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 (T 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 -4 )μ B /formula. This low-temperature state is characterized by a sequence of first-order magnetization jumps