Weak ferromagnetism in κ-(ET)2Cu[N(CN)2]Cl, where (ET) is bis(ethylenedithio)tetrathiafulvalene

Magnetization measurements at ambient pressure on κ-(ET${)}_2$Cu[N(CN${)}_2$]Cl, an organic salt that is superconducting under pressure (${\mathit{T}}_{\mathit{c}}$=12.8 K at 0.3 kbar), reveal an antiferromagnetic transition near 45 K and, for the first time in this class of materials, a transition near 22 K to a state displaying weak ferromagnetic hysteresis with a saturation moment of (8×${10}^{\mathrm{-}4}$)${\mathrm{\mu }}_{\mathit{B}}$/formula. This low-temperature state is characterized by a sequence of first-order magnetization jumps.