Session D40: Invited Session: Graphene Spintronics and Magnetism

Spin transport in epitaxial graphene

Spintronics is a paradigm focusing on spin as the information vector in fast and ultra-low-power non volatile devices such as the new STT-MRAM. Beyond its widely distributed application in data storage it aims at providing more complex architectures and a powerful beyond CMOS solution for information processing. The recent discovery of graphene has opened novel exciting opportunities in terms of functionalities and performances for spintronics devices. We will present experimental results allowing us to assess the potential of graphene for spintronics. We will show that unprecedented highly efficient spin information transport can occur in epitaxial graphene leading to large spin signals and macroscopic spin diffusion lengths ($\sim$ 100 microns), a key enabler for the advent of envisioned beyond-CMOS spin-based logic architectures. We will also show that how the device behavior is well explained within the framework of the Valet-Fert drift-diffusion equations [1]. Furthermore, we will show that a thin graphene passivation layer can prevent the oxidation of a ferromagnet, enabling its use in novel humide/ambient low-cost processes for spintronics devices, while keeping its highly surface sensitive spin current polarizer/analyzer behavior and adding new enhanced spin filtering property [2]. These different experiments unveil promising uses of graphene for spintronics.\\[4pt] In collaboration with B. Dlubak, Unite Mixte de Physique CNRS/Thales, Palaiseau, France \& Universite Paris-Sud, Orsay, France and University of Cambridge; M.-B. Martin, H. Yang, Unite Mixte de Physique CNRS/Thales and Universite Paris-Sud; R. Weatherup, University of Cambridge; M. Sprinkle, GeorgiaTech, Atlanta/Institut Neel; B. Servet, S. Xavier, Unite Mixte de Physique CNRS/Thales and Universite Paris-Sud; C. Berger, W. de Heer, GeorgiaTech, Atlanta/Institut Neel; S. Hoffman, J. Robertson, University of Cambridge; and C. Deranlot, R. Mattana, H. Jaffres, A. Anane, F. Petroff, P. Seneor, A. Fert, Unite Mixte de Physique CNRS/Thales and Universite Paris-Sud.\\[4pt] [1] B. Dlubak et al., Nature Physics 8, 557 (2012); P. Seneor, et al., MRS Bulletin 37, 1245 (2012).\\[0pt] [2] B. Dlubak et al., ACS Nano 6, 10930 (2012); R. Weatherup, et al., ACS Nano 6, 9996 (2012)   

Graphene magnetism due to point defects

Much interest has been generated on intrinsic magnetism in materials without d or f electrons. This is especially true for carbon-based materials and, in particular, graphene. Many theoretical studies have predicted that point defects in graphene should carry a magnetic moment and these can in principle couple either ferromagnetically or antiferromagnetically. However, the experimental evidence for such magnetism remains both scarce and controversial. In this talk we will review our recent experimental results on graphite and graphene where we show that pure graphite exhibits no ferromagnetism or anti-ferromagnetism down to liquid helium temperatures. We will also show that point defects in graphene produced by; (i) fluorine adatoms in concentrations, x, gradually increasing to full stoichiometric fluorographene CFx (x$=$1.0) and (ii) irradiation defects (vacancies) -- carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties, despite the fact that the maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. Our work clarifies the controversial issue of graphene's magnetism and opens the way to novel devices making use of its intrinsic magnetism. We also discuss how the magnetic properties of graphene can be changed by electric fields.   

Colossal Enhancement of Spin-Orbit Coupling in Hydrogenated Graphene

Graphene's extremely small intrinsic spin-orbit (SO) interaction makes the realization of many interesting phenomena such as topological states and the spin Hall Effect (SHE) practically impossible. Recently, it was predicted that the introduction of adatoms in graphene would enhance the SO interaction by the conversion of sp2 to sp3 bonds [1]. However, introducing adatoms and yet keeping graphene metallic, i.e., without creating electronic (Anderson) localization8 is experimentally challenging. Here, we show that the controlled addition of small amounts of covalently bonded hydrogen atoms is sufficient to induce a colossal enhancement of the SO interaction by three orders of magnitude. This results in a SHE at zero external magnetic field even at room temperature, with non-local spin signals up to 100 $\Omega$. The SHE is, further, directly confirmed by the Larmor spin-precession measurements. From this and the length dependence of the non-local signal we extract a spin relaxation length $\sim$ 1 $\mu $m, a spin relaxation time $\sim$ 90 ps and a SO strength of 2.5 meV [2].\\[4pt] [1] Castro Neto, A. H. {\&} Guinea, F. Impurity-induced spin-orbit coupling in graphene. Phys. Rev. Lett. (2009). \\[0pt] [2] Balakrishnan, J., Koon, G. K. W., Jaiswal, M., Castro Neto, A. H., \"{O}zyilmaz, B.; Colossal Enhancement of Spin-Orbit Coupling in Weakly Hydrogenated Graphene; Nature Physics (2013).   

Defect Induced Magnetic Moments in Graphene

We utilize non-local spin transport measurements to detect the presence of defect induced magnetic moments in graphene. As shown in this talk, point defects such as hydrogen adatoms and lattice vacancies generate magnetic moments in graphene that have substantial exchange coupling with the conduction electrons. Therefore, this exchange coupling produces spin relaxation in the conduction electrons. Specifically, a characteristic field dependence of the non-local spin transport signal identifies the presence of the magnetic moments. Furthermore, Hanle spin precession measurements indicate the presence of an exchange field generated by the magnetic moments. The entire experiment including spin transport is performed in an ultrahigh vacuum chamber, and the characteristic signatures of magnetic moment formation appear only after hydrogen adatoms or lattice vacancies are introduced.   

Graphene Spintronics: The Current State of the Art

Graphene has great potential for spintronics and new spintronics applications. Room temperature spin relaxation lengths of 10 micrometer or more have already been achieved, allowing electron spins to be transported (and manipulated) over large distances. However, the basic spin relaxation mechanisms which control spin transport in graphene are still not understood. In this talk I will give an overview of the experimental state of the art, and discuss the role of the various spin relaxation mechanisms in graphene. I will compare results obtained from different graphene systems, ranging from suspended graphene [1], graphene sandwiched between boron nitride layers [2], and epitaxial graphene [3]. I will discuss recent experiments where the spin relaxation is studied in devices with both top and bottom gate devices, which allow the carrier density as well as the electric field to be controlled independently[4]. Finally, I will address the intriguing observation that localized states present in the buffer layer of epitaxial graphene can dramatically change the spin transport parameters [5]. The possible relation with recently observed room temperature ferromagnetism in these systems may make it possible to make ``all-graphene'' spintronics devices.\\[4pt] [1] M. H. D. Guimaraes et al. Nano Lett. 12 (7) 3512 (2012)\\[0pt] [2] P.J. Zomer et al., Phys. Rev. B86, 161416 (2012)\\[0pt] [3] T. Maassen et al., Nano Lett. 12 (3), 1498 (2012)\\[0pt] [4] M.H.D. Guimaraes et al., unpublished\\[0pt] [5] T. Maassen et al., Phys. Rev. Lett. 100, 067209 (2013)