Unconventional magnetic properties of cobalt films in contact with graphene

Owing to its peculiar electronic band structure, high carrier mobility and long spin diffusion length, graphene is a promising two-dimensional material for microelectronics and spintronics. Graphene also shows interesting magnetic properties when in contact with a ferromagnetic metal. For instance, graphene carries a net magnetic moment when deposited on Fe/Ni(111), and a significant spin splitting can be induced in graphene due to proximity with a heavy element.

While these results illustrate potential advantages of integrating graphene within a magnetic stack, the influence of graphene on the magnetic properties of a ferromagnetic metal is still largely unexplored. In particular, non-magnetic overlayers generally affect the magnetic anisotropy energy of thin layers, where interfaces play an important role. We can then wonder how an interface with graphene would influence the magnetic anisotropy of a thin ferromagnetic film.

Using spin-polarized low-energy electron microscopy available at the Lawrence Berkeley National Laboratory in California, we studied how a graphene overlayer affects the magnetic properties of atomically flat, nanometer-thick cobalt films grown on a (111) iridium single crystal. We found several astonishing magnetic properties of graphene-covered cobalt films : 1) Perpendicular magnetic anisotropy is favored over an unusually large thickness range, well above the values reported so far in the literature, 2) Vectorial magnetic imaging reveals an extraordinarily gradual thickness-dependent spin reorientation transition, in sharp contrast with what is usually found in cobalt thin films, 3) During the spin reorientation transition, cobalt films are characterized by an unconventional spin texture, similar to a wheat field blowing in the wind (see Figure), 4) Spectroscopy measurements indicate that incident spin-polarized electrons do not suffer substantial spin-dependent collisions a few electron-Volts above the vacuum level, as if the cobalt films were not ferromagnetic anymore. These properties strikingly differ from those of pristine cobalt films and could open new prospects in surface magnetism and spintronics.

Figure:Figure : Three-dimensional, pixel by pixel representation of the magnetization vector (arrows) in the case of a 16 monolayer-thick cobalt film in contact with graphene. Black and white contrasts give the out-of-plane component of the magnetization. Field of view is 3.2 × 2.4 μm2.
  • Institut Néel (France) : A. D. Vu, J. Coraux, N. Rougemaille
  • Lawrence Berkeley National Laboratory (USA) : G. Chen, A. T. N’Diaye, A. K. Schmid
  • Laboratoire Spintec (France) : H. Yang, A. Hallal, M. Chshiev

Our related publication :

  • Unconventional magnetisation texture in graphene/cobalt hybrids, A. D. Vu, J. Coraux, G. Chen, A. T. N’Diaye, A. K. Schmid and N. Rougemaille, Scientific Reports 6, 24783 (2016).[http://www.nature.com/articles/srep24783]
  • Anatomy and Giant Enhancement of the Perpendicular Magnetic Anisotropy of Cobalt-Graphene Heterostructures, H. Yang, A. D. Vu, A. Hallal, N. Rougemaille, J. Coraux, G. Chen, A. K. Schmid and M. Chshiev, Nano Letters 16, 145 (2016). [http://pubs.acs.org/doi/abs/10.1021...]
  • Perpendicular magnetic anisotropy of cobalt films intercalated under graphene, N. Rougemaille, A. T. N’Diaye, J. Coraux, C. Vo-Van, O. Fruchart and A. K. Schmid, Applied Physics Letters 101, 142403 (2012).[http://scitation.aip.org/content/ai...]
  • Air-Protected Epitaxial Graphene/Ferromagnet Hybrids Prepared by Chemical Vapor Deposition and Intercalation, J. Coraux, A. T. N’Diaye, N. Rougemaille, C. Vo-Van, A. Kimouche, H. Yang, M. Chshiev, N. Bendiab, O. Fruchart and A. K. Schmid, Journal of Physical Chemistry Letters 3, 2059 (2012). [http://pubs.acs.org/doi/abs/10.1021...]

Corresponding author  : J. Coraux N. Rougemaille

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