Magnetic anisotropy and low-frequency dielectric response of weak ferromagnetic phase in kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene

We report a detailed characterization of the magnetism and AC transport in single crystals of the organic conductor kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl by means of magnetic anisotropy measurements and low-frequency dielectric s pectroscopy. Magnetic anisotropy obeys Curie-Weiss law with negative Curie- Weiss temperature in the temperature range 300 K-70 K. An antiferromagnetic transition with concomitant canted antiferromagnetic state is established at 22 K. A large hysteresis in the spin-flop transition and magnetic field reversal of the weak ferromagnetic magnetization are documented for the fir st time. A broad dielectric relaxation mode of moderate strength (Delta eps ilon approximate to 3 x 10(3)) emerges at 32 K, and weakens with temperatur e. The mean relaxation time, much larger than that expected for single-part icle excitations, is thermally activated in a manner similar to the DC cond uctivity and saturates below 22 K. These features suggest the origin of the broad relaxation as an intrinsic property of the weak ferromagnetic ground state. We propose a charged domain wall in a random ferromagnetic domain s tructure as the relaxation entity. We argue that the observed features migh t be well described if Dzyaloshinsky-Moriya interaction is taken into accou nt. A Debye relaxation with similar temperature dependence was also observe d and seems to be related to an additional ferromagnetic-like, most probabl y, field-induced phase. We tentatively associate this phase, whose tiny con tribution was sample dependent, with a Cu2+ magnetic subsystem.