Magnetic resonance in the antiferromagnetic and normal state of NH3K3C60

We report on the magnetic resonance of NH3K3C60 powders in the frequency range 9-225 GHz. The observation of an antiferromagnetic resonance below the phase transition at 40 K is evidence for an antiferromagnetically ordered ground state. In the normal state, above 40 K, the temperature dependence of the spin susceptibilty measured by electron-spin resonance agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic-resonance linewidth has an unusual magnetic-field dependence which is large and temperature independent in the magnetically ordered state and decreases rapidly above the transition. These observations agree with the suggestion that NH3K3C60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.