Magnetic properties and phase transitions in (CH3NH2)K3C60 fulleride:: An 1H and 2H NMR spectroscopic study

The hyperexpanded (CH3NH2)K3C60 fulleride and its perdeuterated analogue were investigated by H-1 and 2 H NMR spectroscopy between room temperature and 4 K. Two phase transitions at T-s = 220 K and T-N = I I K clearly were detected in the temperature dependence of the NMR line shapes, the spectral first (M-1) and second (M-2) moments, and the spin lattice relaxation times, T-1. From H-2 NMR line shape analysis, we found that the structural phase transition at Ts was driven by the freezing out of the discrete Markovian-type jump motion of the entire K+-ND2CD3 unit that is characterized by an activation energy, E-a = 236(51) meV. Below Ts, a sudden change in the temperature dependence of M, suggests the appearance of a small but nonzero spin density on the K+-NH2CH3 unit. This could influence the electronic properties of the fulleride phase by modulating the strength of the exchange interactions between C-60(3-) anions and controlling the width, W, of the t(1u)-derived band. The H-1 NMR spectra below TN show significant line broadening consistent with the onset of long-range antiferromagnetic order. 1H NMR line shape calculations revealed the adoption of a type 11 magnetic structure with an ordering vector, 41, = (1/2, 1/2, 1/2) and individual C-60 (3-) magnetic moments of magnitude (similar to 0.7 mu B) aligned along the crystallographic a-axis.