Anomalous flux jump and supercooling phenomenon around the order-disorder transition in MgB₂ single crystals

We have performed the detailed measurements of temperature dependence of magnetization M(T) on MgB₂ single crystals in a wide range of field H using torque magnetometry. In a field region, where the peak effect in M(H) is observed, we find that M(T) of the warming branch increases monotonously with T and exhibits a small step-like increase at T₁^*, which is related to the order-disorder transition, while M(T) of the cooling branch, showing the larger values than that in warming branch below T₁^*, drops suddenly toward the warming branch at T₂^*. We ascribe this anomalous behavior of M(T) of the cooling branch to a supercooling phenomenon of disordered phase, which transforms to the ordered state at T₂^*.