Sep 14, 2010
Hydrogen vacancies induce stable ferromagnetism in graphane
Graphane, created by hydrogenation on both sides of a single graphene layer, promises a bright future in the emerging field of carbon electronics due to its unusual electronic and magnetic properties. While in graphene there is a π delocalized network created by unsaturated p electrons present on each carbon atom, hydrogenation generates bonding of carbon atoms with atomic hydrogen that ties up those p electrons. This results in removal of the π bands from the band diagram of graphane thereby generating a wide gap. Importantly, introduction of spatially separated hydrogen vacancies in graphane would again generate π electrons, which will be strongly localized on the vacancy site and whose interaction with each other could lead to alignment of their spins generating ferromagnetism.
Influence of the H-vacancy defects on the electronic properties of graphane is in fact, predictable – they turn on spin polarization and generate impurity levels in its band gap. However, the effect of such vacancies on the magnetic properties of graphane is much more complicated. The localized states generated by the H-vacancies are characterized by the non-zero spin and depending on the interference of their spin-density tails (constructive or destructive) the spins would be aligned or have mixed ordering, respectively.
The ordering of spins is one way to control the magnetization of graphane and thus create spin-tunable devices, but there are several issues that need to be addressed before the devices could be made to work.
It has been found that when two localized states induced by the vacancies are created on the same side of graphane, the interference of spin-density tails is constructive and could lead to alignment of their spins. However, it was also found that there are some factors that destabilize the spin alignment. Due to strong localization of the spin-density of the localized states induced by the defects, the interference of their spin-density tails rapidly decreases with increasing distance between the states, thereby destabilizing the spin alignment (defined as the energy difference between states with aligned and mixed spins, E↑↑ – E↑↓).
Another factor is the vacancy concentration whose increase up to a certain point enhances stability (for four vacancies located on the nearest-neighboring carbon atoms E↑↑ – E↑↓=–3.32 eV), but any further increase of vacancy concentration actually leads to lowering of this stability.
In order to create a room-temperature ferromagnet based on graphane, the issues responsible for the stability of the spin alignment should be taken into account. The most obvious spintronics application of graphane distinguished by the non-equivalent concentration of the electrons of different spins would be generation of a spin-polarized current. Moreover, ferromagnetic graphane can also be used in a spin-filtering device, for which graphane can offer the efficient control of the spin-current flow because of significant spin polarization resulting in the generation of potential barriers of different heights for electrons of opposite spins.
Full details can be found in the journal Nanotechnology.
About the author
Dr Julia Berashevich is a research associate in the group of Dr Tapash Chakraborty, professor and Canada research chair at the Department of Physics and Astronomy, University of Manitoba, Canada. Her work is focused on the theoretical investigation of various nanoscale systems, particularly the electronic and magnetic properties of novel materials that are promising for applications in nanoscale electronics.