A one-atom thick carbon film with a hexagonal structure, graphene is one of the hopes for the development of a new form of electronics called spintronics, which might lead to quantum computers, even smaller and faster. In this new world, magnetic information would not be transmitted only by the electric current, as is the case with the current microcomputers, but fundamentally by another property of electrons, namely, spin. As there are only two possible spin values, this state of electrons might prove to be useful to store and propagate data in the form of bits. However, the signal generated by the spin current is extremely weak and tends to propagate in all directions, two characteristics that make its control and detection difficult. According to the recent work of Brazilian theoretical physicists, these drawbacks can apparently be circumvented with graphene, a candidate for taking over the position of silicon. Its electrons’ spin can be magnified and controlled with a mechanism that works like a lens, which might mean this material could be used as a quantum nanotransistor.
“We’ve proven mathematically that graphene can work like a lens and redirect the spin current from a magnetic source to a given region where the receptor unit is to be found,” says the Brazilian physicist Mauro Ferreira, from Trinity College, Dublin, who took part in the study, published in the May issue of the Journal of Physics: Condensed Matter, along with colleagues from the Federal Fluminense University (UFF). “In this way, part of the information that would be lost can be recovered.” None of this has actually been done in a laboratory, but only outlined in theoretical work. After a series of calculations, the researchers state that graphene, which is tougher than steel and a better electricity conductor than copper, might behave as a spin transistor under certain circumstances. The article is the third by the group of physicists that explores, theoretically, the possibilities of using carbon and graphene nanotubes in spintronics. The two prior studies were published last year in Physical Review B.
To transform the graphene spin into a milieu capable of transmitting information in a quantum system, the Brazilians worked with a fairly particular set of circumstances. The creation of a spin current was simulated by inserting a magnetic object in the atomic architecture, shaped like a beehive, of graphene, comprised only of carbons. “Imagine a small magnet in a rotational movement on a sheet of graphene,” compares Ferreira. The presence of this alien object would cause the spin of the carbon electrons to vibrate successively in a similar manner. The vibration of the spin of a given electron would thus be transferred to its neighbor and so forth. The problem is that a spin current disseminates itself, in an uncontrolled way, in all the directions of the graphene. “Like the waves created by a stone dropped in a lake, this current is weaker the further away it is from its origin,” says the researcher.
Small loss of energy
The next step of the simulation consisted in dividing the graphene film into two parts and changing the density of the electric charge in one of them. The procedure would generate in this graphene segment an entryway potential, a path to which the spin current would turn and by means of which it would disseminate itself through the material. “The spin current does not dissipate heat in graphene and the laws of energy in such a system would be minimal. A device running on this current would use very little power,” states the physicist Roberto Bechara Muniz, from UFF, another of the work’s authors. Besides channeling the spin current to a specific region of the graphene and thereby amplifying its signal, the creation of the entryway would work like a key to switch the transistor on and off. It would enable one to block or to allow the flow of the spin current. “Our work makes only a small contribution to this issue, but it shows that it is possible to control the spin current in graphene,” says Muniz. A spintronics expert, José Carlos Egues from the Physics Institute of São Carlos, of the University of São Paulo, who did not take part in the work of Ferreira and Muniz, considers these results interesting, but says that they are still very preliminary. “Further studies are required to explore the viability of the proposal and its relevance for spintronic applications,” comments Egues.
For the sake of explanation, spin is the movement of an electron when it rotates around its own axis, like a top. There are two forms of spin, one with an upward rotation and the other with a downward one. The phenomenon is actually more complex than this, as an electron can simultaneously exhibit the two types of spin. In practical terms, the development of a new form of electronics depends on acquiring a full understanding of the spin current, such as we now have of an electric current, and on having effective means to control conversion from one type of spin to the other. Physicists all over the world have tried to create spin current in semiconductors as well as in graphene, a two-dimensional crystal with a set of unique properties.
In an article published in the April 15, 2011 issue of the American journal Science, Andre Geim and Konstantin Novoselov, the University of Manchester physicists who won the Physics 2010 Nobel Prize for their work on graphene, have produced indications that this material might really be able to transmit a spin current. They applied an electrical field between two electrodes one millionth of a meter away from a sheet of this material and measured the voltage in a region that was 10 millionths of a meter away from the electrodes. When the graphene was exposed to a magnetic field, the voltage became higher. This fluctuation, according to the authors of the study, is evidence that there is a spin current going through the graphene.
GUIMARÃES, F.S.M. et al. Graphene as a non-magnetic spin current lens. Journal of Physics: Condensed Matter. v. 23, n. 17. 4 May 2011.