SPONTANEOUS MAGNETIC-ORDERING IN THE FULLERENE CHARGE-TRANSFER SALT (TDAE)C-60

The zero-field muon spin relaxation technique has been used in the dir ect observation of spontaneous magnetic order below a Curie temperatur e (T-c) of similar to 16.1 kelvin in the fullerene charge-transfer sal t (tetrakisdimethylaminoethylene)C-60 [(TDAE)C-60]. Coherent ordering of the electronic magnetic moments leads to a local field of 68(1) gau ss at the muon site at 3.2 kelvin (parentheses indicate the error in t he last digit). Substantial spatially inhomogeneous effects are manife sted in the distribution of the local fields, whose width amounts to 4 8(2) gauss at the same temperature. The temperature evolution of the i nternal magnetic field below the freezing temperature mirrors that of the saturation magnetization, closely following the behavior expected for collective spin wave (magnon) excitations. The transition to a fer romagnetic state with a T-c higher than that of any other organic mate rial is now authenticated.