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Large Magnetization at Carbon Surfaces Print
Wednesday, 31 August 2011 00:00

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.

Doping Graphene

Graphene has attracted a lot of interest since researchers found an easy way to fabricate thin graphene layers, an effort that was recently rewarded with a Nobel Prize in Physics. Since carbon-based nanostructures like fullerenes (carbon cages), graphene, and nanotubes are easy to fabricate by reproducible methods and are often biocompatible, combining them with magnetism could create nontoxic magnetic nanoparticles used for drug delivery. Looking at the effects of hydrogen doping on graphite, researchers believe that thin magnetic layers of carbon have the potential to improve the delivery of drugs to desired locations within the body.

For the longest time, the occurrence of magnetism was basically restricted to elements like Fe, Ni, or Co and their alloys. Today the entire periodic table is mixed together to produce magnetic systems that have complicated electronic structures. Magnetic carbon provides a playground in which scientists can better understand magnetism in general. Graphite and other carbon-based nanostructures are comparably simple and fairly well understood. By understanding how and why carbon is made to be magnetic, researchers can learn more about magnetism in general.

Proton irradiation of graphite leads to small distortions of the graphite lattice and to incorporation of hydrogen atoms altogether, establishing ferromagnetic order in carbon.

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.

Magnetic hysteresis loops of a proton-irradiated sample. The two loops were acquired using a conventional SQUID (superconducting quantum interference device) magnetometer and a surface-sensitive x-ray absorption technique, called XMCD (x-ray magnetic circular dichroism). The latter was obtained such that the magnetization at the surface induced by hydrogen absorption is detected. The two measurements indicate that the surface exhibits a large magnetic moment that is similar to that of, for example, Ni metal (~55 emu/g).

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).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES), and the German Research Foundation. Operation of the ALS is supported by DOE BES.

Publication about this research: H. Ohldag, P. Esquinazi, E. Arenholz, D. Spemann, M. Rothermel, A. Setzer, and T. Butz, "The role of hydrogen in room-temperature ferromagnetism at graphite surfaces," New J. Phys. 12, 123012 (2011).

ALS Science Highlight #234

 

ALSNews Vol. 323