Nanostructural magnetism of polymeric fullerene crystals
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SORDER, AND PHASE TRANSITIONS IN CONDENSED SYSTEMS
Nanostructural Magnetism of Polymeric Fullerene Crystals E. F. Shekaa, V. A. Zaetsa, and I. Ya. Ginzburgb a
b
Peoples Friendship University, Moscow, 117302 Russia Institute for Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia e-mail: [email protected] Received February 7, 2006
Abstract—The nature of magnetism in all-carbon crystals composed of polymeric layers of covalently bound fullerene (C60) molecules is considered. The results of quantum-chemical calculations performed using the unrestricted Hartree–Fock approximation and the semiempirical AM1 method are presented. It is shown that the exchange integrals J of both a free C60 molecule and a monomer unit of the polymer are too large ensure the required magnetic susceptibility of the fullerene crystal. However, the J value exhibits an approximately n-fold decrease for an oligomer molecule consisting of n C60 units. Therefore, in the case of large n, the exchange integral can be reduced to a low level sufficient to provide for a significant magnetic susceptibility. A nanosize (scaly) model of the observed magnetism is proposed that is consistent with recent experimental data, which are indicative of a nanostructural character of magnetic fullerene samples. PACS numbers: 75.50.Xx, 75.40.Mg, 31.15.Ct DOI: 10.1134/S1063776106110082
1. INTRODUCTION Since the discovery of magnetism in all-carbon crystals consisting of polymeric layers of covalently bound C60 fullerene molecules [1], the still unclear nature of this phenomenon offers an intriguing issue for research. The number of publications devoted to this problem count in dozens that naturally led to a recent monograph [2] systematizing the main concepts and results achieved in this direction. The main issue is still far from being completely clear and this paper offers one more attempt to shed light on this phenomenon. The proposed approach is based on a traditional notion that the magnetism of any crystalline substance must be related to features of the electron structure of the main structural block that supplies unpaired (magnetic) electrons (spins). It is also well known that, in addition to the primary electron structure, the package of these blocks is the second operation factor. As for the structural elements of polymeric fullerene crystals, these are C60 molecules (monomers). It is conventionally assumed that the singlet C60 molecule has no unpaired electrons. This assumption is quite well substantiated in experiment: no significant EPR signals from fullerene samples have been observed either in the gas or in the solid state. Being not subjected to question, this fact underlies all quantum-chemical calculations of the singlet ground state of both C60 molecules and the crystals built of them, which have been per-
formed until recently without allowance for the electron spin, within the framework of the closed shell approximation. In this approximation, a thorough analysis of the possibility that magneti
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