Ferromagnetic hysteresis and aftereffect in polycrystalline TDAE-C-60

The microscopic origin of ferromagnetic ordering as well as the existence of hysteresis in TDAE-C-60 are contradictorily discussed in the literature. At 1.7 K the zero-field magnetization, M(0), extrapolated from the demagnetization curve of a TDAE-C-60 powder, is only a small fraction of the saturation magnetization M-s approximate to 1.2 A m(2)/kg (or emu/g) measured at a field of 4 MA/m. The small value of M(0)IMs may be due to (i) a large amount of small superparamagnetic particles and/or (ii) alignment of a canted magnetic structure at high field. After leaving TDAE-C-60 powder in a sealed quartz tube with He for eight weeks this ratio increased from about 8% to 65% but M-s remained unchanged. This observation points to agglomeration of superparamagnetic clusters increasing the contribution to the ferromagnetic behavior. Typical extrinsic magnetic quantities describing a ferromagnetic material, such as remanence M-r, coercivity H-j(c) and viscosity S, have also been measured. For temperatures 10, 5, and 1.7 K typical values of M-r and H-j(c) are 30, 90, 200 mu T, and 50, 110, and 200 A/m, respectively. The values of coercivity have the order of magnitude of those of low-carbon steel (at room temperature). After leaving a pressed TDAE-C-60 sample in a sealed quartz tube with He for six weeks M-r increased and H-j(c) decreased, both by about 30%. The samples show a logarithmic time-dependence of magnetization Delta M=-S 1n(1+t/t(0)) with large values of the viscosity S. In the remanent state (at H=0) the measured values of S/M-r are 0.025, 0.021, and 0.019 for T = 10, 5, and 2.5 K, respectively. (C) 1997 American Institute of Physics.