Physica B 374–375 (2006) 255–258 mSR and SQUID investigation of ammoniated lithium fullerides M. Ricco` Ã , M. Belli, T. Shiroka, D. Pontiroli, M. Pagliari, F. Gianferrari, A. Gorreri Dipartimento di Fisica, Universita` di Parma & INFM, Parco Area delle Scienze 7/A, 43100 Parma, Italy Abstract The ammoniation of lithium-doped C 60 gives rise to (NH 3 Þ 6 Li x C 60 , a system which is chemically expanded while maintaining C 60 original cubic symmetry and hence the threefold degeneracy of the C x 60 frontier orbitals. Nevertheless, the on-site repulsion U dominates the bandwidth W and, by overcoming the effect of orbital degeneracy, brings to electron localisation. Muon-spin relaxation ðmSRÞ and SQUID magnetometry were used to confirm the presence of a magnetically ordered phase at low temperatures, already reported by Durand and co-workers [Nat. Mater. 2 (2003) 605]. Surprisingly, neither the expected antiferromagnetism nor the even-odd effect on electron band filling were observed down to 2 K. r 2005 Elsevier B.V. All rights reserved. PACS: 81.05.Tp; 76.75.+i; 75.75.+a; 71.27.+a; 81.07.b Keywords: Ammoniated fullerides; Muon-spin rotation; Mott insulator; Strongly correlated systems The discovery of superconductivity in the alkali metal doped fullerides A 3 C 60 (A ¼ K, Rb, Cs) has stimulated a wealth of theoretical and experimental efforts finalised to the understanding of the mechanisms underlying the high correlation effects in these systems. This class of compounds follows fairly well the BCS rule about the dependence of superconducting transition temperature on the density of states at the Fermi level DðE F ) (although with some exceptions as e.g. the ammoniated NaK 2 C 60 [1]). Essentially, an increase in the lattice parameter involves a reduction of the bandwidth, and consequently an increase of DðE F ), as confirmed by the Na 2 CsC 60 case [2], where the intercalation of a neutral molecular spacer in the lattice such as ammonia actually increases T c , coherently with the above picture. This trend is nonetheless invariably terminated by the appearance of electron localisation through a Mott–Hubbard transition to an insulating state. Lithium-doped fullerides differ from the general case of alkali intercalated compounds, since the Li x C 60 family ðx ¼ 3; 4; 5Þ shows a polymerised structure already at room temperature and pressure, which distorts both the symme- try of molecules and lattice, bringing to the disappearance of superconductivity [3]. The synthesis of lithium fullerides directly in liquid ammonia, however, prevents the forma- tion of polymer bonds and leads to the recovery of the cubic symmetry. In the compound (NH 3 ) 6 Li 3 C 60 [4] the presence of ammonia produces a sufficiently large lattice expansion to induce electron localisation, and the reported insulating antiferromagnetic behaviour below 10 K seems consistent with the above picture. With its high sensitivity even to small magnetic moment ordering and its successful use with other ammoniated alkali-metal-doped fullerides [5], the mSR technique is particularly suitable for investigating the true nature of lithium ammoniated compounds. Here we present a magnetic characterisation of this system using both mSR and dc SQUID magnetometry. Surprisingly, none of the techniques offered a clear cut evidence to support the presence of a magnetically ordered state below 20 K. Indeed, the SQUID magnetometry signal follows a Curie–Weiss magnetisation curve down to the lowest temperature, whereas the observed zero-field muon-spin depolarisation is fully compatible with that due to dipolar interactions with local nuclear moments. ARTICLE IN PRESS www.elsevier.com/locate/physb 0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.11.068 Ã Corresponding author. Tel.: +39 0521 905217; fax: +39 0521 905223. E-mail address: Mauro.Ricco@fis.unipr.it (M. Ricco`).