Magnetic resonance in the antiferromagnetic and normal state of NH_3K_3C_60

TL;DR

This study investigates the magnetic resonance properties of NH_3K_3C_60, revealing antiferromagnetic order below 40 K and supporting its characterization as a metal undergoing a Mott-Hubbard transition at that temperature.

Contribution

It provides experimental evidence of antiferromagnetic resonance in NH_3K_3C_60 and links magnetic behavior to a Mott-Hubbard transition at 40 K.

Findings

Antiferromagnetic resonance observed below 40 K.

Normal state susceptibility matches previous measurements.

Magnetic resonance line width behavior supports metallic and insulating phases.

Abstract

We report on the magnetic resonance of NH_3K_3C_60 powders in the frequency range of 9 to 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 ESR agrees with previous static measurements and is too weak to be explained by interacting localized spins in an insulator. The magnetic resonance line width 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 NH_3K_3C_60 is a metal in the normal state and undergoes a Mott-Hubbard metal to insulator transition at 40 K.