Spin-density-wave transition of (TMTSF)₂PF₆ at high magnetic fields

The transverse magnetoresistance of the Bechgaard salt (TMTSF)₂PF₆ has been measured for various pressures, with the field up to 24 T parallel to the lowest-conductivity direction c^*. A quadratic behavior is observed in the magnetic-field dependence of the spin-density-wave (SDW) transition temperature T_SDW. With increasing pressure, T_SDW decreases and the coefficient of the quadratic term increases. These results are consistent with the prediction of the mean-field theory based on the nesting of the quasi-one-dimensional Fermi surface. Using a mean-field theory, T_SDW for the perfect nesting case is estimated as about 16 K. This means that even at ambient pressure where T_SDW is 12 K, the SDW phase of (TMTSF)₂PF₆ is substantially suppressed by the two dimensionality of the system.