Observation of antiferromagnetic resonance in tetramethyltetrathiafulvalene bromide [(TMTTF)${}_2{}^{}{}_{}{}^{}\mathrm{Br}$] shows that the semiconducting ground state is antiferromagnetic, with a Néel temperature of 13±1 K. Two modes are observed as a function of magnetic field, for orientations near the easy ($b^{\prime }$) axis, at a fixed frequency of 9.3 GHz. At 4.2 K the zero-field modes are estimated to have frequencies of 11.7 and 18.2 GHz (4.3 and 6.6 kOe).

• Received 28 June 1982

DOI:https://doi.org/10.1103/PhysRevB.26.6319