Novel phases in the field-induced spin-density-wave state in (TMTSF)_{2}PF_{6}

Magnetoresistance measurements on the quasi-one-dimensional organic conductor $(\mathrm{TMTSF}{)}_{2}{\mathrm{PF}}_{6}$ performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K and under pressures P up to 14 kbar have revealed new phases on its $P\ensuremath{-}B\ensuremath{-}T$ phase diagram. We found a new boundary which subdivides the field induced spin density wave (FISDW) phase diagram into two regions. Whereas a low-temperature region of the FISDW diagram is characterized by a hysteresis behavior typical for the first-order transitions, in a high-temperature region of the FISDW phase diagram, the hysteresis was found to disappear. We also found that the temperature dependence of the resistance (at a constant $B)$ changes sign at about the same boundary. We compare these results with recent theoretical models.