Time decay of the remanent magnetization in TDAE-C60

The time decay of the isothermal magnetization of tetrakis-dimethylaminoethylene (TDAE)-${\mathrm{C}}_{60}$ after switching off a small external magnetic field has been studied between 20 and 4 K. Below ${\mathit{T}}_{\mathit{c}}$ a remanent magnetization ${\mathit{M}}_0$—representing the fraction of the frozen out spins—that becomes unobservably small above 12 K has been found. The relaxing magnetization fraction component ${\mathit{M}}^{\mathrm{*}}$, on the other hand, increases as ${\mathit{T}}_{\mathit{c}}$ is approached from below. The time decay of the magnetization is rather slow below ${\mathit{T}}_{\mathit{c}}$ and can be described by a stretched exponential form M(t)=${\mathit{M}}_0$+${\mathit{M}}^{\mathrm{*}}$exp[-(t/τ${)}^{\mathrm{\alpha }}$]. The exponent α decreases from 0.14 at 4 K to 0.08 at 12 K. This shows that the spectrum of relaxation times slowly changes with temperature and becomes very wide as ${\mathit{T}}_{\mathit{c}}$ is approached from below. No slowly relaxing component is found above ${\mathit{T}}_{\mathit{c}}$. The above features are characteristic of a spin-glass-like freezing process.