HIGH-FIELD MAGNETOTRANSPORT STUDIES OF (ET)2MHG(SCN)4 SALTS (M=K, NH4)

The angle dependence of the magnetoresistance (MR) of (ET)2(SCN)4 (M=K ,NH4) has been measured in steady magnetic fields up to 50 T. In (ET)2 KHg(SCN)4 We observe two SdH frequencies in the MR below the resistive ''kink'' feature at 22 T, and different values of gm* above and belo w the ''kink''; we propose that these effects result from antiferromag netic ordering between the conducting planes. In (ET)2NH4Hg(SCN)4 we h ave found that both the frequency of the Shubnikov-de Haas (SdH) oscil lations and the effective mass of the carriers vary to a high degree o f accuracy as 1/cos(theta) (theta is the angle between the field and t he normal to the conducting layers) as expected for a quasi two-dimens ional system, and in contrast to a previous study by others. Finally w e comment on the validity of the ''Harmonic Ratio'' method for measuri ng effective masses.