IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 22 (2010) 046001 (7pp) doi:10.1088/0953-8984/22/4/046001 The role of sp-hybridized atoms in carbon ferromagnetism: a spin-polarized density functional theory calculation X F Fan 1 , L Liu 1,3 , R Q Wu 2 , G W Peng 2 , H M Fan 2 , Y P Feng 2 , J-L Kuo 1 and Z X Shen 1 1 School of Physical and Mathematical Sciences, Nanyang Technological University, 637616, Singapore 2 Physics Department, Blk. S12, Faculty of Science, National University of Singapore, 2 Science Drive 3, 117542, Singapore E-mail: LiuLei@ntu.edu.sg Received 6 October 2009, in final form 9 November 2009 Published 5 January 2010 Online at stacks.iop.org/JPhysCM/22/046001 Abstract We address the room-temperature (RT) carbon ferromagnetism by considering the magnetic states of low-dimensional carbons linked by sp-hybridized carbon atoms. Based on the spin-polarized density functional theory calculations, we find that the sp ∗ orbitals of carbon atoms can bring magnetic moments into different carbon allotropes which may eventually give rise to the long-range ferromagnetic ordering at room temperature through an indirect carrier-mediated coupling mechanism. The fact that this indirect coupling is Fermi-level-dependent predicts that the individual magnetism of diverse carbon materials is governed by their chemical environments. This mechanism may help to illuminate the RT magnetic properties of carbon-based materials and to explore the new magnetic applications of carbon materials. (Some figures in this article are in colour only in the electronic version) 1. Introduction Ferromagnetism, as is well known, usually has its place in materials with the transition metal elements which possess unpaired d or f electrons [1]. These unpaired electrons can form the local magnetic moments and the coupling between them may result in the collective magnetism. For example, in the RT dilute magnetic semiconductors [2–4], it is expected that the local magnetic moments from the transition metal can be coupled together by the medium carriers (hole or electron) from the semiconductor host. While RT ferromagnetism was routinely recognized as the property of materials containing heavy metallic elements [1], a fundamental question of whether RT ferromagnetism exists in purely light-element materials arises naturally. In the 1980s, the unconventional metal-free ferromagnetism was first seen in pure organic compounds [5, 6]. However, the Curie temperature is just very low, typically much lower than liquid 3 Author to whom any correspondence should be addressed. nitrogen temperature, due to the very weak intermolecular ferromagnetic interaction. Therefore, the recent discovery of room-temperature ferromagnetism in carbon has attracted a great deal of attention from the scientific community [7–9]. So far, many experiments have been carried out to study the high-temperature ferromagnetic behaviors in different forms of carbon materials [8, 10–22]. For example, in 2003, Esquinzai et al have reported that highly oriented pyrolytic graphite samples bombarded with protons show RT ferromagnetic ordering [14]. In 2004, carbon foam, a spongy form of carbon, was found to have strong but temporary ferromagnetism at room temperature [16]. In 2005, nitrogen- and carbon-irradiated nanosized diamond particles were also found to have RT ferromagnetic hysteretic behavior [18]. In 2009, RT ferromagnetism was reported in graphene material prepared from graphene oxide [21]. While the discovery of RT carbon magnets may be a breakthrough in the search for metal-free magnetic materials, the origin of this magnetic behavior remains an open question [21, 23]. A basic issue is 0953-8984/10/046001+07$30.00 © 2010 IOP Publishing Ltd Printed in the UK 1