Observation of magnetization step at order–disorder transition in MgB 2 single crystals

Abstract

We have studied the order–disorder transition in high quality MgB 2 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 M rev( 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 M rev( H). Similar step-like behavior is also observed in the temperature dependence of magnetization M rev( 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.