Magnetic phase diagram of a quasi-two-dimensional organic conductor α-(BEDT-TTF${)}_2$KHg(SCN${)}_4$ is revisited from a viewpoint of magnetic torque measurements in high fields up to 30 T. A phase boundary that is interpreted as a metal-spin density wave (SDW) phase transition is found by using torque measurements. It is shown that this phase boundary is clearly distinguished from so-called kink transition of the magnetoresistance. We demonstrate that the transition temperature defined by the midpoint of the broad phase transition is almost independent on magnetic field up to 23 T. Onset temperature of the transition shifts from about 8 K at H=0 T to higher temperatures with increasing of a magnetic field, and tends to be saturated. The onset line of this transition follows well the theoretical expectation that SDW has to be stabilized by a magnetic field. This allows us to estimate such important band parameters of the quasi-one-dimensional section of the Fermi surface as an effective mass, ${\mathit{m}}^{1\mathrm{D}}$≃(0.5±0.1)${\mathit{m}}_0$, and an upper limit of an imperfect nesting bandwidth ${\mathit{t}}_{\mathit{c}}$′≃(10 ±1) K. The other phase boundaries determined by the position of the kink and hysteresis properties of the magnetoresistance are interpreted as subphases inside the SDW phase. Inside the SDW phase, we find an additional phase boundary at the temperature-independent field of 23 T, which corresponds to the appearance of de Haas–van Alphen oscillations on a magnetic torque curve. At the 23 T boundary, both the effective mass, m*, and the Dingle temperature, ${\mathit{T}}_{\mathit{D}}$, change their values from m*=(1.67 ± 0.05) ${\mathit{m}}_0$ and ${\mathit{T}}_{\mathit{D}}$=3.7–4.0 K in low magnetic field region to (1.95 ± 0.05) ${\mathit{m}}_0$ and 2.5–2.8 K in high field region. The latter phenomenon is discussed in terms of a reconstruction of the Fermi surface due to the SDW formation. Hysteresis of the magnetoresistance observed in one of the subphases inside the SDW phase is studied in detail by measuring both the temperature and the magnetic field dependences. © 1996 The American Physical Society.

• Received 22 April 1996

DOI:https://doi.org/10.1103/PhysRevB.54.12969