Magnetic properties of powder and crystalline TDAE-C60

Magnetic properties of a molecular soft ferromagnet TDAE-C60 (TDAE; tetrakis(dimethylamino) ethylene) are studied by SQUID magnetometer both in powder and crystalline forms. In powder samples, a clear hysteresis between zero-field cooled (ZFC) and field cooled (FC) magnetizations was observed below about 10 K, consistent with our previous observation of a cusp in the temperature dependence of nonlinear ac susceptibility. In addition, an extremely slow relaxation phenomena of magnetization and the stretched exponential type time-dependence was observed at low temperatures. These characteristics are reminiscent of a “spin-glass” with the freezing temperature Tf at about 10 K, although the temperature dependence of magnetic moment at higher temperature above 10 K behaves like a ferromagnet with a transition temperature Tc∼16 K. On the other hand, an extremely sharp phase transition at ∼ 16 K accompanied by a stepwise structure at about 10 K was observed in crystalline samples.