b) shows the three-terminal Hanle curve measured at 10 K obtained with the geometry illustrated in Fig. 3(a). The bias current is 1 mA. The applied magnetic field, which is perpendicular to the injected spins, drives spin precession and dephasing which make the spin polarization lost. This spin relaxation causes the voltage change and the magnitude of Hanle signal is defined as ΔVHanle. The Hanle signal (ΔVHanle) is much smaller than the gate control signal shown in Fig. 2. The reason is that, during the Hanle measurement, the driving force for spin relaxation is not only the external magnetic field but also the Rashba effective field in this channel system. As shown in Fig. 3(c), the Rashba field is constant during the Hanle measurement, so the injected spins precess around the Rashba field even without an applied magnetic field. While the gate field control of spin injection signal corresponds to the full swing of spin precession (Fig. 2(b)), the Hanle signal cannot reflect the total contribution of spin precession. In the four-terminal Hanle measurement, we do not detect any signal because the injected spins fully feel the Rashba field when the carriers move along the x-axis. On the contrary, the carrier path includes both x and z terms, so the non-zero Hanle signal can be detected in the three-terminal Hanle measurement. It is very difficult to estimate how much the Rashba field exerted on the injected spins because the Rashba field depends 11
on the direction of the electron movement. 4. Conclusion We demonstrated the operation of a spin transistor using an InAs quantum well channel with perpendicularly magnetized electrodes by means of an all-electric method. By using the gate electric field, the spin injection and modulation of perpendicularly polarized spins are observed. From the three-terminal Hanle measurement, we also confirmed the spin injection into semiconductor channel with a strong spin-orbit interaction which interacts with the Hanle driving field. This work clearly shows the possibility of all-electric spin transistor which can be applied for the spin logic devices.
Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0017457, 2011-0027905, 2015-004870) and the KIST & KU-KIST Institutional Programs.
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[14] M. Tran, H. Jaffrès, C. Deranlot, J.-M. George, A. Fert, A. Mirad, and A. Lemaître, Phys. Rev. Lett. 107, (2011) 249901. [15] J. Nitta, T. Akazaki, H. Takayanagi, and T. Enoki, Phys. Rev. Lett. 78, (1997) 1335. [16] I. Lo et al., Phys. Rev. B 65, (2002) 161306(R).
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Figure captions
Fig. 1. Spin transistor using perpendicular spins. (a) Relationship between the magnetization direction of ferromagnetic electrodes (source and drain) and the spin precession. S, D and G represent source, drain and gate electrodes, respectively. The arrows inside the channel indicate the spin orientation and the rounded arrows depict the direction of spin precession. BR is the Rashba field.
(b)
Magnetization
behavior
of
the
ferromagnetic
electrodes
consisting
of
Tb20Fe62Co18/Co40Fe40B20 layers. The schematic shows the layer stack of ferromagnetic electrodes with perpendicular magnetization.
Fig. 2. Gate modulation of perpendicular spins. (a) Measurement geometry and scanning electron micrograph of device. For clarity, the image was taken before depositing a gate electrode. (b) Gate control of spin transistor signal. (c) Spin precession angle as a function of the gate electric field. (d) Temperature dependence of gate controlled spin signals.
Fig. 3. Magnetic field driven spin transport. (a) Geometry of three-terminal Hanle measurement. (b) Hanle measurement curve. The spin lifetime from the Hanle curve is 7.48 ps. (c) Interaction between Rashba (BR) and Hanle (Ba) fields.
We realize all-electric spin-FET using perpendicular spins. Gate modulation of spin orientation is detected in a quantum well layer. 15
A gate-controlled spin signal as large as 80 mΩ is obtained at 10 K.
We compare spin injection signals driven by a magnetic field and an electric field.
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