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Spintronics with Graphene

by Sebastian Ringer (08.06.2016)

Graphene has been proposed as a good material for spintronics because of the predicted long spin relaxation time and high charge carrier mobility. However, experiments could not verify the long spin relaxation time, which lead to the search for the correct theory to explain the increased spin scattering in real samples. The goal of this work is to measure the anisotropy of the spin relaxation time in graphene, meaning the difference between the in-plane and out-of-plane component, to identify the spin scattering mechanism. This is achieved by doing non-local Hanle measurements with different orientations of the applied magnetic field. 

The development of better computer chips based on silicon through miniaturization is becoming increasingly challenging. Structures are already small enough that in the not too distant future of 10 years fundamental obstacles like the tunnel effect will make further miniaturization impossible. Because of that, research effort is directed at finding new materials for semiconductor electronics as well as finding entirely new ways to build logic circuits. One approach is to utilize the spin of the conducting electrons, which is named spintronics.

In semiconductors like silicon, a current will lose its spin information after a few hundred nanometers. To make spintronics a success, a material is needed that can transport a spin polarized current over longer distances. Graphene is such a candidate, as the low atomic number of carbon results in a weak spin orbit coupling [1]. The mechanics of spin transport in graphene is not entirely clear so far. The question is what impact the substrate [2], impurities [3], defects and the contacts [4] have on the spin lifetime. The aim of this work is to produce experimental data that helps to solve this problem.

Fig. 1: a)[Bildunterschrift / Subline]: Fig. 1: a) Non-local Hanle measurement with a magnetic field pointing in z direction, the electron spins rotate in the graphene plane.
Fig 1: b)[Bildunterschrift / Subline]: Fig 1: b) Non-local Hanle measurement with a magnetic field pointing in y direction, the electron spins rotate out of the graphene plane.

A lateral spin valve is used to create a pure spin current in graphene, as shown in fig. 1. Two magnetic contacts made of cobalt (orange and blue) are placed on a graphene flake (grey). An electrical current I in the injector circuit creates a spin polarization under the orange contact, the direction of the magnetization controlling the orientation of the spin. Diffusion of electrons extends the spin polarization to the detector contact where the spin sensitive nature of a magnetic contact creates a voltage Unl in the detector circuit.

A magnetic field is used to manipulate the spins diffusing from the injector to the detector, which is then called a non-local Hanle measurement. The spins rotate around the magnetic field (Larmor precession) and the resulting data allows to extract the spin lifetime. For a magnetic field Bz in the z direction (see fig. 1a)), the spins rotate in-plane and only the in-plane relaxation time is measured. For a magnetic field By in the y direction (see fig. 1b)), the spins rotate out-of-plane and the resulting spin relaxation time is a combination of in-plane and out-of-plane. By comparing Bz and By measurements, the out-of-plane contribution can be extracted.

By knowing the ratio of the in-plane to the out-of-plane spin relaxation time, conclusions can be drawn about the mechanism of the spin scattering. For example, for pristine graphene calculations predict that the spin lifetime for out-of-plane orientation is half as long as for in-plane orientation [5]. If no anisotropy for the spin lifetime can be found, this would be an indicator for spin scattering at the contacts.

[1] D. Huertas-Hernando, F. Guinea, and A. Brataas, Phys. Rev. B 74, 155426 (2006)
[2] P. J. Zomer, M. H. D. Guimarães, N. Tombros, and B. J. van Wees, Phys. Rev. B 86, 161416(R) (2012)
[3] A. G. Swartz, J. Chen, K. M. McCreary, P. M. Odenthal, W. Han, and R. K. Kawakami, Phys. Rev. B 87, 075455 (2013)
[4] F. Volmer, M. Drögeler, E. Maynicke, N. von den Driesch, M. L. Boschen, G. Güntherodt, C. Stampfer, and B. Beschoten, Phys. Rev. B 90, 165403 (2014)
[5] J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano and I. Zutic, Acta Phys. Slov. 57, S. 565 (2007)

Sebastian Ringer

Wissenschaftlicher Werdegang
  • seit 2011
  • Promotion bei Prof. Dieter Weiss, Universität Regensburg
  • 2004- 2009
  • Studium der Physik, Universität Regensburg
  • 2003 - 2004
  • Studium der Physik, Technische Universität München

  • S. Ringer, M. Vieth, L. Bär, M. Rührig, and G. Bayreuther, Conductance anomalies of CoFeB/MgO/CoFeB magnetic tunnel junctions, Phys. Rev. B 90, 174401 (2014)