Anisotropy of ESR parameters in organic ferromagnet TDAE-C₆₀

We study the anisotropy of ESR parameters using single crystal TDAE-C ₆₀, which has two polymorphisms: the one is ferromagnetic a-phase and the other is non-ferromagnetic α′-phase. In both α′- and α-phase, the line-widths and g-factors show anisotropic angular dependences at room temperature, such as ΔH_pp^b > ΔH_pp^c ∼ ΔH_pp^a and g^c > g^b = g^a. The anisotropic behaviour of g-factors in both phases suggest the uniaxial rotation of Jahn-Teller distorted C₆₀ molecules around the c-axis, which is the close-packing direction of C₆₀ molecules. In the α′-phase the temperature dependence of g-factors has a clear anomaly around 160 K. Below this temperature, the observed g-factors are in-between the monovalent C ₆₀ and TDAE⁺, indicating the restoration of missing TDAE spins. In the α-phase, the g-factors continuously shift with decreasing temperature, become anisotropic below 230 K, and show no remarkable anomaly at the structural phase transition temperature. These results suggest that the difference exists in the mixing between the wave functions of C₆₀ and TDAE, which is possibly developed by the structural difference between α′- and α-phases. This difference separates the magnetic peculiarities of two polymorphisms in TDAE-C₆₀.