Observation of Magnetic Transition in Quantum Nanomagnet Mn₄Br

Abstract

The magnetization of the polycrystalline manganese cluster Mn₄Br was measured down to 0.13 K under a magnetic field of ±15 kOe. It shows a gradual drop at around 1.5 K and the heat capacity measurement at 0 kOe reveals a sharp anomaly at 1.33 K, indicating the occurrence of a second-order antiferromagnetic (AFM) phase transition. The M-H curve well below the transition temperature T_N shows a step around 2.5 kOe, which probably corresponds to the transition from the AFM phase to metamagnetic (MM) phase. The observed field agrees with that estimated from T_N for the MM transition, using a simple molecular field theory. This is the first observation of a well-defined AFM and also a MM phase among the many manganese clusters with a high-spin ground state that function as molecular nanomagnets.