PORTLAND, Ore.—Pure sheets of carbon—graphene—can be made magnetic by introducing patterns of vacancies into their crystalline lattice, according to researchers at the University of Maryland. By controlling the magnetic properties of graphene semiconductors with vacancy-doping, the researchers hope to enable the pure carbon material to tackle new applications as magnetic sensors and random access memories (MRAMs).
Semiconductors are usually made magnetic by doping with a metallic material such as iron or cobalt, but the University of Maryland team, led by professor Michael Fuhrer, claim that just introducing empty spaces into graphene's otherwise perfect hexagonal pattern—called vacancies—can dope the material for magnetism. Others have used surface treatments to make graphene magnetic, but the new method is said to work better by virtue of eliminating the need for any other material except carbon.
Fuhrer's Lab was one of the first to characterize the carrier mobility in graphene as being more than 10-times higher than silicon (15,000- compared to 1,400-cm2/Vs, square centimeters per volt second). Now the team claims that their newest characterization attempts for the first time explain how magnetic properties can also be introduced into graphene—namely by adding vacancy defects to its crystalline lattice.
Semiconductor defects are usually caused by doping, which in this case are vacancies instead of a different material, each of which acts like a nanoscale magnet with its own "moment." The researchers demonstrated that these vacancy defects strongly interacted with any electrical currents in the material, potentially making is semiconducting properties tunable by virtue of the Kondo effect. The researchers measured the temperature of the Kondo effect in graphene with vacancies and found it to be about the same as in metals with electron densities much higher that un-doped graphene—about 90 degrees Kelvin.
Artist's rendering of a graphene transistor showing gold electrodes (yellow), silicon dioxide (clear), silicon substrate (black) and graphene (red), with inset showing graphene lattice vacancy defects (blue). Source: University of Maryland
Next the researchers are attempting to arrange the vacancies in a pattern that could exhibit ferromagnetism by forcing all the magnetic moments in a domain of vacancies to line up by virtue of the Kondo effect, potentially allowing them to be electrically switched to make pure carbon magnetic memories and sensors.
Funding was provided by the National Science Foundation and the Office of Naval Research.