| Solid State Physics |
 Graphite sample placed on a sample holder  Image obtained from a thermographic camera In addition to typical IBA techniques (PIXE, RBS, ERDA, etc.) for atomic elementary analysis of materials in a spatial scale ranging from nanometers to microns, ion-beam accelerators are also powerful tools for ionic implantation and modification of materials. When an energetic ion impinges on a solid target, important changes in the structure and properties of the solid are produced that can be used for different applications within the fields of physics and engineering, such as biochips or optical and electronic devices. Typical energy ranges employed in ion implantation are very wide, from a few tens of keV, obtained by small ionic sources, up to hundreds of MeV (produced by ion-beam accelerators) which use to be more interesting, though more expensive too, for scientific research. Our research team is currently employing these techniques to address a hot topic within Solid State Physics nowadays: the study of the reported possible magnetic behavior in graphite, graphene and other carbon-based materials. As is well known, graphite, the stable crystalline allotrope of carbon at room temperature and ambient pressure, is known to exhibit a strong and anisotropic "textbook" diamagnetism, due to its delocalized π electrons. Nevertheless, in the last two decades more or less clear evidences of ferromagnetic behavior in carbon at room temperature have been reported, especially the presence of ferromagnetic signals in proton-irradiated Highly-Oriented Pyrolitic Graphite (HOPG) reported [1] by a group at the University of Leipzig (Germany) and led by Prof. Pablo Esquinazi (who was on sabbatical at CMAM during 2007-08). Therefore, we have undertaken a joint research line to study this subject, by making use of the 5 MV tandem ion-accelerator at CMAM and collaborating with other research groups at the Campus of Universidad Autónoma de Madrid and of the University of Leipzig (Germany). At the same time of the ion implantation, the Particle-Induced X-ray Emission (PIXE) technique allows to determine in situ the amount of magnetic impurities in the sample, a crucial issue given the weakness of the reported ferromagnetic signals. The possible existence of ferromagnetism, or other magnetic contributions, in the samples is studied through highly accurate SQUID magnetometry and sometimes through Magnetic Force Microscopy (MFM). [2] We have exhaustively studied [3] the change in the magnetic properties produced on highly oriented pyrolytic graphite (HOPG) samples after irradiation of H-, C- and N- ions in the MeV energy range. The use of specially made sample holders for the magnetic measurements through a SQUID magnetometer provided high reproducibility, allowing us to obtain directly the irradiation effects without any corrections or subtractions. Our results show that three main magnetic phenomena are triggered by the defects produced by the irradiation, namely Curie-like paramagnetism, ferromagnetism and an anomalous paramagnetic state that appears as precursor of the magnetic ordered state. Direct measurements of the surface sample temperature during irradiation and the decrease in the paramagnetic as well as ferromagnetic contributions after irradiation indicate that self-heating effects are one of the causes for the small yield of ferromagnetism. Taking into account the role of hydrogen, our results [3] suggest that the induced ferromagnetism appears when the average vacancy distance is around 2nm in the near surface region. [1] P. Esquinazi, D. Spemann, R. Hohne, A. Setzer, K. H. Han, and T. Butz, Phys. Rev. Lett. 91, 227201 (2003). |
