Element-Specific Probe of the Magnetic and Electronic Properties of Dy incar-Fullerenes F. Bondino,* ,² C. Cepek, ² N. Tagmatarchis, ‡,§ M. Prato, § H. Shinohara, ⊥ and A. Goldoni | Laboratorio Nazionale TASC, INFM-CNR, S.S. 14 km 163.5 in Area Science Park, 34012 Trieste, Italy, Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Vass. Constantinou 48 AVenue, 116 35 Athens, Greece, Dipartimento di Scienze Farmaceutica, UniVersita ` di Trieste, Piazzale Europa 1, 34127 Trieste, Italy, Department of Chemistry, UniVersity of Nagoya, 464 8602 Nagoya, Japan, and Sincrotrone Trieste S.C.p.A., S.S. 14 Km 163.5 in Area Science Park, 34012 Trieste, Italy ReceiVed: October 17, 2005; In Final Form: February 15, 2006 The magnetic and electronic properties of a single atom and a pair of Dy atoms encapsulated inside fullerene carbon cages have been examined using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) as well as resonant photoelectron spectroscopy (RESPES) across the Dy M 4,5 -edge. The comparison of the measured XAS spectra with multiplet calculations indicates that the encaged Dy has a 4f 9 configuration. The presence of Dy 5d spectral weight in the valence band is not detected by RESPES, indicating that Dy is in a formally trivalent state. The evolution of the encaged Dy orbital and spin moments of the 4f orbitals as a function of the applied magnetic field and temperature has been obtained from XMCD measurements. At 6.9 T and 4 K, both the orbital and the spin magnetic moments of the encaged Dy 4f electrons are dramatically smaller than those expected for the free Dy 3+ at saturation. I. Introduction incar-Fullerenes, known also as endohedral fullerenes, are a novel kind of molecular compounds, consisting of metal atoms, clusters, or carbides encapsulated in the inner empty space of a carbon cage. 1 The unique structure of these materials and the possibility to control the properties by changing the size of the cage and the number, nature, and kind of the encaged atoms is expected to lead to new materials with remarkable physical properties and potential applications. For instance, atoms with high magnetic moments, such as Gd, Dy, and Ho, encapsulated in fullerenes or fullerenols, C 60 (OH) n , are materials with potential use as magnetic resonance imaging (MRI) contrast agents, 2-6 diagnostic and therapeutic radiopharmaceuticals, 7 photoelectrochemical cells, 8 and molecular memories. 9 Despite the large number of investigations of these systems, the magnetic properties of the core atoms and the extent of the interaction between the core atom and the carbon cage are still open points. The charge transfer between the core atom and the carbon cage is an important quantity, and several studies have been performed to establish the effective valence of the encaged ion. In several cases, a purely ionic picture was not sufficient to describe the electronic structure of incar-fullerenes. For example, the core ions of La@C 82 and Sc@C 84 , originally thought to be purely trivalent and divalent ions, respectively, were found to be more correctly described by the inclusion of a finite hybridization between the core atom and the fullerene cage. 10,11 The effective valence state of the core atom and the metal- cage interaction play an important role in the magnetic properties of the incar-fullerenes. In Ho@C 82 a dramatic reduction by 40% of the magnetic moment with respect to free Ho 3+ was observed. This behavior was ascribed to the carbon cage crystal field effect and to the partial hybridization between the metal and the carbon cage orbitals. 12 Differently, in Gd@C 82 the magnetic moment of the system is close to that of the free Gd 3+ ion, and photoemission measurements indicate a complete charge transfer from Gd to the C 82 cage. 13 In La@C 82 , superconducting quantum interference device (SQUID) measurements found an average magnetic moment larger than the one expected for free La 3+ ion 14 probably due to the incomplete charge transfer. 10 For Dy@C 82 , Mo ¨ssbauer measurements find paramagnetic relaxation at 4.2 K and an isomer shift consistent with the Dy 3+ state and a full charge transfer to the fullerene cage. 15 Magnetic susceptibility measurements 12 indicate a paramag- netic relaxation even at 1.8 K with the Dy@C 82 system having a magnetic moment reduced by 13-20% with respect to the full moment value of trivalent Dy. It is also interesting to investigate the valence state of the encaged Dy atom since a valence change from the trivalent to the lower valence state is expected to occur when the coordination number is reduced to less than 4 atoms. 16,17 Previous studies using the Dy L 3 -edge X-ray absorption spectroscopy (XAS) of Dy@C 82 indicate a Dy 3+ charge state. 18 Furthermore a variation of the valence state from Dy 3+ to Dy 2+ was found by potassium intercalation into the Dy@C 82 crystals. 19 However, recent calculations indicate the existence of cage-metal hybrid states 20 in contrast to a purely ionic picture of this system. Very recently, an investiga- tion of different lanthanide incar-fullerenes using X-ray mag- netic circular dichroism (XMCD) has been reported. 21 Here we report on XAS, XMCD, and resonant photoelectron spectroscopy (RESPES) measurements performed at the Dy M 4,5 -edge of a Dy@C 82 mono-incar-fullerene and two different Dy 2 @C 88 di-incar-fullerene isomers in the temperature range of 4-300 K under applied magnetic fields from 0 to 6.9 T. * Author to whom correspondence should be addressed. E-mail: bondino@tasc.infm.it. ² Laboratorio Nazionale TASC. ‡ National Hellenic Research Foundation. § Universita ` di Trieste. ⊥ University of Nagoya. || Sincrotrone Trieste S.C.p.A. 7289 J. Phys. Chem. B 2006, 110, 7289-7295 10.1021/jp055938z CCC: $33.50 © 2006 American Chemical Society Published on Web 03/22/2006