Torque measurements on α-(BEDT-TTF)2MHg(SCN)4

We report measurements of the anisotropy of the susceptibility tensor in the quasi-two-dimensional organic metal α-(BEDT-TTF)2KHg(SCN)4, which undergoes a magnetic phase transition below 10K, by means of a capacitive torquemeter in magnetic fields up to 14T and temperatures between 100K and 1.4K. The results are compared with those of another α-phase (M=NH4) which shows no magnetic groundstate. The principal axes of the susceptibility tensor in the KHg-salt are approximately along the crystal a-, b∗- and c-axis (b∗ is the normal to the ac - plane). Above the phase transition (T≥10K) the susceptibility shows a pronounced temperature and field independent anisotropy in all principal directions. Similar anisotropies are found in the NH4-salt. The magnetic groundstate (T<10K) is characterized by a strong increase of |χa−χb∗| and |χb∗−χc| (χa, χb∗ and χc are the eigenvalues of the susceptibility tensor). On the contrary the anisotropy in the ac-plane |χc−χa| remains field- and temperature independent down to 1.4K. These results together with former SQUID-magnetometer measurements show that there exists a complex spin structure in the magnetic groundstate of the KHg-salt.