Weak antiferromagnetic superexchange interaction in fcc C60Hn

Abstract

A recent density-functional calculation for fcc C 60 H n (n = odd) [K. W. Lee and C. E. Lee, Phys. Rev. Lett. 106, 166402 (2011)] proposed the existence of Stoner ferromagnetism based on an itinerant band model. However, our density-functional calculation shows that the antiferromagnetic (AFM) configuration is slightly more stable than the ferromagnetic (FM) one. This preference for antiferromagnetism over ferromagnetism is analogous to the case of a dimer (C 60 H) 2 , where each C 60 H is spin polarized by an intramolecular exchange and the two magnetic moments are antiferromagnetically coupled with each other. The results demonstrate that the underlying mechanism of the magnetic order in fcc C 60 H n is associated with the AFM superexchange between the magnetic moments created by H dopants. The observation of ferromagnetism in fullerenes has attracted intensive attention due to its interest concerning carbon magnetism and potential technological application in the emerging field of spintronics.1-5 However, there has long been controversy about whether the origin of the observed ferromagnetism is associated with extrinsic iron impurities [6][7][8] or intrinsic defects.9-14 The latter intrinsic defects involve carbon vacancies in polymerized fullerenes 9-11 or doped fullerenes 12-14 C 60 R n (R: nonmagnetic elements such as H and O atoms) where doping creates fullerene radical adducts with unpaired spins localized on fullerene. Experimental studies for photo-oxidated fullerenes 3,4 and hydrofullerite 5 C 60 H 24 observed a signal of ferromagnetism at room temperature. However, a density-functional theory (DFT) calculation 13 for C 60 O did not support the existence of ferromagnetism, whereas a DFT calculation 14 for fcc C 60 H n predicted a strong itinerant ferromagnetism with odd-numbered H dopants. From their DFT calculation within the local-density approximation, Lee and Lee (LL) 14 found that H dopants on an fcc C 60 crystal create quasilocalized π electrons leading to a narrow half-filled band, and concluded that a direct overlap of the π electrons between adjacent C 60 H n molecules gives rise to Stoner (itinerant) ferromagnetism with an exchange splitting of ∼0.2 eV. However, this exchange splitting is not due to the Stoner-type FM exchange but to an intramolecular exchange (i.e., Stoner parameter I ), as discussed below. As a matter of fact, the ferromagnetic (FM) and antiferromagnetic (AFM) order is governed by the exchange interaction energy between neighboring magnetic moments (i.e., exchange coupling constant J in the Heisenberg model).Recently, it was reported 15 that the microscopic mechanism of defect-induced magnetism in dilute magnetic semiconductors could be similar to that in carbon-based materials such as C 60 polymers, TDAE-C 60 , graphene ribbons, and irradiated graphite. There are two different mechanisms that describe the exchange interaction in diluted magnetic semiconductors: The double-exchange mechanism favors FM coupling, whereas the superexchange mechanism usually leads to AFM co...

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