Observation of magnetization step at order-disorder transition in MgB2 single crystals

We have studied the order-disorder transition in high quality MgB2 single crystals, using a torque magnetometry combined with a 'vortex shaking' technique. In the wide range of temperature T, field H and the H direction, we succeed in obtaining reversible magnetization curves Mrev(T, H) by shaking the pinned vortices. Especially at low temperatures below 25 K and high fields, where the irreversible magnetization curve exhibits the peak effect due to the order-disorder transition, it is found that the peak is transformed into the clear step in Mrev(H). Similar step-like behavior is also observed in the temperature dependence of magnetization Mrev(T). These results give direct evidence that the order-disorder transition, which is hidden by the large hysteresis of magnetization, has the nature of first-order transition.