|Large Magnetization at Carbon Surfaces|
From organic matter to pencil lead, carbon is a versatile element. Now, another use has been found: magnets. One would not expect pure carbon to be magnetic, but for more than ten years scientists have suspected that carbon can be made to be magnetic by doping it with nonmagnetic materials, changing its order ever so slightly. Years ago, the first x-ray images obtained using the scanning transmission x-ray microscope at ALS Beamline 11.0.2 provided valuable insight into how proton irradiation can cause carbon to transform into a ferromagnetic material. Now, researchers are using x-ray spectroscopy at ALS Beamline 4.0.2 to study the magnetism of proton-irradiated graphite surfaces in order to understand the effects of hydrogen (i.e. protons) on the electronic structure of carbon. In studying the properties of electrons responsible for magnetic order in graphite, researchers found that a very large magnetic moment is essentially switched on when hydrogen atoms are incorporated at the surface of graphite.
Pure carbon comes in two configurations. In the diamond configuration, each carbon atom is surrounded by four others, forming a very stable and "very hard" three-dimensional lattice. Alternatively, if each carbon atom is surrounded by three others, an atomically thin, two-dimensional sheet of carbon atoms called graphene forms. These sheets can be stacked on top of each other to produce a three-dimensional material called graphite.
Scientists agree that pure graphite cannot be ferromagnetic. Each carbon atom has six electrons, three of which exhibit a spin pointing up and the other three pointing down; consequently, the magnetic moment of a carbon atom is zero. It is a perfect "diamagnet," repelled by an external magnetic field. Over the past decade, however, research has indicated that proton irradiation (i.e. hydrogen doping) of carbon can lead to the formation of ferromagnetic order. Scientists hypothesized that hydrogen atoms were incorporated into the graphite lattice during hydrogen doping, distorting the lattice and allowing the spins to align with each other.
Recent magnetic x-ray absorption spectroscopy experiments using an x-ray magnetic circular dichroism (XMCD) technique at ALS Beamline 4.0.2 have corroborated this hypothesis. Researchers directly investigated the electron states in graphite that are responsible for magnetic order by doping graphite with protons. The experiments not only corroborated the existing hypothesis, they also revealed something unexpected.
Using different methods to detect x-ray absorption, researchers distinguished the magnetic properties of the surface from material's bulk magnetic properties, finding that the surface magnetization is much stronger than that of the bulk. The size of the magnetic moment at the surface can be very large—comparable to that of conventional magnetic metals like Fe, Ni, and Co—and the magnetism can exist even at room temperature. This makes carbon's magnetism an interesting natural effect with potential real-world applications if samples are thin enough.
Since carbon-based nanostructures can presently be produced very efficiently and reliably (nanotubes, graphene, bucky balls, and other fullerenes are all made of carbon), finding a way to manipulate nanosized carbon elements to become magnetic would open the door to a completely new class of magnetic devices for magnetic storage, sensors, and data processing. Fortunately, additional research has proven that etching carbon with sulfuric acid can also make the carbon magnetic, opening the door for those who wish to experiment but may lack access to a 2.25-MeV proton accelerator like the one used here, and providing a base for a magnetic carbon research community.
Research conducted by H. Ohldag (SLAC National Accelerator Laboratory); E. Arenholz (ALS); and P. Esquinazi, D. Spemann, M. Rothermel, A. Setzer, and T. Butz (University of Leipzig).
ALS Science Highlight #234