13C NMR study of commensurate antiferromagnetism in (TMTTF)2Br

Abstract

Quasi-one-dimensional organic conductors (TMT$C$F)${}_{2}$$X$ have various electric and magnetic properties. Although theoretical and experimental studies have suggested that (TMTTF)${}_{2}$Br has the properties of commensurate antiferromagnetism, details of the magnetic structure of this compound are unclear. Two types of antiferromagnetism are expected, one due to a localized electron and the other to the nesting of the Fermi surface. We therefore assessed the antiferromagnetic structure of (TMTTF)${}_{2}$Br using ${}^{13}$C NMR. Site assignment of the observed antiferromagnetic spectrum confirmed that there were two magnetic molecular sites, with staggered moments and amplitude of 0.11${\ensuremath{\mu}}_{B}$/molecule, as well as nonmagnetic molecular sites. A commensurate structure with antiferromagnetic ordering of ($\ensuremath{\uparrow}\ensuremath{\circ}\ensuremath{\downarrow}\ensuremath{\circ}$) along a one-dimensional chain would be expected as the freezing of antiferromagnetic fluctuations in the charge-ordered phase of the (TMTTF)${}_{2}$AsF${}_{6}$ salt. The presence of a nodal site is strongly suggestive of the nesting type antiferromagnetism. The fine structure of the antiferromagnetic spectrum suggests superlattice along the interchain direction. We could not observe line broadening due to the charge order above the antiferromagnetic transition.