Antiferromagnetic resonance in methylaminated potassium fulleride (CH3NH2)K3C60

High-frequency magnetic resonance measurements (nu(L)=9.6-420 GHz) were employed to investigate the low-temperature antiferromagnetic ground state of the (CH(3)NH(2))K(3)C(60) fulleride. The frequency and temperature dependence of the intensity, linewidth, and center of the resonance signal detected below T(N) are characteristic of antiferromagnetic resonance (AFMR). The AFMR intensity is consistent with an ordered magnetic moment of mu(eff)=0.7(1)mu(B)/C(60), while the narrowing of the AFMR signal with increasing resonance frequency can be modeled with a spin-flop field of H(sf)=840(80) G and a g-factor anisotropy of delta gamma=710(50) ppm. We stress that the spin-flop field is reduced compared to the ammoniated analog (NH(3))K(3)C(60) on the account of reduced C(60)(3-)-C(60)(3-) exchange interactions. Differences in the level of the anisotropic expansion between CH(3)NH(2) and NH(3) cointercalated fullerides are likely to be responsible for the differences in the electronic structure between the two systems and ultimately may account for the reduced Neel temperature in (CH(3)NH(2))K(3)C(60).