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The static magnetization and magnetic susceptibility anisotropy of (TMTTF)2 X(where X = AsF6 and SbF6) have been investigated between 3 and 300 K for magnetic field strengths up to 50 kG. A spin Peierls phase transition is observed in AsF6 salt around 11 K, whereas in the (TMTTF)2 SbF6 compound an antiferromagnetic (AF) transition is detected around 7 K.

Electrical conductivity and ESR experiments on several new salts of the unsymmetrical molecule dimethylethylenedithiotetrathiafulvalene are described. According to the ESR results, the compounds can be classified in two groups labelled α and β. The α compounds present similar magnetic behavior with an increase of the spin susceptibility at low temperature. The properties of the β samples are reminiscent of those of the TMTTF salts exhibiting either a spin-Peierls or an antiferromagnetic ground state below 20 K.

A new antiferromagnetic organic salt, (DIMET) 2 SbF 6 is presented. The peculiar, strongly dimerized structural arrangement of this salt allows its description in terms of quasi-isolated dimers. In this modelization the exchange coupling constants are deduced from transfer integrals and the anisotropy energy is calculated from dipolar interactions. The good agreement found between our description and experimental results is an asset for discussion about the origin of antiferromagnetism in organic conductors.

The low-temperature phase transitions of tetramethyltetrathiafulvalene thiocyanate are described through magnetic and structural studies. A first transition, which occurs at 160 K, drives the electron gas into a localized state and is shown to be associated with a superstructure formation. Then, the condensation of an unexpected antiferromagnetic ground state is observed around 7 K. We suggest that this striking behavior is related to the peculiar wave vector (${a}^{*}, \frac{1}{2}{b}^{*}, \frac{1}{2}{c}^{*}$) of the 160-K superstructure.

The low-temperature magnetic properties of (DIMET)2SbF6 have been probed by antiferromagnetic resonance experiments and the parameters which characterize the magnetic ordering are determined. Taking advantage of the structure of this compound which presents strongly dimerized dimers, we show that our results can be quantitatively explained using standard expressions for both the exchange and the anisotropy energies. Finally a brief comparison with other antiferromagnetic organic materials is presented.

The anisotropy in the static magnetic susceptibility of some tetramethyltetrathiafulvalene salts (TMTTF)2X (where X- = PF-6, ClO-4, SCN- and Br-) has been investigated with a torque method between 3 K and 300 K for magnetic fields up to 11 kG. The experimental results show that the salts with X- = SCN- and Br- become antiferromagnetic at about 8 and 13 K respectively, whereas for the two other salts above 1 kG no field dependence of the magnetic susceptibility is observed down to 3 K.