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 25K 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.