Kemajuan Terbaru dalam Spintronics Dua Dimensi

Fungsi Dasar dari Operasi Perangkat Spintronic 2D

Perkembangan terbaru dalam material 2D yang muncul dan beberapa teknik karakterisasi lanjutan telah memungkinkan bidang spintronics 2D berkembang pesat [51,52,53]. Isu kunci untuk realisasi perangkat spintronic termasuk konversi spin-charge, transportasi spin, dan manipulasi spin. Pembangkitan dan pendeteksian arus putaran yang efisien merupakan tantangan utama untuk mengembangkan perangkat spintronik 2D yang menggantikan perangkat listrik. Transportasi putaran menginginkan saluran transportasi yang sesuai dengan masa pakai putaran yang lama dan propagasi putaran jarak jauh. Manipulasi putaran diperlukan untuk mengontrol arus putaran dan mencapai fungsionalitas perangkat.

Konversi Spin–Charge

Banyak metode yang diusulkan untuk mencapai konversi spin-to-charge, seperti dengan injeksi/deteksi spin listrik atau dengan memanfaatkan efek spin Hall dan efek Edelstein, yang berasal dari SOC [54,55,56]. Namun, efek spin Hall biasanya terjadi pada material massal, sedangkan efek Edelstein biasanya dianggap sebagai efek antarmuka [55].

Pengukuran “nonlokal” dan “lokal” biasanya digunakan untuk melakukan injeksi/deteksi spin listrik ke dalam bahan saluran [14]. Untuk pengukuran nonlokal (Gbr. 4a), elektroda E2 adalah logam feromagnetik sebagai injektor spin, dan E3 adalah elektroda feromagnetik sebagai detektor spin. Arus yang diterapkan mengalir dari elektroda E1 ke E2, dan E3 dan E4 digunakan untuk mendeteksi sinyal arus putaran murni yang tersebar. Polaritas tegangan terukur antara E3 dan E4 tergantung pada konfigurasi magnetisasi elektroda E2 dan E3. Metode ini dapat memperoleh arus spin murni tanpa arus pengisian, sedangkan pengukuran "lokal" mendapatkan sinyal campuran arus spin dan arus pengisian (Gbr. 4b). Perbedaan tegangan antara magnetisasi paralel dan antiparalel dari elektroda E2 dan E3 dianggap sebagai sinyal transpor spin.

Direproduksi dengan izin dari Han et al., Nat. nanoteknologi. 9, 794 (2014). Hak Cipta 2018 Nature Publishing Group [14]. c , d Direproduksi dengan izin dari Mendes et al., Phys. Pdt. Lett. 115, 226601 (2015). Hak Cipta 2015 American Chemical Society [68]. e , f Direproduksi dengan izin dari Shao et al., Nano Lett. 16, 7514 (2016). Hak Cipta 2016 American Chemical Society [71]

Putar dan isi daya konversi dalam materi 2D. a Injeksi dan deteksi putaran listrik dengan geometri pengukuran nonlokal. b Injeksi dan deteksi putaran listrik dengan geometri pengukuran lokal. c Konversi spin-to-charge dalam graphene pada YIG, isolator feromagnetik. Arus putaran dihasilkan dari pemompaan putaran dari YIG dan diubah menjadi arus pengisian di graphene. d Ketergantungan medan magnet dari tegangan pemompaan putaran yang diukur pada YIG/Grafena e Pengukuran SOT untuk MX₂ /CoFeB bilayer. MX₂ mewakili MoS₂ dan WSe₂ . f Ilustrasi akumulasi putaran yang diinduksi oleh efek Rashba–Edelstein (REE) pada antarmuka MX₂ /CoFeB di bawah medan listrik eksternal. a , b

Bukit dkk. pertama kali melaporkan injeksi spin ke graphene dengan menggunakan elektroda NiFe magnetik lunak [57]. Namun, efisiensi injeksi spin diperkirakan relatif rendah, sekitar 10%, yang dapat dikaitkan dengan ketidaksesuaian konduktansi antara logam feromagnetik dan graphene. Kemudian, beberapa peneliti mengusulkan menggunakan penghalang isolasi seperti Al₂ O₃ atau MgO sebagai lapisan untuk menyesuaikan resistivitas spin-dependent antarmuka dan meningkatkan efisiensi injeksi putaran [58,59,60], tetapi menumbuhkan lapisan oksida berkualitas tinggi merupakan tantangan utama. Beberapa metode telah digunakan untuk meningkatkan teknik pertumbuhan lapisan oksida atau mengubah lapisan oksida antarmuka lain, seperti lapisan TiO₂ atau HfO₂ [61, 62]. Namun, resistivitas bergantung-putaran antar muka masih menjadi masalah mendasar, yang menyebabkan efisiensi injeksi putaran rendah. Satu bahan insulasi 2D, boron nitrida heksagonal (h-BN), memiliki struktur kristal yang mirip dengan grafena. Studi teoretis dan eksperimental telah menunjukkan bahwa menggunakan h-BN sebagai penghalang terowongan dapat menghasilkan antarmuka berkualitas tinggi dan sangat meningkatkan efisiensi injeksi-spin graphene. Beberapa lapisan h-BN menunjukkan kinerja injeksi putaran yang lebih baik daripada h-BN monolayer [63, 64]. Namun demikian, hasil penelitian ini masih menyisakan celah besar yang harus diisi sebelum aplikasi praktis dimungkinkan. Pada akhirnya, untuk mencapai injeksi putaran (100%) yang sempurna membutuhkan banyak penelitian, dan bahan 2D memberikan arah yang menjanjikan, seperti struktur heterostruktur 2D yang terdiri dari bahan feromagnetik 2D, penghalang terowongan 2D, dan saluran transportasi 2D.

Efek (terbalik) Rashba–Edelstein adalah efek antarmuka yang berasal dari SOC yang kuat, yang dapat digunakan untuk mencapai konversi spin-charge [65]. Meskipun graphene intrinsik memiliki SOC yang agak lemah, ia dapat mencapai konversi spin-charge yang efisien dengan menggunakan SOC yang kuat dari material yang berdekatan melalui efek kedekatan [66, 67]. Seperti ditunjukkan pada Gambar. 4c, ketika graphene berdekatan dengan isolator feromagnetik YIG, arus spin dihasilkan di lapisan YIG melalui pemompaan spin, kemudian diubah menjadi arus muatan di graphene oleh efek Edelstein terbalik [68]. Gambar 4d menunjukkan kurva tegangan pemompaan putaran sebagai fungsi medan pada perangkat YIG/graphene. Tegangan pemompaan putaran dapat dideteksi dalam medan magnet yang tegak lurus terhadap saluran graphene. Selain itu, ketika medan magnet eksternal diputar di sepanjang saluran graphene, tidak ada tegangan pemompaan putaran. Selanjutnya, saluran cairan ionik yang diterapkan pada permukaan graphene jelas dapat memodulasi sifat graphene untuk mengubah efisiensi konversi spin-to-charge dari YIG/graphene [56].

Tidak seperti graphene, TMDCs dengan SOC yang kuat dianggap sebagai bahan yang menjanjikan untuk mencapai konversi spin-charge [69, 70]. Torsi putaran–orbit (SOT) besar di TMDC monolayer (MoS₂ atau WSe₂ )/struktur bilayer CoFeB dihasilkan melalui akumulasi putaran yang diinduksi arus yang disebabkan oleh efek Rashba-Edelstein (Gbr. 4e, f) [71]. Torsi seperti medan dan torsi seperti redaman ditentukan melalui pengukuran harmonik kedua, dan hasilnya menunjukkan bahwa TMDC monolayer area luas memiliki aplikasi potensial karena efisiensinya yang tinggi untuk pembalikan magnetisasi. Selain itu, teknik spin-torque ferromagnetic resonance (ST-FMR) telah digunakan untuk menyelidiki konversi putaran dan muatan di TMDC. Sebagai contoh, hasil ST-FMR yang menarik menunjukkan bahwa SOT dapat dikontrol melalui simetri kristal WTe₂ di WTe₂ / Lapisan ganda Permalloy. Saat arus diterapkan di sepanjang sumbu simetri rendah WTe₂ , torsi anti-redaman out-of-plane dapat dihasilkan [72]. Properti penguncian spin-momentum dalam status permukaan TI berguna untuk mencapai injeksi arus putaran ke bahan yang berdekatan melalui SOT. Karena korelasi yang kuat antara arah polarisasi putaran dan arah arus muatan, arah putaran dapat dimanipulasi oleh arus muatan di TI. Teknik pengukuran yang berbeda telah digunakan untuk menyelidiki konversi spin-charge, termasuk pengukuran harmonik kedua, pemompaan spin, dan ST-FMR. Hasil pengukuran ini menunjukkan bahwa dimungkinkan untuk menghasilkan SOT yang efisien dalam material 2D seperti TMDC dan TI.

Transportasi Putar

Kunci untuk transportasi putaran adalah untuk mendapatkan saluran transportasi putaran yang menguntungkan dengan panjang difusi putaran yang panjang dan waktu relaksasi putaran. Relaksasi spin disebabkan oleh hamburan momentum, sehingga graphene dengan SOC lemah dianggap sebagai bahan yang ideal untuk transportasi spin [14, 73]. Tombros dkk. [74] merealisasikan transpor spin elektronik dan presesi spin dalam katup spin graphene tunggal lateral pada suhu kamar dengan pengukuran nonlokal pada tahun 2007. Seperti yang ditunjukkan pada Gambar 5a, b, katup spin nonlokal terdiri dari kobalt feromagnetik empat terminal sebagai elektroda, Al₂ thin yang tipis O₃ lapisan oksida sebagai penghalang, dan lembaran graphene sebagai saluran transpor spin. Sinyal pengukuran pada Gambar 5c menunjukkan bahwa jika elektroda feromagnetik untuk injeksi spin dan deteksi spin memiliki magnetisasi paralel, resistansi nonlokal yang diukur oleh kontak 1 dan 2 memiliki nilai positif. Jika elektroda feromagnetik untuk injeksi spin dan deteksi spin memiliki magnetisasi antiparalel, maka resistansi nonlokal menunjukkan nilai negatif. Presesi putaran Hanle dapat digunakan untuk menentukan panjang difusi putaran dan masa pakai putaran. Seperti yang ditunjukkan pada Gambar. 5d, masa pakai putaran (τ _sf ) dan panjang relaksasi putaran (λ _sf ) adalah 125 ps dan 1,3 m, masing-masing, dalam katup spin graphene tunggal lateral pada suhu kamar. Selanjutnya, gerbang dapat digunakan untuk meningkatkan panjang relaksasi putaran dan umur putaran [75, 76]. Teori memperkirakan bahwa masa pakai putaran di graphene murni dapat mencapai 1 s, sedangkan nilai eksperimen yang dilaporkan berkisar dari picoseconds hingga beberapa nanoseconds.

Direproduksi dengan izin dari Tombros et al., Nature 448, 571 (2007 ). Hak Cipta 2007 Nature Publishing Group [74]. e –h Direproduksi dengan izin dari Avser et al., Nat. fisik 13, 888 (2017). Hak Cipta 2017 Nature Publishing Group [84]

Transportasi spin di katup spin lateral. a Geometri pengukuran transpor putaran nonlokal. Arus disuntikkan dari elektroda 3 melalui Al₂ O₃ penghalang menjadi graphene dan diekstraksi pada kontak 4. b Memindai mikrograf elektron dari katup putar empat terminal dengan graphene satu lapis sebagai saluran transpor spin dan Co sebagai empat elektroda feromagnetik. c Sinyal spin valve nonlokal pada 4.2 K. Konfigurasi magnetik elektroda diilustrasikan untuk kedua arah sapuan. d Presesi putaran Hanle dalam geometri nonlokal, diukur sebagai fungsi medan magnet tegak lurus B _z untuk konfigurasi paralel. e Skema katup spin fosfor hitam. Inset menunjukkan skema heterostruktur. f Gambar optik perangkat. g Sinyal spin valve nonlokal sebagai fungsi medan magnet dalam bidang. Magnetisasi relatif dari elektroda injektor dan detektor diilustrasikan oleh panah vertikal, dan panah horizontal mewakili arah medan magnet. h Presesi putaran Hanle dalam geometri nonlokal, diukur sebagai fungsi medan magnet tegak lurus B _z untuk konfigurasi paralel dan antiparalel. Sisipan menunjukkan presesi putaran di bawah medan magnet yang diterapkan. a –d

Banyak metode yang ditingkatkan digunakan untuk meningkatkan panjang difusi putaran dan umur putaran, dan beberapa perangkat sudah menunjukkan panjang difusi putaran panjang dalam kisaran mikrometer [13, 77, 78]. Sebagai contoh, graphene yang ditumbuhkan secara epitaksial pada SiC memiliki mobilitas tinggi, menunjukkan efisiensi transpor spin hingga 75% dan panjang difusi spin melebihi 100 m [79]. Heterostruktur h-BN/graphene/h-BN menunjukkan kinerja transpor spin jarak jauh, di mana panjang difusi spin dapat mencapai 30,5 m pada suhu kamar [13]. Transportasi spin dalam material 2D dapat dipengaruhi oleh difusi/drift, yang dapat dimodulasi dengan menerapkan medan listrik. Ingla-Aynes dkk. [80] melaporkan panjang relaksasi putaran hingga 90 m dalam graphene bilayer enkapsulasi h-BN dengan menggunakan drift pembawa. Namun, SOC yang lemah dan celah pita nol dalam graphene intrinsik membatasi prospeknya untuk perangkat spin semikonduktor. Fosfor hitam memiliki celah pita langsung yang cukup besar dan mobilitas suhu ruangan 1000 cm ² V ⁻¹ s ⁻¹ , yang membuatnya menjadi bahan spintronik semikonduktor yang ideal [81,82,83]. Avsar dkk. [84] membangun katup spin lateral berdasarkan lembaran fosfor hitam ultra tipis dan mengukur sifat transpor spinnya pada suhu kamar melalui geometri nonlokal (Gbr. 5e, f). Transpor spin elektronik pada Gambar 5g menunjukkan bahwa sebagai arah magnetisasi dari saklar feromagnet, hambatan nonlokal memiliki perubahan ΔR 15Ω. Selain itu, presesi putaran Hanle menunjukkan waktu relaksasi putaran hingga 4 ns dan panjang relaksasi putaran melebihi 6 m (Gbr. 5h). Transpor putaran dalam fosfor hitam berkaitan erat dengan konsentrasi pembawa muatan, sehingga sinyal putaran dapat dikontrol dengan menerapkan medan listrik.

Manipulasi Putar

Menyadari manipulasi putaran adalah kunci fungsionalisasi perangkat yang efektif. Menerapkan tegangan gerbang dapat mengontrol konsentrasi pembawa dalam material, yang dapat digunakan untuk memanipulasi sinyal putaran [85, 86]. Berbagai material 2D sebagai saluran transpor putaran telah diselidiki untuk mewujudkan penyesuaian parameter transpor putaran melalui penerapan tegangan gerbang. Sebagai contoh, graphene yang diinduksi bias bisa mendapatkan spin-injection dan polarisasi deteksi hingga 100% pada heterostruktur feromagnet/bilayer h-BN/graphene/h-BN [64]. Katup spin yang dapat disetel gerbang berdasarkan fosfor hitam dapat mencapai waktu relaksasi putaran dalam rentang nanodetik dan panjang relaksasi putaran yang panjang [84]. Untuk MoS semikonduktor₂ channel, applying a gate voltage can still get a relatively long spin-diffusion length, larger than 200 nm [70]. However, a suitable spin field-effect device requires a clear switching ratio, which is a challenge for graphene and even for semiconducting 2D materials [87, 88].

To solve this issue, a vdW heterostructure based on atomically thin graphene and semiconducting MoS₂ has been developed to achieve a spin field-effect switch via applying a gate voltage (Fig. 6a) [89]. In this structure, the superior spin transport properties of graphene and the strong SOC of MoS₂ are combined. The applied gate voltage can change the conductivity of MoS₂ and spin absorption during the spin transport, which results in switching of the spin current between ON and OFF states in the graphene channel (Fig. 6b). Another research effort produced a similar report about the graphene/MoS₂ vdW heterostructure. In this report, an electric gate control of the spin current and spin lifetime in the graphene/MoS₂ heterostructure was achieved at room temperature [90]. Moreover, that report pointed out that the mechanism of gate tunable spin parameters stemmed from gate tuning of the Schottky barrier at the MoS₂ /graphene interface and MoS₂ channel conductivity.

source contact. b Effective B _SO sebagai fungsi dari V _g . The inset shows the surface carrier distribution in the Cr-TI layer under V _g  = − 10 V, + 3 V, and + 10 V. a –c Reproduced with permission from Yan et al., Nat. Commun. 7, 1 (2016). Copyright 2016 Nature Publishing Group [89]. d , e Reproduced with permission from Fan et al., Nat. Mater. 13, 699 (2014). Copyright 2014 Nature Publishing Group [95]. f , g Reproduced with permission from Fan et al., Nat. Nanotechnol. 11, 352 (2016). Copyright 2016 Nature Publishing Group [96]

Spin manipulation. a Schematic illustration of a 2D spin field-effect switch based on a vdW heterostructure of graphene/MoS₂ with a typical nonlocal magnetoresistance measurement. b The nonlocal resistance R _nl switches between R _P dan R _AP for parallel and antiparallel magnetization orientations of the Co electrodes. The spin signal is calculated as ΔR _nl  = R _P − R _AP . c The plot with blue circles shows the gate modulation of the spin signal ΔR _nl . The solid black line represents the sheet conductivity of the MoS₂ sebagai fungsi dari V _g . The insets show the spin current path in the OFF and ON states of MoS₂ . d Schematic illustration of SOT-induced magnetization switching in a Cr-doped TI bilayer heterostructure. The inset shows illustrations of the Hall bar device and the measurement setting. e Experimental results of SOT-induced magnetization switching by an in-plane direct current at 1.9 K while applying a constant in-plane external magnetic field B _y during the measurement. The inset shows an enlarged version of the circled part in the figure. f 3D schematic of the Hall bar structure of the Al₂ O₃ /Cr-TI/GaAs stack with a top Au gate electrode. A gate voltage of V _g can be applied between the top gate and the

Current-induced SOT is regarded as another efficient strategy to manipulate spin. The spin current, generated by the spin Hall effect within the heavy metals or the Rashba effect at the interfaces, can exert a spin torque to ferromagnets and thereby realize magnetization switching [91,92,93]. Efficient current-induced magnetization switching via SOT may lead to innovative spintronic applications. Due to strong SOC and time-inversion symmetry breaking, magnetically doped TIs are being considered as a promising material to manipulate spin signals via SOT [94]. Wang’s group first experimentally demonstrated a magnetization switching induced by an in-plane current in an epitaxial Cr-doped TI (Bi_0.5 Sb_0. 5)₂ Te₃ /(Cr_0.08 Bi_0.54 Sb_0.38 )₂ Te₃ bilayer film (Fig. 6c) [95]. The spin Hall angle in the Cr-doped TI film, ranging from 140 to 425, is almost three orders of magnitude larger than that in heavy metal/ferromagnetic heterostructures, and the critical switching current density is below 8.9 × 104 A cm ⁻² at 1.9 K (Fig. 6d). Furthermore, this team also reported an effective electric field control of SOT in a Cr-doped (Bi_0.5 Sb_0.42 )₂ Te₃ thin film epitaxially grown on GaAs substrate (Fig. 6e) [96]. The gate effect on the magnetization switching was investigated by scanning gate voltage under a constant current and an applied in-plane magnetic field in the film (Fig. 6f). The SOT intensity depends strongly on the spin-polarized surface current in the thin film, and it can be modulated within a suitable gate voltage range. The effective electric field control of SOT in the TI-based magnetic structures has potential applications in magnetic memory and logic devices.

In addition, electrical control of emerged 2D magnets has also been investigated. For example, utilizing electric fields or electrostatic doping can achieve the magnetic conversion of bilayer CrI₃ antiferromagnetic to ferromagnetic [97]. The coercivity and saturation field of few-layer Cr₂ Ge₂ Te₆ can be modulated via ionic liquid gating [98]. In contrast to magnetic semiconductor, electrostatic doping can be used to control the carrier concentrations of the ferromagnetic metal, and the ferromagnetic transition temperature of Fe₃ GeTe₂ can be dramatically raised to room temperature via an ionic gate [99]. The emergence and research of 2D magnets provide a new platform for engineering next-generation 2D spintronic devices.

Applications of 2D Spintronics

2D materials exhibit great potential for the engineering of next-generation 2D spintronic devices. Graphene with high electron/hole mobility, long spin lifetimes, and long diffusion lengths is a promising candidate for a spin channel. Moreover, graphene can gain magnetism by introducing adatoms, or magnetic proximity effect [23, 25]. The carrier density in proximity-induced ferromagnetic graphene can be modulated by gating, allowing to observe Fermi energy dependence of the anomalous Hall effect conductivity. This result can help understand the physical origin of anomalous Hall effect in 2D Dirac fermion systems. Realizing a ferromagnetic surface state in a TI is predicted to allow several prominent phenomena to emerge, such as the interfacial magnetoelectric effect [33], and the electric field-induced image magnetic monopole [34]. However, the current technology of inducing magnetism in TI is confined to low temperatures, which restrict its potential for applications. A key requirement for useful applications is the generation of room temperature ferromagnetism in the TI. The PNR result shows that the Bi₂ Se₃ /EuS bilayer has a ferromagnetic order at the interface, and this topologically enhanced interfacial ferromagnetism can persist up to room temperature [32]. The topological magnetoelectric response in such an engineered TI could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

The STT, and tunnel magnetoresistance (TMR) effects offer alternative approaches for write and read-out operations. The STT effect refers to the reorientation of the magnetization of ferromagnetic materials via the transfer of spin angular momenta. Efficient current-induced magnetization switching via SOT may lead to innovative spintronic applications [71, 100]. Due to strong SOC and time-inversion symmetry breaking, magnetically doped TIs are being considered as a promising material to manipulate spin signals via SOT [93]. TMR refers to magnetization-dependent magnetoresistance behavior. A high TMR ratio is the key to achieve spintronic devices with higher sensitivity, lower energy consumption. 2D materials with high-quality crystal and sharp interfaces can achieve some new functionalities such as spin filtering. The 2D vdW MTJ consists of a 2D magnetic CrI₃ layer as a spin filtering tunnel barrier, which reaches a value of TMR up 19,000% [101]. Progress in the fabrication of graphene-based and other 2D heterostructures has led to the optimization of long-distance spin diffusion (up to tens of micrometres), as well as directional guiding of the spin current [13, 64]. Spin manipulation, electrical gating [56], electrical field induced drift [80], SOT-induced switching [95, 96], and the magnetic proximity effect [25, 32] have been explored to develop next-generation MRAM.

2D Spintronic Devices for Memory Storage and Logic Applications

Great efforts have been made to search for new 2D spintronic devices. According to the function, 2D spintronic devices can be classified as memory storage or logic devices. Here we focus on several important 2D spintronic devices, including the 2D magnetic tunnel junction (MTJ), 2D spin field-effect transistor (sFET), and 2D spin logic gate.

2D MTJ

The discovery of the GMR opens the door for 2D spintronics. However, TMR has a stronger magnetoresistance ratio than GMR, so TMR holds greater potential in magnetic storage applications. The TMR structure consists of two ferromagnetic layers and an intermediate insulating layer, which is called the MTJ. The tunneling probability is related to the density of states near the Fermi energy in the ferromagnetic layers. When the two magnetic layers are parallel, the similar density of states for each spin-state can provide more available states for tunneling, resulting in a low resistance state. On the other hand, when the layers are antiparallel, a mismatch between spin channels of the source and sink will result in a high resistance state. Some issues in traditional thin-film MTJs limit the achievement of a high TMR ratio, such as the quality of the insulation barrier and the thermal stability [102]. 2D materials with high-quality crystal and sharp interfaces may offer promising routes to address these issues and even achieve some new functionalities such as spin filtering.

Karpan et al. first explored graphene layers as the barrier in vertical MTJ by computational means in 2007 [103]. They proposed a match between the band structure of graphene and that of nickel, predicting a large spin polarization close to 100%, which can result in a large TMR up to 500%. However, the subsequent experimental results show that the MTJs based on graphene exhibit a very low TMR. Compared to monolayer or bilayer graphene, the few-layer MTJ holds the highest recorded TMR signal of up to 31% in graphene-based MTJs [11, 15]. In addition to graphene, some other 2D materials have been explored as tunneling barrier layers, including insulating h-BN and semiconducting TMDCs [104, 105]. Piquemal-Banci et al. [63] fabricated Fe/h-BN/Co junctions where the h-BN monolayer was directly grown on Fe by using the chemical vapor deposition (CVD) method, observing large spin signals of TMR and the spin polarization of P  ~ 17%. MTJs based on MoS₂ or WSe₂ were reported to have only a few percent of the TMR signal; further exploration is needed to achieve a high TMR ratio.

Emerging 2D magnetic materials exhibit many surprising properties. When the magnetizations in bilayer CrI₃ are switched to different magnetic configurations (Fig. 7a), the MTJ based on CrI₃ exhibits a giant TMR produced by the spin-filtering effect [101, 106, 107]. As demonstrated in Fig. 7b, the 2D vdW MTJ consists of a 2D magnetic CrI₃ layer as a spin filtering tunnel barrier, graphene as a contact electrode, and h-BN as an encapsulation layer to prevent device degradation. The transport result shows that the TMR is enhanced as the CrI₃ layer thickness increases, and it reaches a value of 19,000% in four-layer CrI₃ based MTJ at low temperature (Fig. 7c) [101]. Subsequently, Xu’s group also reported gate-tunable TMR in a dual-gated MTJ structure based on four-layer CrI₃ . The TMR can be modulated from 17,000 to 57,000% by varying the gate voltages in a fixed magnetic field [108, 109]. Moreover, with few-layer Fe₃ GeTe₂ serving as ferromagnetic electrodes, the TMR in Fe₃ GeTe₂ /h-BN/Fe₃ GeTe₂ heterostructures can reach 160% at low temperature [110]. More interestingly, Zhou et al. reported a theoretical investigation of a VSe₂ /MoS₂ /VSe₂ heterojunction, where the VSe₂ monolayer acts as a room-temperature ferromagnet, and the large TMR can reach 846% at 300 K [111]. On the other hand, the strong spin Hall conductivity of MoS₂ holds potential to switch the magnetization of the VSe₂ free layer by SOT. Therefore, they put forward the concept of SOT vdW MTJ with faster reading and writing operations, which offers new opportunities for 2D spintronic devices.

Reproduced with permission from Song et al., Science (2018). Copyright 2018 The American Association for the Advancement of Science [101]. c , d Reproduced with permission from Wen et al., Phys. Rev. Appl. 5, 044003 (2016). Copyright 2016 American Chemical Society [118]

2D spintronic Devices. a Magnetic states of bilayer CrI₃ with different external magnetic fields. b Schematic illustration of a 2D spin-filter MTJ with bilayer CrI₃ sandwiched between graphene contact. c Tunneling current of a bilayer CrI₃ sf-MTJ at selected magnetic fields. The top inset shows an optical image of the device, and the bottom inset shows the schematic of the magnetic configuration in different magnetic fields. d Diagram of a proposed 2D XOR spin logic gate, where A, B, and M are ferromagnetic electrodes on top of a spin transport channel. Aku _s dan Aku _keluar denote the injection and detection currents, respectively. The magnetizations of the electrodes are input logic 1 and 0. The detected current I _keluar serves as the logic output. e Aku _keluar measured as a function of H . Vertical arrows indicate the magnetization states of A and B. The top-left inset shows the table of XOR logic operation. a , b

2D sFET

Datta and Das first proposed the idea of the sFET in 1990 [112]. The sFET consists of the source and sink ferromagnetic electrodes, and a two-dimensional electron gas (2DEG) channel which can be controlled by an electrical gate. The gate voltage can result in a spin precession and, consequently, a change in the spin polarization of the current on the channel. Since switching the current through the device requires only little energy and a short time, sFET is expected to be a 2D spintronic device with low power consumption and high computing speed.

As mentioned in the previous section, graphene with high carrier concentration and weak SOC is considered to be a promising candidate as a spin transport channel [113]. Michetti et al. [76] designed a double-gate field-effect transistor structure, where bilayer graphene acts as the transport channel. Theoretical analysis shows that the spin precession of carriers in the graphene channel can be turned on and off by the application of a differential gate voltage. Experimentally, Avsar et al. first reported a dual-gated bilayer graphene structure with h-BN as a dielectric layer, where the spin current propagation in bilayer graphene channel can be controlled by exerting a vertical electric field [114]. The transport results show that the spin-relaxation time decreases monotonically as the carrier concentration increases, and the spin signal exhibits a rapid decrease, eventually becoming undetectable close to the charge neutrality point. A suitable spin field-effect device requires a clear switching ratio, which is a challenge for graphene.

To solve this issue, a graphene/MoS₂ heterostructure has been developed to achieve a spin field-effect switch via applying a gate voltage. Two independent groups demonstrated that the applied gate voltage can change the conductivity of MoS₂ and result in spin absorption during the spin transport, which gives rise to switching the spin current between ON and OFF states in the graphene channel [89]. Due to the low spin injection efficiency and rapid spin relaxation in channels, it is a challenge to achieve a large high-to-low conductance ratio in 2D sFET device. However, the discovery of 2D magnetic crystals provides new opportunities to explore new 2D spintronic devices. Kin Fai Mak’s group reported a spin tunnel field-effect transistor (sTFET) based on a dual-gated graphene/CrI₃ /graphene heterostructure [115]. By using bilayer CrI₃ as a magnetic tunnel barrier, the applied gate voltage can switch magnetization configurations of bilayer CrI₃ from interlayer antiferromagnetic to ferromagnetic states under a constant magnetic field near the spin-flip transition. Distinct from conventional spin transistors, these devices rely on electrically controlling the magnetization configuration switching rather than the signal of spin current in the channel. This technique allows the sTFET devices to achieve a large high–low conductance ratio approaching 400%, which provides a new approach for exploring memory applications.

2D Spin Logic Gate

Dery and Sham first reported a spin logic device based on an “exclusive or” (XOR) gate [116]. The XOR logic gate structure includes a semiconductor channel and three ferromagnetic terminals. An XOR logic operation can be implemented by different spin accumulations, which is caused by different magnetization configurations of the input terminals [117]. Experimentally, the proposed three-terminal XOR logic gate achieved logical operations in a graphene spintronic device at room temperature [117,118,119]. As shown in Fig. 7c, the device includes single-layer graphene as the channel, and three ferromagnetic terminals composed of A, B, and M Co electrodes with MgO tunnel barriers. The magnetization of the electrodes A and B represents the input states 0 or 1, and the current of the electrode M acts as the output state. The magnetizations of input electrodes A and B will be switched by varying an applied external magnetic field, which results in a different spin accumulation in the M electrode, corresponding to a different output current. If the input A and B electrodes have identical contributions to the output M electrode, then the current in the output ferromagnetic terminal has a detectable value only when the magnetization of input ferromagnetic terminals are antiparallel (01 or 10). When the magnetizations of the input ferromagnetic terminals are parallel (00 or 11), the output current is almost zero. Therefore, the XOR logic operation can be achieved (Fig. 7d).

Dery et al. further designed a reconfigurable magnetologic gate with five-terminal structure combining two XOR gates-XOR (A, X) and XOR (B, Y) with a shared output terminal, M [119]. Similar to the three-terminal XOR logic gate, the different magnetic configurations of input electrodes give rise to the different spin accumulation in the output terminal M, which results in different output signals. By analogy, a finite number of these XOR gates can be used to implement any binary logic function. Subsequently, other groups extended this theoretical design to experimental studies by constructing graphene spin logic gates [120,121,122]. Various modeling, simulation, and experimental explorations of 2D spin logic gates have helped to accelerate the progress toward building practical spin logic applications. However, two key issues remain in the research of graphene spin logic gates. The first one is to balance the contributions of two input terminals to the output one. The other one is to eliminate the influence of background signals on the output.

Challenges and Opportunities in 2D Spintronics

As discussed above, much theoretical and experimental research has been carried out to explore spintronics based on 2D materials, and considerable progress has been achieved [15, 123, 124]. However, great challenges still need to be addressed for the practical application of 2D memory and logic applications. We now discuss three of these:physical mechanisms, materials science, and device engineering.

Physical Mechanisms

Due to the complexity of the experiments, the proposed theoretical research and experimental results usually have large discrepancies. For example, based on the mechanism of spin relaxation, theory predicted that the spin lifetime for pristine graphene would be up to 1 μs, whereas experimental values range from tens of picoseconds to a few nanoseconds [14, 57, 103]. Furthermore, the spin injection efficiency of graphene measured experimentally ranges from a few percent to 10%, which is far smaller than the theoretical prediction value of 60–80% [125]. These differences indicate that more in-depth physical mechanisms and accurate theoretical models need to be proposed and developed to better guide the research direction and analyze the experimental results.

Materials Science

2D materials provide an ideal platform to construct various heterostructures for spintronic applications. However, there are still many major problems in 2D materials. For example, stability is a great challenge for 2D materials. Most 2D materials of thickness close to the atomic level are susceptible to moisture, oxygenation, and temperature, especially the recently emerging 2D magnetic materials, which must be peeled off in a glove box with ultra-low water and oxygen content. Besides this, most currently available 2D magnets rely on mechanical exfoliation, and their low magnetic transition temperature is far below room temperature. These are significant limitations because stability in air, convenient wafer-scale synthesis, and operation above room temperature are prerequisites for 2D materials used in practical applications.

Device Engineering

Breakthroughs have been made in the fundamental study of 2D spintronics, such as enhanced spin injection efficiency by using 2D tunnel barriers h-BN, long spin diffusion length up to 30 μm at room temperature in graphene-based 2D heterostructures [13], and high TMR up to 19,000% by using 2D magnets as spin filter barriers [101]. Based on the study of 2D spintronic devices, it is promising to develop the low-power device applications, including advanced magnetic memories and spin logic circuits, which are compatible with the existing complementary metal-oxide semiconductor (CMOS) electronics. However, the design and application of functional 2D spintronic devices are still in the early theoretical prediction and proof-of-concept stage.

Opportunities

2D spintronics is an important scientific research field with many potential applications for future technologies. As mentioned above, considerable challenges currently remain, but there are also many opportunities. Spin valves based on graphene as the spin transport channel can exhibit a long spin diffusion length up to 30 mm at room temperature [13]. Magnetic tunnel junctions with four-layer CrI₃ as spin filter tunneling barriers show giant TMR up to 19,000% at low temperatures [101]. The magnetic transition temperature of Fe₃ GeTe₂ can reach above room temperature via an ionic liquid gate or when tailored by a TI [99, 126]. Spin-polarized current can be injected from WTe₂ into magnetic substrates by SOT switching [127]. New concepts of spin tunneling field-effect transistors based on 2D magnets CrI₃ have been proposed as well. The demonstration of giant TMR, the efficient voltage control of 2D magnetism, and the magnetization switching in 2D magnets by STT or SOT all open up opportunities for potential next-generation spintronic devices based on atomically thin vdW crystals [21, 100].

Kesimpulan

The study about the magnetic properties of 2D materials is of great significance to the development of 2D spintronics. The magnetic interaction in graphene and TIs has scarcely been explored, and recently discovered 2D magnets also provide an ideal platform to study 2D magnetism. Great progress has been made in 2D spintronics in recent decades, especially in graphene spintronics. However, the origin of spin relaxation in graphene is still a major open question, and further improvement in the spin lifetime and spin diffusion length remains an important research direction for graphene-based spintronic devices. The practical application of 2D spintronic devices still requires meeting great challenges, including related physical mechanisms, materials science, and device engineering. The development of technology, the improvement of theoretical models, and the exploration of new materials all provide more opportunities for new-generation 2D spintronic device applications in the future.

Ketersediaan data dan materi

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Singkatan

2D:

Dua dimensi

GMR:

Giant magnetoresistance effect

STT-MRAM:

Spin-transfer-torque magnetoresistive random-access memory

vdW:

Van der Waals

SOC:

Spin–orbit coupling

TMDCs:

Dichalcogenides logam transisi

TIs:

Topological insulators

SQUID:

Perangkat interferensi kuantum superkonduktor

YIG:

Yttrium iron garnet

QAHE:

Quantum anomalous Hall effect

PNR:

Spin-polarized neutron reflectivity

MOKE:

Efek Kerr magneto-optik

MBE:

Epitaksi berkas molekul

h-BN:

Boron nitrida heksagonal

SOT:

Spin–orbit torque

ST-FMR:

Spin-torque ferromagnetic resonance

MTJ:

Magnetic tunnel junction

sFET:

Spin field-effect transistor

TMR:

Tunneling magnetoresistance

CVD:

Deposisi uap kimia

2DEG:

Two-dimensional electron gas

sTFET:

Spin tunnel field-effect transistor

XOR:

Exclusive or

CMOS:

Complementary metal-oxide semiconductor

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