Detailed electron spin resonance experiments have been performed on single crystals of $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6,$ $(\mathrm{TMTSF}{)}_2{\mathrm{AsF}}_6,$ and $(\mathrm{TMTTF}{)}_2\mathrm{Br}.$ These low-dimensional organic compounds undergo transitions to incommensurate and commensurate antiferromagnetic ground states, respectively. From the antiferromagnetic resonances observed at low temperatures we obtain information on the anisotropy energies, the zero-field spin-wave frequency, and the spin-flop field. The anisotropy energies of the different AFM ground states depend strongly on the spin-orbit coupling constant of selenium or sulfur. Well below the phase transition, the order parameter of $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6$ and $(\mathrm{TMTSF}{)}_2{\mathrm{AsF}}_6$ is only weakly temperature dependent indicating visible deviations from the behavior expected in mean-field theory. In contrast, the sublattice magnetization of $(\mathrm{TMTTF}{)}_2\mathrm{Br}$ is a function of temperature down to 4.2 K suggesting the excitation of thermal magnons. The temperature dependence of the spin susceptibility in the paramagnetic state can be described by the Hubbard model in the limit of strong Coulomb repulsion. Near the phase transitions the spin dynamics are characterized by antiferromagnetic fluctuations.

• Received 19 January 2000

DOI:https://doi.org/10.1103/PhysRevB.62.6512