|First Proof of Ferromagnetic Carbon|
Although it has long been suspected that carbon belongs on the short list of materials that can be magnetic at room temperature, attempts to prove that pure carbon can be magnetized have remained unconvincing. However, using a proton beam and an advanced x-ray microscope at the Advanced Light Source, a multinational team of researchers from the SSRL, the University of Leipzig, and the ALS finally put to rest doubts about the existence of magnetic carbon.
Ferromagnetism is an “ordering phenomenon” in which the spins of neighboring electrons are coupled together such that they point in the same direction. If the temperature of the sample is elevated above a certain point, called the “Curie-temperature,” however, the disorder caused by the thermal motion of the atoms takes over and destroys the magnetic ordering. In fact, many different materials show ferromagnetic behavior at low temperatures, below 5 Kelvin for example, but only iron, cobalt, nickel and some alloys are useful ferromagnets above room temperature and can be manufactured in large quantities. The key challenge of showing that a clean carbon sample can exhibit ferromagnetism has thus lingered.
A particularly promising approach to making carbon magnetic emerged from a group led by Pablo Esquinazi at the University of Leipzig, Germany, in 2003. They irradiated clean carbon films with an intense proton beam focused to a tiny spot of 2 mm diameter. The proton irradiation caused small distortions in the carbon lattice, which in turn caused electron spins on neighboring atoms to align parallel and order ferromagnetically. The SSRL and ALS researchers collaborated with the Leipzig group and built upon this approach, studying proton-irradiated samples using scanning transmission x-ray microscopy (STXM) at ALS Beamline 11.0.2. Their studies revealed the carbon sample’s intrinsic magnetism.
The STXM microscope addresses the magnetic properties of different elements in a sample by using x-ray magnetic circular dichroism (XMCD) in x-ray absorption (XAS). In STXM, an incident x-ray beam is focused on the sample by a “zoneplate” lens, and the intensity of the transmitted x rays is measured on the detector. The sample is simultaneously scanned perpendicular to the beam, ultimately yielding a full field-of-view image. The absorption of x rays is strongly enhanced when their energy is chosen to excite a core-level electron into an empty valence state. These core-level resonances appear at characteristic photon energies for different elements, revealing information about element distribution in an unknown sample. In addition to elemental specificity, the transmission of circular polarized x rays at the resonance depends on the presence and direction of a ferromagnetic moment (XMCD). It is therefore possible to obtain information about the magnetism of the sample as well. A thin sample of carbon (t = 200 nm) was irradiated with a focused proton beam, leaving behind a magnetic ring. The images acquired using STXM at the carbon, iron, cobalt, and nickel resonances revealed that the magnetic ring only appears at the carbon resonance and not the others. The detected magnetic signal was very small, so only the use of a modern scanning transmission x-ray microscope at a state-of-the-art x-ray source providing x-ray beams of high brilliance with variable polarization made it possible to observe these tiny effects. These results underline the crucial importance of modern x-ray science and instruments in basic research.
Harnessing the magnetic properties of carbon could one day revolutionize a range of fields from nanotechnology to electronics. Magnetic carbon nanodevices could be built one atom at a time, leading to miniaturized machines and lightweight electronics. Magnetism, which forms the basis of information storage and processing in computer hard drives, could be employed in novel ways in tomorrow's electronic devices.
Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES); the German Research Foundation (DFG); and the European Union.
Publication about this research: H. Ohldag, T. Tyliszczak, R. Höhne, D. Spemann, P. Esquinazi, M. Ungureanu, and T. Butz, "π-Electron ferromagnetism in metal-free carbon probed by soft x-ray dichroism," Phys. Rev. Lett. 98, 187204 (2007).