Graphene

Graphene is interesting for basic science, because it has a highly atypical band structure. It has raised notions like massless Dirac fermions, pseudospin, Berry phase, Klein paradox, Zitterbewegung and the spectacular manifestation of the integer and fractional quantum Hall effects.

But beyond the beauty for the basic research, graphene is even more interesting for applications. It is considered as a promising building block for prospective electronic technologies among which is spintronics. This is partially based on the high mobility of charge carriers and on a low spin-orbit coupling which is synonymous with a long spin lifetime.  The goal of our research is to study the magnetic properties of graphene, and to extract the spin relaxation time by Electron Spin Resonance (ESR) measurements. It turns out that it is not so easy to pin down this quantity, because it depends on the environment of graphene, on the substrate by which it is supported, and last but not least, on the production method of it. One of the non-negligible goals of our work is to identify which production route of graphene gives the appropriate quality for spintronic applications and to find the intrinsic spin life-time of a real material.

ESR study of graphene

ESR is an excellent method to perform the quality control of graphene produced by various methods. For the best samples (where a Pauli contribution is observed and very little of defects are present) one can measure the spin relaxation time, important parameter for spintronic applications.

Representative images of graphene samples studied in the group and were produced by various methods: a) Mechanically exfoliated graphene [K. S. Novoselov et al., Proc. Natl. Acad. Sci. (2005); b) Reduced graphene oxide [C. Gomez-Navarro et al., Nano Lett., (2010)]; c) CVD grown graphene [A. Reina et al., Nano Res., (2009)]; d) Epitaxially grown on SiC [S. Sonde, et al., Phys. Rev. B, (2009)]. The images represent surfaces is in the 100 x 100 µm2 range.

The major results are the following:

The intrinsic spin relaxation time measured on the mechanically exfoliated graphene is close to 10 ns – promising for applications. This value is much longer than any other reported in the literature measured in a spin valve configuration.

Apart from the mechanically exfoliated graphene sample, no other methods give an acceptable structural/electronic quality for applications in electronics. In many samples even a moderate quantity of defects creates Anderson localization of the electronic states. No ferromagnetism was detected in any of the graphene samples.

References:
[1] Luka Ciric, Electron Spin Resonance Study of Graphene,.PhD dissertation EPFL (2011).
[2] L. Ciric et al., Size dependence of the magnetic response of graphite oxide and graphene flakes – an electron spin resonance study, Physica Status Solidi B-Basic Solid State Physics, 247, 2958-2961 (2010).
[3] L. Ciric et al., Towards electron spin resonance of mechanically exfoliated graphene, Physica Status Solidi B-Basic Solid State Physics, 246, 2558-2561(2009).
[4] L. Ciric et al., Defects and localization in chemically-derived graphene monolayers, submitted to Phys. Rev. B