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

We report measurements of the anisotropy of the susceptibility tensor in the quasi-two-dimensional organic metal alpha-(BEDT-TTF)(2)KHg(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 alpha-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 greater than or equal to 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 /chi(a)-chi(b)*/ and /chi(b)*-phi(c)/(chi(a), chi(b)* and chi(c) are the eigenvalues of the susceptibility tensor). On the contrary the anisotropy in the ac-plane /chi(c)-chi(a) 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.