Comparative torque studies of alpha-(BEDT-TTF)(2)MHg(SCN)(4) (M=K, Tl, NH4)

We report comparative studies of oscillatory (de Haas-van Alphen effect) and steady (anisotropy of the static susceptibility) torque in the quasi-two-dimensional organic metals alpha-(BEDT-TTF)(2)MHg(SCN)(4) (M=K, Tl, NH4). The measurements were carried out with a torque-magnetometer in magnetic fields up to 28 T and temperatures between 0.4 and 4.2 K. Up to the highest fields, no anomalous behaviour of the steady or oscillatory torque is observed in the NH4 salt. In contrast the K and Tl salts undergo a phase transition the so-called ''kink transition'' at around 23 and 26 T respectively. These two compounds show very similar behaviour. For Theta less than or equal to 60 degrees where Theta is the angle between the magnetic field and the normal to the conducting planes the phase transition is characterized by a strong increase in the oscillation amplitude. For Theta greater than or equal to 60 degrees, the kink transition can no longer be clearly seen but up to the highest fields, several anomalies appear together with strong hysteresis of the steady torque.