Magnetic properties of polymerizedC60: The influence of defects and hydrogen

Abstract

A combination of reactive force field molecular dynamics and hybrid-exchange density functional theory (DFT) generates a defective structure of Rh-C 60 possessing an inter-cage link. Hybrid-exchange DFT is used within periodic boundary conditions to investigate the long-range magnetic coupling between the resulting defects. Inelastic neutron scattering experiments highlight the presence of hydrogen chemically bonded to carbon in the magnetic samples. A simple spin model previously applied to studies of planar conjugated electron systems is used to illustrate the mechanism through which chemically bonded hydrogen leads to a ferromagnetic ground state for this system. The recent observation of high temperature ferromagnetism in carbon materials is of great interest both as it introduces a class of potentially highly tunable materials for use in magnetic devices and because it presents a major challenge to our current understanding of magnetism. At elevated pressures and temperatures cubic C 60 fullerenes form well ordered two-dimensional polymerized phases. Below 9 GPa three distinct phases occur, with orthorhombic, tetragonal ͑T͒ and rhombohedral (Rh) symmetries.1 Further heating of these phases beyond ϳ900 K results in the collapse of the C 60 cages and the formation of hard "graphitic" phases. The ferromagnetic phases occur close to this phase boundary.2,3 A remarkably high Curie temperature ͑T c ͒ of ϳ500 K was reported in initial studies, 3 with recent data indicating an even higher T c of ϳ820 K. 4 Magnetic force microscopy (MFM) measurements 5 and additional experimental evidence 3 imply that the magnetism is an intrinsic property of carbon and not due to metallic impurities such as iron.The experimental characterization of the magnetic phases has proven difficult and currently the detailed atomic structure is not known. In situ x-ray diffraction measurements reveal a thermally activated process which converts the Rh phase (Fig. 1) into the highly disordered graphite-like phase, which displays very broad Bragg peaks. 6 The detailed structure of the magnetic phase cannot be determined from this data. Transmission electron microscopy (TEM) reveals an apparently well ordered crystalline structure in which the C 60 cages are largely intact and still in a Rh-C 60 -like arrangement.2 MFM studies established that only ϳ30% of the material is magnetic with the magnetism occurring in well defined domains. 5 As the characterization of the magnetic phase is problematic, theoretical calculations have an important role to play in determining possible local geometries. A number of previous theoretical studies have addressed the origins of magnetism in nonplanar carbon systems. Tight-binding molecular dynamics and cluster ab initio calculations have recently been used to analyze a single C 60 cage with a carbon vacancy. 7 Density functional theory (DFT) calculations have been used to examine a fullerene molecule during the transition to a nanotube segment via a series of Stone-Wales transformations 8 and a fragment with n...

Modeling spin interactions in carbon peapods using a hybrid density functional theory

“…At these separations, the intermolecular spin interaction is therefore via direct exchange between fullerenes, with a negligible contribution from interactions via the nanotube. This intermolecular coupling is much larger than the classical dipole coupling of Na@ C 60 26 and even larger than that computed for defective fullerenes with intercage links, 27 ensuring Ͼ10 3 two-qubit gate operations within the decoherence time. This surprising result follows from the HOMOs in the Sc@ C 82 chain being very extended as illustrated in Fig.…”

Structure-dependent exchange in the organic magnets Cu(II)Pc and Mn(II)Pc

“…In particular, the B3LYP functional, [35][36][37] which mixes about one-quarter Hartree-Fock exchange, has been found to give good results for dinuclear molecules, organic biradicals, and spins localized at defects in carbon-containing materials. [32][33][34] In Sec. IV, we perform DFT with B3LYP exchangecorrelation functional to calculate exchange interactions in Cu͑II͒Pc dimers based on this broken-symmetry concept.…”

Electronic structure and magnetic properties of graphitic ribbons

“…This approach has also been used succesfully in recent studies of magnetic ordering in wide band-gap semiconductors, 37 Prussian blue analogs, 38 and in fullerene based metal-free systems. 38,39 It appears that the hybrid exchange approach allows the balance between electron localization and delocalization, which is crucial in the current work, to be described more reliably than in the LSDA and PBE. The authors, however, are not aware of a systematic study of the perfomances of B3LYP for systems containing localized ͑in the sense of nonbonding or atomic͒ orbitals of only s-p nature.…”