Magnets have always been associated with metals or metallic structures. Since the beginning of science, nobody proved anything else to be magnetic than some iron composites, be them natural or man-made.
A team of physicists from the Autonomous University along with Francisco Guinea from the Institute of Materials Science from Madrid, Spain, have discovered that by removing individual atoms from a graphite surface, local magnetic moments can be created in the graphite.
The first application is on graphene, a relatively new discovered material that is made of only one sheet of carbon atoms. Graphene is planned to be used in various nanoelectronic devices, and proving it can be magnetic would make tremendous waves in the electronics industry.
“It is a pressing challenge of nanotechnology to be able to integrate graphene in real electronic devices,” Ivan Brihuega, one of the researchers, told PhysOrg.com. “To this end, it is mandatory to understand how the presence of single atomic defects modifies its properties. In our work, we use a scanning tunneling microscope in ultra-clean environments to address such a fundamental question for a graphene-like system, a graphite surface. Our main result is our capability to examine at the atomic scale the intrinsic impact that each single carbon atom removed from the surface has in the electronic and magnetic properties of the system.”
The researchers used highly ordered pyrolytic graphite in their experiments, consisting of a graphene sheet slightly shifted with respect to the upper layer in such a way that half of the carbon atoms of the upper sheet have a carbon atom located exactly below them, while the other half do not (called the “AB-AB” stacking sequence).
By creating atomic holes in substances like graphene the researchers created a strong impact on its mechanical, electronic and magnetic properties of the material. Other older studies on this field only investigated the effects of atomic vacancies on the properties of the material as a whole, while this study probed the effects on each individual atomic vacancy.
The atomic vacancies were created by applying low-energy ion irradiation, using just enough energy to remove the surface atoms and produce atomic point imperfections. Then, using a low-temperature scanning tunneling microscope, the researchers identified the presence of a sharp resonance peak on above the top of individual atomic holes. The resonance peaked around the Fermi level, predicted in many theoretical studies, but never experimentally observed before.
“In a pristine carbon system, one would never expect to find magnetism because of the tendency of its electrons to couple in pairs by forming covalent bonds,” Brihuega explained. “The association of electrons in pairs runs against the existence of a net magnetic moment, since the total spin of the electronic bond will be zero. By removing one carbon atom from the graphite surface, what we do precisely is to break these covalent bonds and as a result we create a localized state with a single unpaired electron that will generate a magnetic moment.”
“To create a magnet from a pure carbon system is a tantalizing possibility since this would be a metal-free magnet and thus optimal for applications in biomedicine,” Brihuega said. “In addition, it should be much cheaper to produce than conventional magnets since, to give some numbers, a ton of carbon costs around a thousand times less than a ton of nickel ($16 vs. $16,000), a commonly used material in actual magnets. In the case of graphene systems, one would also have flexibility and lightness as additional advantages; but to date, the total magnetization reported for these systems is very low when compared with the strongest existing magnets.
“In my opinion,” he added, “the brightest future in terms of applications stems in the emerging field of spintronics, i.e. in trying to exploit the ‘spin’ of the unpaired electron for creating new spin-based devices.”
Creating metal-free magnets would also benefit the electric cars industry, since electric cars use electric motors, that have magnets inside. Being made out of carbon, future electric motors would be several orders of magnitude cheaper and lighter.