Critical exponents at the ferromagnetic transition were measured for the
first time in an organic ferromagnetic material tetrakis(dimethylamino)ethylene
fullerene[60] (TDAE-C60). From a complete magnetization-temperature-field
data set near Tc=16.1±0.05, we determine the susceptibility and
magnetization critical exponents γ=1.22±0.02 and β=0.75±0.03 respectively, and the field vs. magnetization exponent at Tc of
δ=2.28±0.14. Hyperscaling is found to be violated by Ω≡d′−d≈−1/4, suggesting that the onset of ferromagnetism can be
related to percolation of a particular contact configuration of C60
molecular orientations.Comment: 5 pages, including 3 figures; to appear in Phys. Rev. Let