Observation of vortex phase transition in MgB2 single crystals by a magnetic torque method

The magnetic torque measurements were performed on two MgB2 single crystals with the different values of upper critical fields Hc2 (3.7 T and 3.1 T at 4.2 K). At low temperatures, the irreversible torque-vs-field curves τ(H) in the both samples show the peak effect, accompanied by the field-history dependence, close to Hc2 for all the field directions measured. With increasing temperature, the peak in τ(H) becomes less pronounced, and disappears around T = 27 K. In the sample with smaller Hc2, τ(H) curves display diamagnetic steps at the higher temperatures. On the other hand, in the sample with larger Hc2, such a step is not observed. Our results suggest that the order-disorder transition around the peak transforms to the first-order melting transition, which is very sensitive to the degree of disorder.