Ferromagnetic hysteresis and aftereffect in polycrystalline TDAE-C60

The microscopic origin of ferromagnetic ordering as well as the existence of hysteresis in TDAE-C₆₀ are contradictorily discussed in the literature. At 1.7 K the zero-field magnetization, M(0), extrapolated from the demagnetization curve of a TDAE-C₆₀ powder, is only a small fraction of the saturation magnetization M_s≈1.2 A m²/kg (or emu/g) measured at a field of 4 MA/m. The small value of M(0)/M_s 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-C₆₀ 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 _JH_c and viscosity S, have also been measured. For temperatures 10, 5, and 1.7 K typical values of M_r and _JH_c are 30, 90, 200 μT, 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-C₆₀ sample in a sealed quartz tube with He for six weeks M_r increased and _JH_c decreased, both by about 30%. The samples show a logarithmic time-dependence of magnetization ΔM=-S ln(1+t/t₀) 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. © 1997 American Institute of Physics.