Magnetization measurements of a two-leg spin-1/2 ladder with strong leg interactions

We have measured the magnetization of a single crystal of the spin-1/2 ladder compound bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY), using micromechanical force magnetometry down to 30mK. DIMPY undergoes a quantum phase transition from a gapped phase with no long--range magnetic order to a gapless phase, at critical field Hc. Recent specific heat results by Hong et al. show that the gapless phase is a Tomonaga-Luttinger liquid (TLL). Low temperature magnetic susceptibility as a function of field exhibits a maximum near Hc, in consistence with the divergence of the zero temperature susceptibility at Hc expected for an ideal 1-D system. A clear minimum in the magnetization of the gapless phase as a function of temperature is observed, a minimum that marks the limit below which the TLL exists. In addition the field dependence of the TLL parameter K is studied.