EPR analysis of spin susceptibility and line width in the hyperexpanded fulleride (CH₃NH₂)K₃C₆₀

The electron paramagnetic resonance (EPR) technique is used to study the temperature evolution of the electronic and magnetic properties of the hyperexpanded fulleride (CH₃NH₂)K₃C ₆₀ in the temperature range 4-294 K. Unambiguous signatures for the development of large internal magnetic fields and a transition to an antiferromagnetic ground state below T_N = 11 K are provided by the temperature dependence of the spin susceptibility, the EPR line width, and the position of the EPR line. The EPR results in the paramagnetic regime are indicative of an insulating electronic state, with localized S=1/2 moments on the C₆₀^3- ions. The accompanying complex evolution of the spin susceptibility, EPR line width, and g-factor anisotropy furthermore reveals that effects of electronic origin (t_lu orbital order-disorder transition, crossover from static to dynamic Jahn-Teller distortions) in the vicinity of the structural transformation are important for the understanding of the exchange interactions between the fulleride ions.