Magnetic Studies of (TMTTF)2X (X=PF6, ClO4, and Br)

We investigated the magnetic properties of the organic spin-chain compounds (TMTTF)2PF6, (TMTTF)2ClO4, and (TMTTF)2Br in the temperature range 2 K ≤ T ≤ 500 K by electron-spin-resonance measurements. At high temperatures the spin susceptibility of the compounds can be well described by a spin 1/2 antiferromagnetic Heisenberg chain with exchange constants J ≈ 420 K, 430 K and 500 K (X =PF6, ClO4, and Br). In (TMTTF)2ClO4 the first-order structural phase transition at tao = 72.5 K is accompanied by a dimerization of the spin chain with a singlet-triplet gap Δ ≈ 82 K. At tafm = 12 K we find an additional transition to an antiferromagnetic ordered state. (TMTTF)2PF6 undergoes a spia-Peierls transition at tsp = 19 K which can be well described using mean field theory with Δ(0) ≈ 34.5 K. The phase transition in the spin-density-wave state in (TMTTF)2Br at tsdw ≈ 13.5 K is accompanied by visible fluctuations.