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The ESR spectra of a TDAE 1 -C 2 60 single crystal show the existence of antiferromagnetic (AFM) correlations between the unpaired spins on neighboring C 2 60 ions along the c axis and spin canting which leads, below T c , to weak ferromagnetism along a direction perpendicular to the c axis. This suggests that a Dzyaloshinsky-Moriya type mechanism is responsible for ferromagnetic ordering rather than itinerant ferromagnetism. The incomplete orientational ordering of the C 2 60 ions leads to a distribution of exchange coupling constants resulting in spin-glass type behavior which coexists with long range ferromagnetic ordering below T c . The observation of AFM correlations may help to understand why the same basic molecule C 60 can support such different phenomena as ferromagnetism (in TDAE-C 60 ) and superconductivity (in, e.g., K 3 C 60 ).

To check on the nature of the weak magnetic order in polycrystalline magnetoelectric Pb(Fe1∕2Nb1∕2)O3 the X-band, Q-band, and far infrared electron paramagnetic resonance (EPR) spectra have been measured between 4 and 600K and compared with magnetic susceptibility and magnetization data. The asymmetric line shapes can be simulated at higher temperature by thermally fluctuating superparamagnetic nanoclusters. The pronounced temperature dependence of the position of the spectra demonstrates the presence of an internal magnetic field which is small but nonzero even at room temperature, i.e., far above the antiferromagnetic transition. The electronic spin-spin exchange has been found to be in the terahertz range. The magnetization data reveal a weak ferromagnetism even above 300K and a break in the temperature dependence of susceptibility at the paramagnetic to ferromagnetic transition.

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