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First Proof of Ferromagnetic Carbon Print

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.

Carbon’s Magnetic Personality Attracts Attention

Most materials exhibit weak forms of magnetism—diamagnetism, which repels an external magnetic field, or paramagnetism, which attracts it. However, take away the external magnetic field, and the material is no longer magnetized. So-called real magnets like refrigerator and horseshoe magnets are ferromagnets. They’re stronger because of long-range ordering, which allows domains of aligned electrons to have a high degree of magnetization. Left on their own, these domains cancel each other out. But when even a small magnetic field is applied to a ferromagnetic material, these domains line up with each other, and the material becomes fully magnetized. In addition, once magnetized, it retains some of its magnetism.

Recent tests found ferromagnetic “readings” in carbon samples. In order to determine whether these findings were originating from the carbon itself or possible impurities, researchers used x-ray microscopy at the ALS to determine if the magnetism was coming from iron, nickel, cobalt, or carbon. They proved that the magnetism originated only from the carbon, and that it was sustained above room temperature, where only a few materials stay magnetized. These findings have given researchers a way to understand and control magnetism in nanodevices such as graphene sheets and carbon nanotubes.

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.

In a scanning transmission x-ray microscope (STXM), x rays are focused onto a sample via a zoneplate. The sample can be moved perpendicular to the x-ray beam, while at the same time the transmitted intensity is detected to produce a 2D map (image, far right) of the x-ray absorption cross section of the sample using computer software.

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.

A carbon film is hit by a high-energy proton beam, causing the magnetic moments of the atoms to align around the beam impact area, creating a ring-shaped magnetic pattern that can be imaged with a magnetic-force microscope (left). The x-ray microscope can then be used to "scan" the sample for magnetism associated with other elements. The absence of a ring pattern in scans for cobalt, nickel and iron prove that the sample contains only carbon (bottom right).




Research conducted by H. Ohldag (Stanford Synchrotron Radiation Laboratory); T. Tyliszczak (Advanced Light Source); and R. Höhne, D. Spemann, P. Esquinazi, M. Ungureanu, and T. Butz (Institut für Experimentelle Physik II, Universität Leipzig, Linnéstraße, Germany).

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).