Eur. Phys. J. B 40, 499–504 (2004) DOI: 10.1140/epjb/e2004-00286-6 T HE EUROPEAN P HYSICAL JOURNAL B Defects in graphite may be magnetic and magnetostrictive as revealed by scanning tunneling microscopy H. Wang, A.C. Papageorgopoulos a , and N. Garcia Laboratorio de Fisica de Sistemas Peque˜ nos y Nanotecnologia CSIC, Serrano 144, 28006 Madrid, Spain Received 12 February 2004 / Received in final form 25 May 2004 Published online 24 September 2004 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2004 Abstract. We use scanning tunneling microscopy to measure magnetic field induced strains in highly oriented pyrolytic graphite. This is done by using a scanning tunneling microscope with some magnetic components, which however do not produce an observable response within our resolution in the case of pure (99.999%) paramagnetic or diamagnetic metals (at the low field strengths applied). We study also ferromagnetic metals with this method for comparison. We find a relatively large (similar to that of permalloy) magnetostrictive response of graphite for the low applied field. The data shows saturation of the strain and also that the strain observed is localized and is not the cumulative strain from the mounted edge of the sample to the position of measurement, implying that volume is not conserved with the strains. We believe that the observed strains correspond to a signal of a ferromagnetic material and in this case may be due to the defects observed on the graphite planes. PACS. 68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM) – 75.80.+q Magnetomechanical and magnetoelectric effects, magnetostriction – 81.05.Uw Carbon, diamond, graphite In the last several years, the search for, and study of new magnetic materials has witnessed substantial growth. This is due to the technological importance of such materials as well as their interesting potential properties [1–3]. Ma- terials comprised of carbon are of special interest, such as polymerized fullerine [4] and highly oriented pyrolytic graphite (HOPG) [5]. From the purely scientific stand- point, ferromagnetism in systems containing exclusively p- and s-electrons, such as materials comprised of the light elements (C, H, N, O, S) is a topic that challenges cur- rent assumptions that such ferromagnetism is only possi- ble in materials containing metallic 3d and 4f elements. Poor reproducibility, insufficient characterization of the impurity concentration and the relatively low quality of graphite samples in the past, however, may have con- tributed to the conclusion that for magnetic polymers most of the claimed high-temperature ferromagnetism in magnetic ion-free compounds in the past ended up be- ing of extrinsic origin [6]. Despite the controversy, there has been some very interesting research done recently into the possible magnetic properties of HOPG. In particu- lar, there have been a variety of experiments conducted where ferromagnetic-like signals in HOPG that were not due to the presence of impurities were detected [5,7]. a e-mail: apapageo@fsp.csic.es Though the perfect graphene layer does not show ferro- magnetic instabilities [8], the electronic structure of the graphite π-system differs considerably for carbons within the graphite sheet as opposed to those at its edges [9,10]. Edge states may, therefore, lead to an increase in the den- sity of states at the Fermi level, and if such edge states occur at a high enough density, a ferromagnetic spin po- larization may result. Characterization techniques complementing those uti- lized so far can bring the possibility of metal-free carbon ferromagnets further toward fruition. One such technique is the measurement of the magnetostriction of the sam- ple in mention. Magnetostriction occurs in ferromagnetic materials and results in a change in the dimensions of the specimen when a magnetic field is applied, which is usu- ally on the order of a few parts per million (ppm) [11–13]. We have recently developed a novel method to measure magnetostriction of specimens using Atomic Force Mi- croscopy (AFM) and an electromagnet, positioned around the sample during measurement [14]. The method allows the direct observation of magnetically induced unidirec- tional topographic changes in the samples’ dimensions in a profoundly illustrative manner. Strains as small as 5×10 -8 have been observed on metallic wires, and the topographic data clearly shows the mechanism of magnetostriction in the nanometer regime. In this study, we perform similar