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Graphene+X

Contact Aaron Sharpe (aaron.sharpe@) for more information.

 

Graphene is a remarkable atomically thin two-dimensional (2D) material that is, by some metrics, the most efficient electrical conductor ever discovered while also being tunable with electric fields. Being 2D, graphene is effectively all surface and no bulk. Therefore, graphene can “inherit” electronic properties of other 2D materials when placed in intimate contact. Since graphene’s discovery in 2004, researchers have developed techniques to make atomically clean interfaces between graphene and many other materials which opened up new avenues for research. Our work is a sort of alchemy by proximity, where we can create van der Waals interfaces by pairing graphene with other special 2D materials to form a hybrid electron system with new emergent functionalities.

Figure 1: Fabrication of van der Waals heterostructures of different 2D materials. Figure taken from Ref. 1

 

Graphene has high electron mobility and gate tunability, but does not have an electronic band gap or strong correlations (such as magnetism and spin-orbit coupling). Chalcogenide layered compounds have been extensively explored in recent years. They can preserve the high electronic quality of graphene sandwiched with them [2], and in some cases display correlated behavior [3, 4, 5].

One important electronic property that we can manipulate via proximity effects is electron spin, by combining graphene with a magnet. Most magnets are metals. If we were to pair graphene with a normal magnet, like iron, current would flow through the iron because it is also conducting. However, we can use one of a few known ferromagnetic insulators (FMI). In such a stack, the ferromagnetism is predicted to leak into the graphene, producing a spin split band structure, while preserving current flow through the graphene because the FMI is insulating. Theoretical estimates for exchange energy J of the splitting possible in graphene/FMI heterostructures range from 5 to 70 meV, depending on the FMI material chosen and the method used to estimate the coupling [6, 7, 8]. Particularly, we can use the 2D chalcogenide layered material, Cr2Ge2Te6 ­ (CGT). CGT is easy to cleave to create atomically smooth surfaces onto which we can stack graphene. The heterointerface of Bi2Te3/CGT was shown to have a significantly enhanced Curie temperature and the anomalous Hall effect [9], demonstrating a strong exchange coupling across the interface and supporting our idea that a strong magnetic polarization may be attainable in graphene on CGT.

 

Figure 2: Spin-split band structure of graphene/FMI interface

 

Proximity-induced ferromagnetism in graphene could offer an alternative to spin injection by ferromagnetic contacts: a region of proximitized graphene as the source injecting spin into an adjacent normal region. By injecting current from proximitized, ferromagnetic graphene, an injected current pulse will have a spin polarization equal to the net equilibrium polarization of the ferromagnet. Net polarization near 100% could in principle be achieved by tuning to the Dirac point for the minority carrier. Estimates of power consumption predict that our proposed device could operate at as little as one thousand times lower voltage and power, compared to the case where spin is injected into graphene from metallic ferromagnets. Additionally, recent work [10] quantitatively calculates graphene spin switch performance, predicting subnanosecond switching times with dissipation of ~fJ energy to switching loss per one flip of a non-volatile bit, which would be highly competitive with the state of the art in spin torque transfer magnetic random access memory. Competitive low-power spin switching using graphene could be the key mechanism which opens the door to novel all-spin architectures. Replacing conventional transistors with switches with sharper on/off switching could lower power consumption of computers.

 

[1] A. K. Geim et al., Nature 499, 419-425 (2013)

[2] A. V. Kretinin et al., Nano Lett 14, 3270-3276 (2014)

[3] J. M. Vandenberg-Voorhoeve, “Structural and Magnetic Properties of Layered Chalcogenides of the Transition elements,” Optical and Electrical Properties (1976)

[4] V. Carteaux et al., J. Phys.: Condens. Matter 7, 69-87 (1995)

[5] H. Ji et al. J. Appl. Phys. 114, 114907 (2013)

[6] H. Haugen et al. Phys. Rev. B 77, 115406 (2008)

[7] H. X. Yang et al., Phys. Rev. Lett. 110, 046603 (2013)

[8] Z. Qiao e tal., PRL 112, 116404 (2014)

[9] L. D. Alegria et al., Appl. Phys. Lett. 105, 053512 (2014)

[10] L. Su et al., Appl. Phys. Lett. 106, 072407 (2015)