Spintronics presents a pathway to energy-efficient nanoelectronics beyond CMOS by utilizing the spin degree of freedom in addition to charge for memory and logic. One of the emerging approaches is the use of 2D materials, such as graphene, transition metal dichalcogenides (TMDs), and 2D magnets. Graphene is ideal for spin transport due to its long spin lifetime and record-high spin diffusion lengths (> 30 µm at room temperature)3. Monolayer TMDs (MX2: M = Mo, W; X = S, Se) have optical selection rules and spin-orbit interaction that couple the helicity of photons to the valley-spin polarization with 100% polarization efficiency4. 2D magnets provide tunable magnetic ordering with strong perpendicular magnetic anisotropy and efficient spin filtering5.
A key strength of 2D materials for spintronic functionality lies in combining different layers to create van der Waals (vdW) heterostructures without compromising any individual properties, while the strengths of each constituent layer can compensate for the weakness of the other(s).
Previous work highlights:
Opto-valleytronic spin injection in MoS2/graphene hybrid spin valves
We report the first coherent spin transfer across a vdW interface based on monolayer MoS2/graphene hybrid spin valves. This is achieved by exciting spin-valley populations in MoS2 using a circularly polarized laser beam, and then detecting an electrical spin signal using a ferromagnetic electrode (Co) with a high-quality MBE grown tunneling barrier (MgO). We observe that the magnitude and direction of the spin polarization are controlled by both helicity and photon energy, which is consistent with the unique spin-polarized band structure of monolayer MoS2. We also perform Hanle spin precession to precisely extract the photon-to-spin conversion efficiency of 5%. This work opens the door for many opportunities using light to read and write coherent spins in vdW heterostructures.
We discover a strong and highly-tunable spin-lifetime anisotropy with a long out-of-plane spin lifetime up to 7.8 ns at 100 K in dual-gated bilayer graphene. This demonstrates the successful manipulation of spins in graphene by electrically-controlled spin-orbit fields, which is unexpected due to graphene’s weak intrinsic spin-orbit coupling (∼12 μeV). Near the charge neutrality point, we show that the application of a perpendicular electric field opens a band gap and generates an out-of-plane spin-orbit field that stabilizes out-of-plane spins against spin relaxation, leading to a large spin-lifetime anisotropy (defined as the ratio between out-of-plane and in-plane spin lifetime) up to ∼12 at 100 K. This work establishes the high electrical tunability of graphene-based spintronic devices, which also builds on our earlier work of developing current-based spin detection in graphene spin valves for circuit integrations.
– J. Xu, T. Zhu, Y. K. Luo, Y.-M. Lu, and R. K. Kawakami, “Strong and tunable spin lifetime anisotropy in dual-gated bilayer graphene“, Physical Review Letters 121, 127703 (2018) Editors’ Suggestion. Featured in Physics.
Future directions:
Spin-magnon and photon-magnon interactions a 2D material/FM insulator interfaces
Magnons have gained substantial interest in quantum information science (QIS) as a means of coherently coupling spin qubits over a large distance. Substantial effort has been made on using the quantum state of Nitrogen-vacancy (NV) spins to excite magnons in FM insulators such as YIG. Here I propose using vdW heterostructures such as TMDs and graphene as a new platform for coupling spins and photons with magnons in FM insulators, which has substantial advantages over NVs in terms of control, tunability, and device integration. Building on the spin transparency of vdW interfaces established in our prior work (Nano Lett. 17, 3877–3883 (2017)), we will develop near-field imaging to probe magnon thermalization and inter-mode scattering, excitation, and absorption processes, and also domain-wall deformations in emerging FM and AFM insulators such as TmIG and 2D magnets. This effort will provide the key scientific foundation to unlock magnons for spin-based information processing.
Electric control of spin and valley currents and proximity spin-orbit interaction
Exciting advances are being made using electrical fields and proximity effects to generate efficient new mechanisms for controlling the intrinsic properties of 2D materials1. For example, our prior work (PRL 121, 127703 (2018)) demonstrated that bilayer graphene under an external vertical electric field exhibits an out-of-plane (OOP) spin-orbit field which changes the spin-lifetime anisotropy (ratio of OOP spin lifetime over in-plane) from zero up to ~12, despite the fact that graphene by itself has a weak intrinsic spin-orbit coupling. Our recent demonstration of ferromagnetism at the monolayer limit6, together with our demonstrated efforts of controlling spins in non-magnetic 2D systems, will position our group uniquely to explore many exciting opportunities such as voltage-controlled magnetic switching, spin field-effect transistors, tunable magneto-tunnel junctions, and magnetic proximity effects.
References: [1] Žutić, I., Matos-Abiague, A., Scharf, B., Dery, H. & Belashchenko, K. Proximitized materials. Mater. Today 22, 85–107 (2019). [2] Wolf, S. A., Awschalom, D. D., Buhrman, R. A., Daughton, J. M., Molnár, S. von, Roukes, M. L., Chtchelkanova, A. Y. & Treger, D. M. Spintronics: a spin-based electronics vision for the future. Science 294, 1488–1495 (2001). [3] Han, W., Kawakami, R. K., Gmitra, M. & Fabian, J. Graphene spintronics. Nature Nanotechnol. 9, 794–807 (2014). [4] Mak, K. F., He, K., Shan, J. & Heinz, T. F. Control of valley polarization in monolayer MoS2 by optical helicity. Nat. Nanotechnol. 7, 494–498 (2012). [5] Mak, K. F., Shan, J. & Ralph, D. C. Probing and controlling magnetic states in 2D layered magnetic materials. Nat. Rev. Phys. 1, 646–661 (2019). [6] O’Hara, D. J., Zhu, T., Trout, A. H., Ahmed, A. S., Luo, Y. K., Lee, C. H., Brenner, M. R., Rajan, S., Gupta, J. A., McComb, D. W. & Kawakami, R. K. Room temperature intrinsic ferromagnetism in epitaxial manganese selenide films in the monolayer limit. Nano Lett. 18, 3125–3131 (2018).