The easiest way to achieve that is to use a paramagnetic anion like FeCl4-, i.e. by adding FeCl3 to some chloride ionic liquid precursor. See, for example, in
PCCP, 2010,12, 8919–8925, DOI 10.1039/b920652n
However, which approach and IL to use depends on your requirements. You might consider using magnetic nanoparticles stabilized in ionic liquids (e.g. by reacting [P66614][FeCl4] with Fe powder in the presence of some base like ammonia).