Nov 17, 2011
Graphene goes magnetic
Graphene can be made magnetic by simply forming honeycomb-like arrays of hydrogen-terminated nanopores on it. So say researchers in Japan who have observed room-temperature ferromagnetism in the graphene nanopore arrays, caused by, they believe, electron spins localized at the zigzag-shaped atomic-structured nanopore edges. The phenomenon, only predicted by theory until now, might help make magnets that are rare-element free, extremely light, transparent and flexible, as well as novel spintronics devices that exploit edge-polarized spins.
Graphene is a 2D layer of carbon just one atom thick. It is very strong and an excellent conductor of heat and electricity. As such it is often described as a "wonder material" with many possible technological applications from ultra-fast transistors to DNA sequencing.
"Graphene is made up of only carbon and does not contain any magnetic atoms like iron, nickel or chromium but graphene nanopore arrays show large-magnitude ferromagnetism even at room temperature," team member Junji Haruyama of Aoyama Gakuin University in Kanagawa told nanotechweb.org. Although the effect had been predicted by theory, methods to make graphene edges – that until now relied on lithography – invariably introduced a large amount of defects into the material. These hinder the formation of the pure zigzag edges needed for magnetism in graphene.
Avoiding too many defects
Haruyama and colleagues avoided lithography by developing a new method that exploits nanopore templates. The researchers began by placing a porous alumina template (PAT) on graphene and etching the carbon-based material using the PAT as an etching mask. The PAT is made up of a honeycomb-like array of hexagonal nanopores so the pore array is used to entirely transcript this shape to graphene. "Since this is a non-lithographic method, it introduces only minute amounts of defects to the graphene edges," explained Haruyama. "Moreover, the etching was performed using low-power argon gas, which also induces only a small number of defects."
After the nanopore array were formed, the team annealed the structures at 800 °C, first in vacuum and then under hydrogen. This step produces zigzag-shaped atomic-structured pore edges by so-called edge reconstruction, and the hydrogen terminations – predicted to be responsible for the magnetism in graphene. "A high density of the zigzag pores in the graphene nanopore arrays induce this ferromagnetism thanks to a large ensemble of zigzag edges," said Haruyama.
The researchers confirmed that the material was magnetic by conventional SQUID (superconducting quantum interference device) measurements.
Magnets and spintronics devices
According to Haruyama, the graphene arrays could be used as single-layer magnets that are rare-element free, extremely light, transparent and flexible. They might also be used to make novel spintronics devices that utilize edge-polarized spins.
The team, which includes researchers from SPINTEC at UJF Grenoble, France, would now like to make spin transistors using graphene as a magnetic semiconductor material by exploiting the quantum spin Hall and "spin rectification" effects. "We would do this by modulating the energy bands of the pore edges and control polarized spins just by applying voltage," revealed Haruyama.
The work was detailed in Applied Physics Letters. Further results on this research will soon be published in Physical Review Letters, says the team.
About the author
Belle Dumé is contributing editor at nanotechweb.org