INTERPLAY OF THE SPIN-DENSITY-WAVE STATE AND MAGNETIC-FIELD IN THE ORGANIC CONDUCTOR ALPHA-(BEDT-TTF)(2)KHG(SCN)(4)

Magnetic phase diagram of a quasi-two-dimensional organic conductor al pha-(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 bounda ry is clearly distinguished from so-called kink transition of the magn etoresistance. 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 tra nsition follows well the theoretical expectation that SDW has to be st abilized by a magnetic field. This allows us to estimate such importan t band parameters of the quasi-one-dimensional section of the Fermi su rface as an effective mass, m(1D) similar or equal to (0.5 +/- 0.1)m(0 ), and an upper limit of an imperfect nesting bandwidth t(c)(') simila r or equal to (10 +/- 1) K. The other phase boundaries determined by t he position of the kink and hysteresis properties of the magnetoresist ance are interpreted as subphases inside the SDW phase. Inside the SDW phase, we find an additional phase boundary at the temperature-indepe ndent field of 23 T, which corresponds to the appearance of de Haas-va n Alphen oscillations on a magnetic torque curve. At the 23 T boundary , both the effective mass m, and the Dingle temperature, T-D, change their values from m = (1.67 +/- 0.05) m(0) and T-D = 3.7 - 4.0 K in l ow magnetic field region to (1.95 +/- 0.05) m(0) and 2.5 - 2.8 K in hi gh field region. The latter phenomenon is discussed in terms of a reco nstruction of the Fermi surface due to the SDW formation. Hysteresis o f the magnetoresistance observed in one of the subphases inside the SD W phase is studied in detail by measuring both the temperature and the magnetic field dependences.